JP2009260746A - Image encoder, and control method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for efficiently coding a coefficient after quantization in image encoding processing for performing frequency conversion and quantization with a pixel block as a unit. <P>SOLUTION: Image data is supplied to a boundary bit position decision section 116 through a block division section 102, a sequence transformation section 103, and a coefficient quantization section 104. The boundary position decision section 116 decides a boundary bit position B for separating each coefficient into upper and lower bit sections based on the distribution of the coefficient. A first coefficient encoding section 117 separates each coefficient into upper and lower bit sections according to the boundary bit position B to encode each of them. A redundancy determination section 118 determines redundancy in a plane at the boundary bit position B, and allows a block recoding section 122 to start encoding as a boundary bit position B-1, when the plane is redundant. An encoding selection section selects smaller data from among coding data generated by a block recoding section 122 and that generated by the first coefficient encoding section 117 for output. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image encoding technique, and more particularly to a technique for generating encoded image data by converting image data into frequency space data and encoding the obtained conversion coefficient.

  Conventionally, as an image encoding method, transform encoding is used in which image data is converted into frequency space data and encoded. JPEG recommended as an international standard system for still image coding is a representative example of transform coding using DCT as a transform method to a frequency space. The JPEG encoding process will be briefly described below.

  The image data to be encoded is divided into rectangular area blocks, and a discrete cosine transform is applied to each block. The coefficient obtained by the conversion is quantized according to a desired image quality level and code amount, and the quantized conversion coefficient is entropy encoded. JPEG Baseline employs Huffman coding as a coefficient entropy coding method. Data of 8 × 8 = 64 frequency bands are encoded by being divided into one DC component and 63 AC components. The DC component is encoded as a difference value from the immediately preceding block, and the AC component is Huffman encoded by a combination of a zero consecutive number (zero run) and a non-zero coefficient value.

  As can be seen in JPEG Baseline, in an encoding method that encodes a continuous number of zeros, a coefficient that becomes 0 must exist to some extent in order to perform encoding efficiently. As an extreme example, when all 63 AC coefficients are non-zero, encoding is performed by combining run 0 and non-zero coefficient values, which is not efficient.

  For example, when JPEG encoding is performed with a small quantization step value, each coefficient is separated into an upper bit part and a lower bit part to efficiently perform the encoding, A method has been proposed in which run-length coding is applied only.

An example of an image processing apparatus and an image processing method using such a conventional technique is disclosed in Patent Document 1.
JP 2000-13797 A

  In the coding for separating the upper bit part and the lower bit part as described above, the bit position at which the coefficient is separated is an important factor that determines the compression performance. If the boundary bit position is set low, the length of the zero run is shortened and run-length encoding does not function effectively. On the other hand, if the boundary bit position is set too high, a lot of redundant data is included in the lower bit part, and the encoding performance of the lower bit part affects the overall performance. For example, in Patent Document 1 shown above, the lower bit part is not compressed, but in such a case, high encoding performance cannot be realized.

  In order to obtain the boundary bit position where the code amount is the smallest, it is necessary to perform the encoding process by overlapping each coefficient data many times at different boundary bit positions. Especially, the encoding process by software takes time. There is. Further, when receiving and holding encoded data generated elsewhere, the boundary bits are not always set at an appropriate position, and efficient compression may not be performed.

  The present invention has been made in view of the above-mentioned problems, and in image coding processing that performs frequency conversion and quantization in units of pixel blocks, a technique for more efficiently coding the quantized coefficients. Is to provide.

In order to solve this problem, for example, an image encoding device of the present invention has the following configuration. That is,
An image encoding device that performs frequency conversion and quantization processing in units of pixel blocks from multivalued image data, and encodes each multivalued coefficient obtained by the quantization processing,
For each coefficient obtained from the pixel block of interest, the number of bits from the bit position that first becomes “1” from the most significant bit to the least significant bit to the least significant bit is detected as the effective bit number, and each effective bit number Counting means for counting the number of occurrences of
Boundary bit position determining means for determining the boundary bit position B based on the distribution of the number of occurrences of the number of effective bits counted by the counting means;
The coefficients are separated into an upper bit part higher than the boundary bit position B and a lower bit part below the boundary bit position B, and the upper bit part of each coefficient is run-length encoded, and the lower bit A first coefficient encoding means for encoding a bit plane in units;
Redundancy determining means for determining whether the bit plane at the boundary bit position B of each coefficient has a large or small redundancy by using a preset threshold;
When the redundancy determining means determines that the redundancy is high, the coefficients are separated into an upper bit part higher than the boundary bit position B-1 and a lower bit part below the boundary bit position B-1. A second coefficient encoding means for performing run length encoding on the upper bit part of each coefficient and encoding the lower bit part on a bit plane basis;
If it is determined by the redundancy determining means that the redundancy is low, the encoded data obtained by the first coefficient encoding means is selected and output as the encoded data of the block of interest;
When the redundancy determining means determines that there is a lot of redundancy, the code amount of the encoded data obtained by the first coefficient encoding means and the second coefficient encoding means Selecting means for comparing the code amount of the encoded data and selecting and outputting the encoded data having a small code amount as the encoded data of the block of interest;

  According to the present invention, in an image encoding process that performs frequency conversion and quantization in units of pixel blocks, it is possible to encode the quantized coefficients more efficiently.

  In addition, according to another invention, it is possible to re-encode existing encoded image data at least at a higher compression rate without changing the image quality.

  Embodiments according to the present invention will be described below in detail with reference to the accompanying drawings.

[First Embodiment]
FIG. 1 is a block configuration diagram of an image encoding device according to the embodiment.

  The image encoding apparatus according to the present embodiment receives image data to be encoded from the outside, and performs sequence conversion such as frequency conversion (DCT) and quantization for each pixel block. Then, each quantized coefficient obtained from the pixel block of interest is separated into an upper bit part and a lower bit part and encoded. The input source of the image data is an image scanner, but it may be a storage medium that stores image data as a file, and the type thereof is not limited.

  The image data to be encoded is assumed to be monochrome multivalued image data having only a luminance component, and the luminance component is assumed to be 8 bits (256 gradations from 0 to 255). However, this is to simplify the description of the embodiment, and one pixel may be a plurality of components (for example, RGB or CMYK), the type of the color space is not limited, and the number of bits of one component is also The number of bits is not limited to 8 bits and may be more than 8 bits. Furthermore, the image to be encoded is composed of horizontal W pixels and vertical H pixels. For simplicity of explanation, it is assumed that W and H are multiples of 8.

  Hereinafter, the encoding process in the image encoding apparatus in FIG. 1 will be described.

  First, image data to be encoded is sequentially input from the image input unit 101. The input order of the pixel data is a raster scan order. One pixel is a non-negative integer value from 0 to 255 expressed by 8 bits. Let the upper left corner of the image be coordinates (0, 0), and let the value of the pixel at the pixel position x in the horizontal right direction and the pixel position y in the vertical down direction be represented by P (x, y). For example, when the pixel at the position (x, y) = (3,4) has a luminance value of 128, it is expressed as P (3,4) = 128. Further, in the following description, the expression “P (x, y)” is also used when expressing the “pixel” at the position (x, y).

  The block dividing unit 102 appropriately stores pixel data input from the image input unit 101 in an internal buffer (not shown), and sequentially outputs the rectangular blocks having a size of 8 × 8 pixels as a unit. The block at the upper left corner of the image is numbered 0, identification numbers are given in ascending order in the raster scan order, and rectangular block data is extracted and output in this order.

The series conversion unit 103 performs frequency conversion on the rectangular block data output from the block division unit 102. Here, the discrete cosine transform used in JPEG (ISO / IEC 15444-1) is applied as the series conversion process, and is converted into DCT coefficient data. In another embodiment of the present invention, in order to reduce the distortion of the block, the conversion to the frequency space can be performed using the overlapping orthogonal transformation, the overlapping bi-orthogonal transformation, or the like that performs the transformation process while overlapping the blocks. . Note that the JPEG discrete cosine transform processing is described in the following international standard recommendations and the like, and will not be described here.
ITU-T Recommendation T.81 | ISO / IEC 10918-1: 1994, Information technology-Digital compression and coding of continuous-tone still images: Requirements and guidelines.
As in the case of JPEG, level shift processing for subtracting the intermediate value is performed prior to the application of the discrete cosine transform. In this embodiment, since each pixel of the input image data to be encoded is 8-bit data in the range of 0 to 255, 128 is subtracted from the pixel value P (x, y).

  The coefficient quantization unit 104 performs quantization processing on the DCT coefficient data obtained by the series conversion unit 103 according to the required image quality level, and each coefficient value after quantization (64 in the embodiment) Is stored in the coefficient buffer 105. Hereinafter, when it is not particularly necessary to distinguish the presence or absence of quantization, a quantized coefficient value stored in the coefficient buffer 105 is simply referred to as a coefficient value. In the embodiment, one coefficient value is described as being represented by 9 bits (of which 1 bit is a positive or negative sign), but may be 10 bits or more in order to improve accuracy.

  The coefficient buffer 105 stores coefficient values for one rectangular block (64 in the case of the embodiment).

  The boundary bit position determination unit 116 examines the distribution of the number of effective bits, which will be described later, for each coefficient value for one rectangular block stored in the coefficient buffer 105, and determines the boundary bit position B used in the subsequent coefficient encoding unit 117. To do.

  FIG. 2 is a block configuration diagram of the boundary bit position determining unit 116. The boundary bit position determination unit 116 includes a valid bit number determination unit 106, a frequency table generation unit 107, a frequency table buffer 108, and a boundary bit selection unit 109.

The effective bit number determination unit 106 sequentially reads each coefficient value Cn stored in the coefficient buffer 105 and in the target rectangular block, and detects the effective bit number B (Cn). The number of effective bits indicates a bit position where “1” appears first when a search is performed from the higher order to the lower order bits when the absolute value of the coefficient value Cn of interest is expressed in a natural binary number. For example, when the coefficient value Cn of interest is “−5”, the natural value of the absolute value “5” is “0... 0101”. Since the most significant “1” is the third digit from the lower order, B (5) = 3. In other words, it can be said that the effective number of bits of the coefficient value having the absolute value “5” is 3 bits from the least significant bit. When Cn = 0, there is no “1” bit, so the number of effective bits B (Cn) is zero. In short, it can be said that the effective bit number B (Cn) is the smallest positive integer value P that satisfies the condition | Cn | <2 ^P with respect to the absolute value | Cn | of the coefficient.

  The frequency table generation unit 107 updates the occurrence frequency table held in the frequency table buffer 108 for B (Cn) detected by the valid bit number determination unit 106. The frequency table buffer 108 has a capacity for storing the appearance frequencies N (0) to N (8) of the valid bit numbers 0 to 8, respectively. The frequency table buffer 108 is initialized with “0” when the frequency of one rectangular block is obtained. When the effective bit determination unit 106 determines the number of effective bits B (Cn) of the target coefficient value Cn, the frequency table generation unit 107 increases N (B (Cn)) in the frequency table buffer 108 by “1”. Let That is, the frequency table generation unit 107 updates the frequency table buffer 108 every time the valid bit determination unit 106 determines a valid bit of one coefficient value.

  FIG. 4 shows an example in which the appearance frequency (count value) of the effective bits of all coefficient values in the block of interest obtained as described above is shown as a graph. FIG. 4 shows an example in which 0 to 7 exist as the number of effective bits, and the number of appearances of 8 effective bits is 0.

  When the effective bit number determination and the frequency table update processing are performed for all coefficient values of the target rectangular block, the boundary bit selection unit 109 selects the boundary bit B in the target rectangular block. Specifically, it is as follows.

The boundary bit selection unit 109 first calculates the number N (b) of occurrences of each effective bit number stored in the frequency table buffer 108 (b is an arbitrary integer value indicating the effective bit number) from the maximum value of the effective bit number. By performing cumulative addition toward the minimum value, conversion is made into a table S (b) of cumulative frequency as shown in FIG. In the rectangular block of interest, when the maximum value of the number of effective bits having an appearance count of 1 or more is bmax (bmax = 7 in the case of FIG. 4), the boundary bit selector 109 sets the variables b = bmax, bmax−1. , Bmax-2..., And a cumulative frequency table S (b) represented by the following equation is obtained.
Sb (b) = N (b) When b = bmax Sb (b) = N (b) + S (b + 1) When b ≠ bmax

Conversely, when the variable b = 0, 1, 2,..., Bmax is updated, it can be expressed as follows using the summation function Σ.
Sb (bmax−b) = ΣN (bmax−i) (i = 0, 1,..., B)

  In short, the cumulative frequency table S (b) indicates that when b ≠ 0, the cumulative frequency table S (b) represents the number of coefficient values whose absolute value is 2 ^ b or more. S (0) is equal to the number of all coefficient values in the target rectangular block.

  After obtaining the cumulative frequency table S (b), the boundary bit selection unit 109 checks the value of S (b) from the maximum value to the minimum value of the number of effective bits, that is, in descending order, and sets a preset threshold value. A maximum integer b exceeding Th1 is searched. Then, the boundary bit selection unit 109 outputs the obtained b as the boundary bit position B. Although details will become clear from the description to be described later, when S (1) does not exceed the threshold value Th1, separation processing is not performed for all the coefficients in the target rectangular block, and all are run-length encoded. .

  Here, the threshold value Th1 is set to be a predetermined ratio with respect to the total number of coefficient values (in the case of the embodiment, since the rectangular block has a size of 8 × 8 pixels, the total number of coefficient values is 64). This threshold value Th1 can be set or selectable by the user as appropriate. From a statistical point of view, this threshold value Th1 is suitably 1/4 to 1/3 of the total number of coefficients in the rectangular block. is there. For example, if the ratio 1/3 is adopted, the threshold Th1 is “21” (≈64 / 3).

  As will be described later, in the encoding process of each coefficient value in the target rectangular block in the later-stage coefficient encoding unit 117, the coefficient value is separated into upper and lower by the boundary bit B, and zero is continuous with respect to the upper bit. Apply number coding, ie run-length coding. Therefore, the role of the threshold value Th1 here is for adjusting the ratio of the coefficient whose non-zero high-order bit part is zero and the coefficient value of zero, and aims to make run-length encoding function effectively. It can be said that it is set as. In the example shown in FIG. 5, “5” corresponds to the maximum b exceeding the threshold Th1, and “5” is output as the boundary bit position.

  When the boundary bit position determination unit 116 determines the boundary bit position B, the boundary bit position B is converted into a first coefficient encoding unit 117 and a second coefficient encoding unit in the block re-encoding unit 122 described later. It outputs to 119.

  The first coefficient encoding unit 117 reads out each coefficient value stored in the coefficient buffer 105 in accordance with a preset order and performs an encoding process.

  The coefficient values stored in the coefficient buffer 105 are read (scanned) up to three times. The first scan is for the boundary bit position B determination process by the boundary bit position determination unit 116 already described. The second scan is performed by the first coefficient encoder 117, and the third scan is performed by the block re-encoder 122. The scan order of the first scan may be any order. On the other hand, the second and third scans are in a zigzag scan order from the viewpoint of encoding efficiency, but are not necessarily limited thereto. Although details will be described later, in the first coefficient encoding unit 117 and the second coefficient encoding unit 119 in the block re-encoding unit 122, the higher-order bit part of the coefficient is set as a zero consecutive number. Encode with a combination of zero coefficient values. At this time, any scanning method may be used as long as the number of consecutive zeros is increased. For example, an appropriate method may be selected from several scan patterns. In addition, the scan order may be dynamically changed so that the coefficients are arranged in order from the largest to the smallest with reference to the coefficient distribution in the previously encoded block.

  First, the first coefficient encoding unit 117 will be described, and then the block re-encoding unit 122 will be described.

  FIG. 3 is a block configuration diagram of the first coefficient encoding unit 117. The first coefficient encoding unit 117 includes an upper / lower bit separation unit 110, an upper bit encoding unit 111, a lower bit encoding unit 112, and a code buffer 113. The first coefficient encoding unit 117 according to the present embodiment encodes each coefficient value in the coefficient buffer 105 using the boundary bit position B supplied from the boundary bit position determining unit 116. A specific example will be described below.

  For the coefficient value Cn of interest read out from the coefficient buffer 105, the upper / lower bit separation unit 110 includes an upper bit part higher than the boundary bit position B in the code absolute value expression, and a lower bit part below the boundary bit position B. To separate. However, positive and negative sign bits are not processed. More specifically, the boundary bit selection unit 109 includes the upper bit part Un (= Cn >> B) and the lower bit part Ln (= | Cn | & ((1 << B) −) of the coefficient value Cn of interest. 1)). Here, Y >> X indicates that the value Y is shifted in the lower direction by X bits, and Y << X indicates that the value Y is shifted in the upper direction by X bits. | Y | represents the absolute value of the value Y, and X & Y represents the logical product (AND) of the value X and the value Y in bit units.

  The upper / lower bit separation unit 110 outputs the bit part Un of the target coefficient value Cn to the upper bit encoding unit 111 and outputs the lower bit part Ln to the lower bit encoding unit 112.

  It is assumed that the sign representing the sign of the coefficient Cn is also passed to the lower bit coding unit 112 for the coefficient Cn whose value of the upper bit part Un excluding the sign bit is “0”.

  FIG. 6 shows an example in which “−41, −7, 28, −1,...” Is input as an example of the coefficient value Cn, and the upper bit part Un and the lower bit part Ln are separated at the boundary bit position B = 4. .

  In the case illustrated, the upper / lower bit separation unit 110 outputs 64 upper bit units Un including one bit of the positive / negative code to the upper bit encoding unit 111.

  When the value represented by 4 bits excluding the sign bit of the input upper bit part Un is “0”, the upper bit coding unit 111 does not code the sign bit here. In the upper bit part Un, the value represented by 4 bits excluding the sign bit is “0” in the case of FIG. 6 in the coefficient values Cn = “− 7” and “−1”. When the value represented by 4 bits excluding the sign bit of the upper bit part Un is non- “0”, the upper bit part Un including the sign bit is directly subjected to run length encoding.

  Therefore, in the case of FIG. 6, the upper bit encoding unit 111 performs run-length encoding in the order of multi-value data “−2”, “0”, “1”, “0”. Become. As a result, the probability that the upper bit part is “0” is increased, and the encoding efficiency can be increased.

  Note that one DC coefficient and 63 AC coefficients are obtained from one rectangular block. The upper bit encoding unit 111 in the embodiment uses the same method as the encoding of DC and AC coefficients in JPEG Baseline. That is, when the upper bit part Un of the DC coefficient of the rectangular block of interest is encoded, the upper bit part Un of the DC coefficient of the immediately preceding block (which is also an already encoded block) is used as a predicted value. Then, the difference between the upper bit part Un of the block of interest and the predicted value is calculated, and Huffman coding is performed based on the difference. Then, the Huffman coding is performed on the upper bit part Un of the AC coefficient by combining the number of consecutive zeros and the non-zero coefficient. Similar to JPEG Baseline, a special code representing EOB (End Of Block) is used when the end of the block ends with a sequence of zeros.

  On the other hand, the upper / lower bit separation unit 110 outputs the 64 lower bit parts Ln of the target rectangular block to the lower bit encoding unit 112. However, as described above, the upper / lower bit separation unit 110, for the coefficient Cn whose 4-bit value except the sign bit of the upper bit part Un is “0”, converts the sign bit to the lower bit. It is added to the part Ln and output to the lower bit encoding part 112.

  In the example of FIG. 6, the coefficient values “−7” and “−1” indicate that the value of the upper bit part Un excluding the sign bit is “0”. Therefore, the addition position of the plus / minus sign bit is set to a position where “1” first appears from the high order to the low order of the low order bit part Ln. In the case of FIG. 6, the position indicated by the diagonal lines is the position where the positive and negative bits are inserted.

  The lower bit encoding unit 112 encodes the lower bit part Ln output from the upper / lower bit separation unit 110. Since the lower bit portion Ln is generally hard-to-compress data with respect to the upper bit portion Un, the lower bit encoding portion 112 of the present embodiment outputs the lower bit portion without compression (however, in the case of pursuing compression performance) Arithmetic codes etc. may be applied). Since the data is not compressed, the input data is basically encoded data as it is, but the bit data is arranged according to the degree of influence on the image quality, in other words, the decoding apparatus can perform progressive reproduction. Therefore, the data are rearranged in the order of bit planes and output.

  In the case of FIG. 6, the lower bit encoding unit 112 stores the input lower bit unit Ln in an internal buffer (not shown), and is configured with the most significant bit digit (Bth digit) of each lower bit unit Ln. Output a bit plane. This process is repeated up to the least significant bit digit (first digit).

  An example of data output by the lower bit encoding unit 112 in the case of the coefficient value sequence “−41, −7, 28, −1,...” (Boundary bit position B = 4) illustrated in FIG. Shown in

  The plane (bit information at the same bit position) output at the beginning of the lower bit part Ln is a B-digit bit plane “1010...”. The coefficient values “−41” and “28” correspond to “1”. Since the value represented by the upper bit part Un excluding the sign bits of the coefficient values “−41” and “28” is non-zero, the positive / negative sign bit is not inserted.

  Focusing on the next bit plane of B-1 digits, the first “1” bit representing the coefficient value “−7” appears (see FIG. 6). Further, the value of the upper bit part Un excluding the sign bit of the coefficient value “−7” is “0”. Therefore, as shown in the figure, positive and negative sign bits are inserted at positions following the first “1” representing the coefficient value “−7”. Hereinafter, the B-2 digit bit plane and the B-3 digit bit plane are output. In the B-3 digit bit plane, since the first “1” representing the coefficient value “−1” appears, the sign bit is inserted subsequently.

  The encoding buffer 113 stores encoded data in units of rectangular blocks output from the upper bit encoding unit 111 and the lower bit encoding unit 112.

  The above is the processing content of the first coefficient encoding unit.

  Returning to the description of FIG. When the first coefficient encoding unit 117 completes the encoding process for the block of interest, the redundancy determining unit 118 includes boundary bits in the encoded data of the code buffer 113 in the first coefficient encoding unit 117. The redundancy of the bit plane data at position B is determined. The bit plane data at the boundary bit position B to be determined is generated by the lower bit encoding unit 112.

  The redundancy determination unit 118 in this embodiment counts the number of times Nc that changes from 0 to 1 by sequentially checking the binary data of the bit plane at the boundary bit position B.

  As a simple example, consider the binary sequence “0, 0, 1, 1, 0, 0, 0, 1, 0”. In this case, the value changes from 0 to 1 from the second bit to the third bit, and the value changes from 0 to 1 instead of the seventh bit to the eighth bit. . Therefore, in this example, the number of inversions Nc is “2”.

The redundancy determining unit 118 compares the number of changes Nc with a preset threshold Th2 and determines that Nc ≧ Th2, and determines that the redundancy of the bit plane at the boundary bit position B of interest is small. The control signal “0” is output to the block re-encoding unit 122 and the code selection unit 121. If it is determined that Nc <Th2, the redundancy determining unit 118 determines that the bit plane at the target boundary bit position B has a high redundancy, and transmits the control signal “1” to the block re-encoding unit 122, When the control signal from the redundancy determination unit 118 is “0”, the code selection unit 121 outputs the code stored in the code buffer 113 in the first coefficient encoding unit 117. Data is selected and output to the code string forming unit 114 as encoded data of the rectangular block of interest. At this time, the code selection unit 121 also outputs the boundary bit position B notified from the boundary bit position determination unit 116 to the code string formation unit 114.

  Also, the code selection unit 121 unconditionally generates the encoded data generated by the first coefficient encoding unit 117 when the boundary bit position B determined by the boundary bit position determination unit 116 is “1”. Select and output. This is because when the boundary bit position B is “1”, all bits of each coefficient are run-length encoded as the upper bit part. This is also because the boundary bit position B-1 does not exist.

  On the other hand, when the boundary bit position B determined by the boundary bit position determination unit 116 is larger than “1” and the control signal from the redundancy determination unit 118 is “1”, the code selection unit 121 will be described later. The encoded data is not selected or output until the determination result of the code amount comparison unit 120 in the block re-encoding unit 122 is received. In other words, the code selection unit 121 receives a control signal from the redundancy determination unit 118 in the block re-encoding unit 122 and then performs processing related to selection and output of encoded data, which will be described later. Hereinafter, the block re-encoding unit 122 will be described.

  The block re-encoding unit 122 operates when the control signal from the redundancy determining unit 118 is “1”. This is because when the control signal from the redundancy judgment unit 118 is “1”, the bit plane at the boundary bit position B is also likely to be used for run-length encoding.

  The block re-encoding unit 122 includes a second coefficient encoding unit 119 and a code amount comparison unit 120. Here, the second coefficient encoding unit 119 has the same internal structure as the first coefficient encoding unit 117. However, the second coefficient encoding unit 119 is different from the first coefficient encoding unit 117 in that the boundary bit position B-1 output from the boundary bit position determining unit 116 is not used, but the boundary bit position B-1 Is used to separate each coefficient into an upper bit part and a lower bit part. That is, the second coefficient encoding unit 119 separates each coefficient into an upper bit part higher than the boundary bit position B-1 and a lower bit part below the boundary bit position B-1, and encodes each. Therefore, the second coefficient encoding unit 119 performs the same processing as the first coefficient encoding unit except for this point, and thus detailed description thereof is omitted.

  When the encoding process by the second coefficient encoding unit 119 ends, the code amount comparison unit 120 includes the code amount CL1 stored in the code buffer 113 in the first coefficient encoding unit 117, and the second The code amount CL2 stored in the code buffer inside the coefficient encoding unit 119 is compared.

  If the code amount comparison unit 120 determines that CL1 ≦ CL2, the control signal “0” is output to the code selection unit 121. When the code amount comparison unit 120 determines that CL1> CL2, the control unit 120 outputs a control signal “1” to the code selection unit 121.

  When the control signal from the code amount comparison unit 120 is “0”, the code selection unit 121 selects the encoded data from the first coefficient encoding unit 117 and outputs it to the code string formation unit 114. At this time, the code selection unit 121 outputs the boundary bit position B to the code string formation unit 114.

  Further, when the control signal from the code amount comparison unit 120 is “1”, the code selection unit 121 selects the encoded data from the second coefficient encoding unit 117 and outputs it to the code string formation unit 114. To do. At this time, the code selecting unit 121 outputs the boundary bit position B-1 to the code string forming unit 114.

  The code sequence forming unit 114 generates encoded data of a predetermined format while storing the encoded data of the rectangular block of interest supplied from the code selecting unit 121 in a buffer included in the encoding forming unit 114. And output to the code output unit 115. The code output unit 115 generates a file header at an output destination (for example, a hard disk device) when inputting image data to be encoded, and then outputs the code sequence generated by the code sequence forming unit 114. Thus, one encoded data file is generated.

  FIG. 8 shows the structure of an encoded data file output from the image encoding apparatus of this embodiment. In the file header code string, the number of pixels in the horizontal direction of the image, the number of pixels in the vertical direction, the number of color components (the number of color components is one because it is an example of a monochrome image in the embodiment), the number of bits of each pixel, Various information necessary for decoding is added as a header. This header includes information on the boundary bit position B for all rectangular blocks. Therefore, the decoding apparatus can obtain the boundary bit position of each block by analyzing this header. After the file header, the encoded data of the upper bit part Un is arranged for all rectangular blocks of the image, and then the lower bit encoded data is arranged. In the figure, N represents the total number of blocks, and L represents the maximum value of the boundary bit position B searched for all blocks. The higher-order bit encoded data is configured by concatenating the higher-order bit portion encoded data of each rectangular block in the order of raster scan in units of rectangular blocks. The low-order bit encoded data is configured by arranging the low-order bit part encoded data of each rectangular block in the order of bit planes. The bit plane i is configured by collecting the data of the bit plane i of each block in the raster scan order, but does not include the data from the block where the boundary bit B <i, and therefore the data is not necessarily collected from all the blocks. It is not done.

  The code structure shown in FIG. 8 is merely an example, and any code may be used as long as it includes boundary bit position information and at least a part of higher-order bit encoded data. Further, FIG. 8 may be a macro block composed of a plurality of blocks or a data structure of a tile including a plurality of macro blocks.

  The encoded data generated by the image encoding apparatus of the present embodiment shifts the upper bit part to the upper part by the number of bits represented by the boundary bit position B, and restores the lower bit part according to the required image quality level. It can be reproduced by decoding in the reverse process of encoding.

  As described above, in the image coding apparatus according to the present embodiment, the effective bit number of the coefficient after series conversion and quantization in a pixel block (8 × 8 pixel size in the embodiment) composed of a plurality of pixels. Find the frequency distribution for. Then, boundary bits were obtained in which the number of non-zero upper bit portions Un is a predetermined ratio with respect to the number of coefficients of the entire rectangular block. Using this, the coefficient was separated into an upper bit part and a lower bit part, and the upper bit part was encoded by combining a continuous number of zeros and a non-zero coefficient. Thereby, run-length encoding can be made to function effectively and encoding efficiency can be improved.

[Modification of First Embodiment]
An example in which processing equivalent to that in the first embodiment is realized by a computer program will be described below as a modification of the first embodiment.

  FIG. 13 is a block configuration diagram of an information processing apparatus (for example, a personal computer) in the present modification.

  In the figure, reference numeral 1401 denotes a CPU which controls the entire apparatus using programs and data stored in a RAM 1402 and a ROM 1403 and executes image encoding processing and decoding processing described later. A RAM 1402 includes an area for storing programs and data downloaded from the external device via the external storage device 1407, the storage medium drive 1408, or the I / F 1409. The RAM 1402 also includes a work area used when the CPU 1401 executes various processes. Reference numeral 1403 denotes a ROM which stores a boot program, a setting program for the apparatus, and data. Reference numerals 1404 and 1405 denote a keyboard and a mouse, respectively. Various instructions can be input to the CPU 1401.

  A display device 1406 includes a CRT, a liquid crystal screen, and the like, and can display information such as images and characters. Reference numeral 1407 denotes a large-capacity external storage device such as a hard disk drive device. The external storage device 1407 stores an OS (operating system), a program for image encoding and decoding processing described later, image data to be encoded, encoded data of an image to be decoded, and the like as files. The CPU 1401 loads these programs and data into a predetermined area on the RAM 1402 and executes them.

  A storage medium drive 1408 reads programs and data recorded on a storage medium such as a CD-ROM or DVD-ROM and outputs them to the RAM 1402 or the external storage device 1407. In addition, a program for image encoding and decoding processing described later, image data to be encoded, encoded data of an image to be decoded, and the like may be recorded on the storage medium. In this case, the storage medium drive 1408 loads these programs and data into a predetermined area on the RAM 1402 under the control of the CPU 1401.

  Reference numeral 1409 denotes an I / F, which connects an external device to the present apparatus through the I / F 1409 and enables data communication between the present apparatus and the external apparatus. For example, image data to be encoded, encoded data of an image to be decoded, and the like can be input to the RAM 1402, the external storage device 1407, or the storage medium drive 1408 of this apparatus. A bus 1410 connects the above-described units.

  In the above configuration, when the power of this apparatus is turned on, the CPU 1401 loads the OS from the external storage device 1407 to the RAM 1402 in accordance with the boot program stored in the ROM 1403. As a result, the keyboard 1404 and the mouse 1405 can be input, and the GUI can be displayed on the display device 1406. When the user operates the keyboard 1404 or the mouse 1405 to give an instruction to start an application program for image encoding processing stored in the external storage device 1407, the CPU 1401 loads the program into the RAM 1402 and executes it. As a result, the present apparatus functions as an image encoding apparatus.

  The processing procedure of the image processing application program executed by the CPU 1401 will be described below with reference to the flowchart of FIG. Basically, this program includes functions corresponding to the components shown in FIG. However, areas such as the coefficient buffer 105, the frequency table buffer 108, and the code buffer 113 in FIG. 1 are secured in the RAM 1402 in advance.

  First, in step S200, initialization processing before starting encoding is performed. Here, a variable i holding the number of the pixel block to be processed is initialized to 0.

  Next, in step S201, image data to be encoded is sequentially input from an external device connected by the I / F 1409, and the i-th block data is formed while being stored in the RAM 1402 (image input unit 101, block dividing unit). 101 equivalent). At this time, the total number of pixel blocks included in the image is calculated by referring to the header of the image data.

  Next, in step S202, the target pixel block MB (i) is subjected to series conversion processing such as discrete cosine transformation, and the corresponding DCT coefficient data (block coefficient value) is stored in the RAM 1402 (processing of the series conversion unit 103). Equivalent).

  Subsequently, in step S203, the DCT coefficient data is quantized and stored as a coefficient value in the RAM 1402 (corresponding to the process of the coefficient quantization unit 104).

  Next, in step S204, the boundary bit position B is obtained by referring to each coefficient value of the target block MB (i) (corresponding to the processing of the boundary bit position determination unit 116).

  FIG. 10 shows a processing flow for determining the boundary bit position B in step S204.

  First, in step S301, the frequency table and the number n for identifying the coefficient inside the block MB (i) of interest are initialized. Specifically, the number of appearances is initially set to 0 for all effective bits, and 0 is set to the coefficient number n.

  Next, in step S302, the coefficient Cn represented by the number n is acquired from the RAM 1402.

  In the subsequent step S303, the effective bit number B (Cn) is obtained for the coefficient Cn of interest (corresponding to the processing of the effective bit number determination unit 106).

  In step S304, the number of valid bits B (Cn) obtained in step S303 is updated by incrementing the value of the corresponding frequency table N (B (Cn)) (corresponding to the processing of the frequency table generating unit 107).

  In step S305, it is determined whether or not the coefficient Cn of interest is the last coefficient of the block. If it is the last coefficient (yes), the process proceeds to step S307. If not (no), the process proceeds to step S307. The process moves to S306. Since the size of the pixel block is 8 × 8 pixels, whether the coefficient Cn is the last coefficient of the block can be determined by comparing the variable n with “64”.

  If the coefficient Cn is not the last coefficient in the block, the value of n is incremented and updated in step S306 in order to move the processing target to the next coefficient. After step S306, the process returns to step S302, and the process is performed for the coefficient Cn to be newly focused.

  On the other hand, if it is determined in step S305 that Cn is the last coefficient of the block, in step S307, the frequency table N (b) (b is an arbitrary integer representing the number of effective bits) is changed to the cumulative frequency table S (b). And convert. This corresponds to part of the processing of the boundary bit selection unit 109.

  Next, in step S308, b in the cumulative frequency table S (b) generated in step S307 is searched in order from the maximum value of the effective bits to the minimum value, and is selected as the boundary bit position B (of the boundary bit selection unit 109). Equivalent to part of the process).

  With the above processing, the boundary bit position B of the block MB (i) of interest is determined.

  Returning to the description of FIG. When the boundary bit position B of the block MB (i) of interest is determined as described above, the process proceeds to step S205, and the coefficient value of the block of interest MB (i) is converted to the upper bit portion Un and the lower bit portion. Separate into Ln and perform encoding respectively. This corresponds to the processing of the first coefficient encoding unit 117. The encoded data obtained by encoding is stored in the RAM 1402.

  In step S209, the redundancy of the bit plane B is examined to determine whether there is much redundancy (corresponding to the processing of the redundancy determining unit 118). If it is determined that the boundary bit position B is greater than “1” and the redundancy is high (the determination in step S209 is yes), the process branches to step S210. If it is determined that the boundary bit position B is “1” or the redundancy is low (No in step S209), the process proceeds to step S206.

  If it is determined that the redundancy is high, in step S210, the block of interest is bit-separated at the boundary bit position B-1, and the encoding process is performed again (corresponding to the process of the second coefficient encoding unit 119). The encoded data obtained by encoding is stored in the RAM 1402.

  Subsequently, in step S211, the encoded data generated in step S205 is compared with the encoded data generated in step S210, and the smaller code quantity is selected (the larger code quantity is deleted from the RAM 1402). ). This corresponds to the processing of the code amount comparison unit 120 and the code selection unit 121.

  In step S206, it is determined whether or not the processed block is the last block of the image. If it is the last block, the process proceeds to step S208, and if not, the process proceeds to step S207.

  In step S207, in order to move the block to be processed to the next block, the variable i is updated by adding 1, and the encoding process is continued from step S201 for the next block.

  On the other hand, when the processing is completed for all the blocks of the image, the process proceeds to step S208, where final encoded data is generated from the encoded data of all the blocks stored in the RAM 1402, and the I / F 1409 is solved. And output to an external device (corresponding to processing of the code string forming unit 114 and the code output unit 115).

  As described above, it will be apparent that the present modification can also provide the same operational effects as those of the first embodiment. In other words, the frequency distribution of the number of effective bits of the coefficient value after quantization is obtained in units of blocks composed of a plurality of pixels, and appropriate boundary bit positions at which the coefficients whose non-zero upper bit part is non-zero have a desired ratio To decide. As a result, encoding efficiency can be improved.

  In particular, according to this modification, the bit at the boundary bit position B in the lower bit Ln obtained by the first encoding process (step S205) is not always performed twice for one block. Since it can be determined whether or not to perform the second encoding process (step S210) according to the presence or absence of the redundancy of the plane, it can be said that it is advantageous also in terms of processing speed.

[Further Modification of First Embodiment]
In the first embodiment, after performing the encoding process by the first coefficient encoding unit 117, the redundancy determining unit 118 determines the redundancy of the boundary bit position B in the encoded data, and the second It was determined whether or not to perform encoding based on the boundary bit position B-1 by the coefficient encoding unit 119. That is, the timing for starting the encoding process of the second coefficient encoding unit 119 is after the encoding process of the first coefficient encoding unit 117 is completed.

  However, since the bit plane at the boundary bit position B is basically uncompressed, it can be determined before encoding by the first coefficient encoding unit 117. That is, in parallel with the encoding process by the first coefficient encoding unit 117, the redundancy of the boundary bit position B of all the coefficient values Cn in the target rectangular block may be determined. In order to realize this, the redundancy determination unit 118 in FIG. 1 is not located downstream of the first coefficient encoding unit 117, but the coefficient buffer 105 can be directly accessed, and the redundancy determination is performed. The boundary bit position B from the boundary bit position determination unit 116 may be supplied to the unit 118.

  As a result, the block re-encoding unit 122 can start the process without waiting for the end of the encoding process by the first coefficient encoding unit 117.

  It can be easily understood that this modification is effective particularly when implemented by hardware.

[Second Embodiment]
In the first embodiment and its modification, unencoded image data is input and compression-encoded. Encoded data generated by another device does not necessarily have an appropriate boundary bit position set. Therefore, in the second embodiment, encoded image data generated by another device is received or input, and whether or not boundary bit settings are appropriate, that is, the compression rate is further increased with the same image quality. An example will be described in which it is determined whether or not it is possible, and if possible, re-encoding is performed.

  In the second embodiment, the target image data is monochrome image data for the sake of simplicity, but may be applied to RGB image data or CMYK color image data. The image is assumed to be composed of horizontal W pixels and vertical H pixels.

  FIG. 11 shows a block diagram of an image coding apparatus according to the second embodiment. Hereinafter, the processing of the second embodiment will be described with reference to FIG.

  First, encoded image data to be processed is input from the encoded data input unit 1101. The description will be made assuming that the encoded data targeted in this embodiment has the file structure shown in FIG. However, at this stage, it is not known whether the boundary bit position is appropriate.

  The encoded data input from the encoded data input unit 1101 is temporarily stored in the code string buffer 1102.

  The encoded data analysis unit 1103 analyzes the header of the encoded data stored in the code string buffer 1102, acquires information necessary for decoding, interprets the structure, and extracts the encoded data in block order. And stored in the code buffer 1104.

  FIG. 12 shows an example of encoded data extracted by the encoded data analysis unit 1103 for the block n of interest and stored in the code buffer 1104. When storing the block n in the code buffer 1104, the encoded data analysis unit 1103 extracts the boundary bit position B of the block n from the header portion, and the boundary bit position B is extracted from the coefficient re-encoding unit 1111 and the redundancy. The data is output to the determination unit 1105. Note that the following description is processing for block n and is performed for all blocks.

  The redundancy determination unit 1105 determines redundancy when the encoded data of the block of interest is stored in the code buffer 1104 by the encoded data analysis unit 1103.

The determination process of the redundancy determination unit 1105 is as follows.
i) When the boundary bit position B of the target block is “0”, that is, when all the bits of all the coefficient values of the target block are run-length encoded, the redundancy determining unit 1105 controls the control signal “0”. "Is output to the block re-encoding unit 1106 and the code selection unit 1107.
ii) When the boundary bit position B of the block of interest is greater than “0”, the redundancy determining unit 1105 determines the redundancy for the bit plane data at the boundary bit position B. In the second embodiment, the binary data of the bit plane at bit position B is viewed in order, and the first-order Markov model entropy referring to the immediately preceding value is measured. When this entropy is higher than the preset threshold Th3, it is considered that the redundancy is low. When the entropy is lower than the threshold Th3, it is determined that a lot of redundancy remains when the entropy is low. When it is determined that the redundancy is high, the redundancy determining unit 1105 outputs “1” as a control signal for operating the block re-encoding unit 1106. When the redundancy is low, “0” is output as the control signal. Accordingly, when the boundary bit position B of the target block is “0”, the block re-encoding unit 1106 does not operate.

  The block re-encoding unit 1106 starts its operation when the control signal output from the redundancy determining unit 1105 is “1”. The block re-encoding unit 1106 includes a coefficient decoding unit 1109, a coefficient buffer 1110, a coefficient re-encoding unit 1111, a code amount comparison unit 1112, and a code buffer 1113.

  The coefficient decoding unit 1109 decodes the coefficient data from the upper bit encoded data and the lower bit encoded data stored in the code buffer 1104 in a procedure that makes a pair with the coefficient encoding unit 116 described in the first embodiment. And stored in the coefficient buffer 1110.

  The coefficient re-encoding unit 1111 is the same as the second coefficient encoding unit 119 in the first embodiment. That is, the coefficient re-encoding unit 1111 re-encodes each coefficient value of the coefficient buffer 1110 according to the boundary bit position B-1 obtained by subtracting “1” from the boundary bit position B output from the encoded data analysis unit 1103. Process. The coefficient re-encoding unit 1111 outputs the generated encoded data to the code buffer 1113.

  Here, the code amount of the encoded data of the block of interest stored in the code buffer 1104 is defined as CL1, and the code amount of the encoded data of the block of interest stored in the code buffer 1113 is defined as CL2.

  When the encoded data of the block of interest is stored in the code buffer 1113, the code amount comparison unit 1112 compares the code amounts CL1 and CL2. In the case of CL1 ≦ CL2, the code amount comparison unit 1112 outputs the control signal “0” to the code selection unit 1107. On the other hand, when CL1> CL2, the code amount comparison unit 1112 outputs a control signal “1” to the code selection unit 1107.

The processing of the code selection unit 1107 is as follows.
i) When the control signal from the redundancy judgment unit 1105 is “0”:
The code selection unit 1107 selects the encoded data in the code buffer 1104 and outputs it to the code string forming unit 114.
ii) When the control signal from the redundancy judgment unit 1105 is “1” and the control signal is “0” from the code amount comparison unit 1112:
The code selection unit 1107 selects the encoded data in the code buffer 1104 and outputs it to the code string forming unit 114.
iii) When the control signal from the redundancy judgment unit 1105 is “1” and the control signal is “1” from the code amount comparison unit 1112:
The code selection unit 1107 selects the encoded data in the code buffer 1113 and outputs it to the code string forming unit 114.

  The code string forming unit 114 forms a code string as in the first embodiment while appropriately storing the encoded data output from the code selecting unit 1107 in an internal buffer (not shown). The configuration of the code string is the same as the structure of the input code string shown in FIG. Although not particularly illustrated, it is assumed that header information acquired by the encoded data analysis unit 1103 is delivered to the code string formation unit 114. However, the changed boundary bit position of each block is adopted.

  The code string output from the code string forming unit 114 is output from the code output unit 115 to the outside of the apparatus.

  As described above, according to the second embodiment, encoded image data generated by separating the upper and lower bits outside the apparatus is input, and the boundary bit position of each block may be inappropriate. It is possible to perform re-encoding when it is determined whether or not there is a possibility. In addition, the image quality before and after re-encoding does not change at all, and it becomes possible to generate an encoded data file having a higher compression rate. In addition, since blocks that have been determined to have appropriate boundary bit positions are not re-encoded, processing can be performed at a higher speed than when all blocks are re-encoded.

  In addition, since it is clear from the modified example of 1st Embodiment demonstrated previously that it can implement | achieve the process equivalent to this 2nd Embodiment with a computer program, it abbreviate | omits about the description.

[Third Embodiment]
In the image processing apparatuses according to the first and second embodiments, it is determined from the information of the “bit plane” corresponding to the boundary bit position whether there is a possibility that the setting of the boundary bit is inappropriate. Based on the information, it is also possible to determine the possibility of inappropriate boundary bit positions. In the third embodiment, an example will be described in which it is determined whether there is a possibility that the boundary bit position is inappropriate based on the frequency distribution of the number of effective bits of the coefficient.

  In the third embodiment, the image data to be processed is multi-valued monochrome image data for the sake of simplicity, but may be applied to RGB image data or CMYK color image data. The image is assumed to be composed of horizontal W pixels and vertical H pixels.

  FIG. 14 is a block diagram of an image encoding device according to the third embodiment. Blocks that are the same as those in FIGS. 1 and 11 described in the first and second embodiments are assigned the same numbers, and descriptions thereof are omitted.

  In the following, portions that differ in operation from the second embodiment will be described.

  The encoded data input from the encoded data input unit 1101 is temporarily stored in the code buffer 1102. The encoded data analysis unit 1103 analyzes the data structure of the encoded data stored in the code buffer 1102, extracts the encoded data in units of blocks in order, stores the encoded data in the code buffer 1104, and the encoded data The boundary bit position B1 is output to the boundary bit comparison unit 1401.

  The coefficient decoding unit 1109 decodes the coefficient value of the block of interest from the encoded data stored in the code buffer 1104, stores it in the coefficient buffer 1110, and outputs it to the boundary bit position determining unit boundary bit position determining unit 116. .

  As described in the first embodiment, the boundary bit position determination unit 116 obtains the distribution of the number of effective bits for the coefficient Cn in the input block of interest and determines the boundary bit position. Here, in order to distinguish from the boundary bit position output from the encoded data analysis unit 1103, in FIG. 14, the boundary bit position output from the encoded data analysis unit 1103 is B1, and the boundary bit position output from the boundary bit position determination unit 116 The boundary bit position to be performed is represented as B2.

  The boundary bit comparison unit 1401 compares B1 and B2, and outputs a control signal for operating the block re-encoding unit 1402 when B1 and B2 are different. The boundary bit comparison unit 1401 outputs a control signal “1” when B1 and B2 are different, and outputs a control signal “0” when B1 = B2. When the control signal is “1”, the boundary bit position determination unit 116 outputs the boundary bit position B2 to the coefficient re-encoding unit 1111 in the block re-encoding unit 1402.

  When the control signal output from the boundary bit comparison unit 1401 is “1”, the block re-encoding unit 1402 starts re-encoding the coefficient data of the block stored in the coefficient buffer 1101. At the time of re-encoding, B2 output from the boundary bit comparison unit is used as the boundary bit position. The coefficient re-encoding unit 1111 performs the same process as the first coefficient encoding unit 117 in the first embodiment, and stores the generated encoded data in the code buffer 1113.

  The code amount comparison unit 1112 compares the code amount CL1 of the encoded data stored in the code buffer 1104 with the code amount CL2 of the encoded data stored in the code buffer 1113, and selects a code using the comparison result as a control signal. Output to the unit 1107. This control signal is “0” when CL1 ≦ CL2 and “1” when CL1> CL2.

The processing of the code selection unit 1107 is as follows.
i) When the control signal from the boundary bit comparison unit 1401 is “0”:
The code selection unit 1107 selects the encoded data stored in the code buffer 1104 and outputs it to the code string forming unit 114.
ii) When the control signal from the boundary bit comparison unit 1401 is “1” and the control signal from the code comparison unit 1112 is “0”:
The code selection unit 1107 selects the encoded data stored in the code buffer 1104 and outputs it to the code string forming unit 114.
ii) When the control signal from the boundary bit comparison unit 1401 is “1” and the control signal from the code comparison unit 1112 is “1”:
The code selection unit 1107 selects the encoded data stored in the code buffer 1113 and outputs it to the code string formation unit 114.

  The code string forming unit 114 forms a code string for output in the same manner as in the first embodiment while appropriately storing the encoded data output from the code selecting unit 1107 in an internal buffer (not shown). The configuration of the code string is the same as the structure of the input code string shown in FIG. Although not particularly illustrated, it is assumed that header information acquired by the encoded data analyzing unit 1103 is delivered to the code string forming unit 114. However, the changed boundary bit position of each block is adopted.

  As described above, according to the third embodiment, re-encoding is performed only when the boundary bit position of the decoded coefficient value does not match the boundary bit position determined by the algorithm of the present embodiment. As a result, an increase in burden on re-encoding can be suppressed. In addition, it is promised that the finally obtained encoded data does not exceed the data amount of the original encoded data.

  It is obvious that the processing corresponding to the third embodiment can be realized by a computer program, as in the modification of the first embodiment described above.

[Other Embodiments]
In each of the above embodiments, an example in which a method similar to JPEG Baseline is used for encoding the upper bit part has been described. However, it is easy to understand that various modifications are possible as long as run-length coding is used to transmit zero and non-zero information. In the above-described embodiment, the boundary bit position is determined using 8 × 8 pixels as a block. However, a minimum coding unit is configured by several groups of 8 × 8 blocks, such as MCU in JPEG, and each of which has a boundary bit. May be set. Alternatively, the image may be divided into tiles in advance, and the boundary bit position may be determined for each tile.

  In the embodiments, encoding of monochrome image data has been described as an example. However, it is apparent that the color space of an image can be applied to various types of image data such as RGB, CMYK, Lab, and YCrCb. The present invention is not limited by the number of color components and the type of color space.

  Further, the computer program is usually stored in a computer-readable storage medium such as a CD-ROM, and can be executed by setting it in a computer reader (CD-ROM drive) and copying or installing it in the system. Become. Therefore, it is obvious that such a computer-readable storage medium falls within the scope of the present invention.

It is a block block diagram of the image coding apparatus which concerns on 1st Embodiment. It is a block block diagram of the boundary bit position determination part in 1st Embodiment. It is a block block diagram of the 1st coefficient encoding part in 1st Embodiment. It is a figure which shows the example of a frequency table. It is a figure which shows the example of a cumulative frequency table. It is a figure which shows the outline of the bit separation of a coefficient. It is a figure which shows the example of low-order bit encoding data. It is a figure which shows the data structure of the encoding data file produced | generated in 1st Embodiment. It is a flowchart which shows the encoding process sequence in the modification of 1st Embodiment. It is a flowchart which shows the process sequence of boundary bit position determination of the modification in 1st Embodiment. It is a block block diagram of the image coding apparatus which concerns on 2nd Embodiment. It is a figure which shows the structure of the encoding data of 1 block in 2nd Embodiment. It is a block block diagram of the information processing apparatus which concerns on the modification of 1st Embodiment. It is a block block diagram of the image coding apparatus which concerns on 3rd Embodiment.

Claims (12)

An image encoding device that performs frequency conversion and quantization processing in units of pixel blocks from multivalued image data, and encodes each multivalued coefficient obtained by the quantization processing,
For each coefficient obtained from the pixel block of interest, the number of bits from the bit position that first becomes “1” from the most significant bit to the least significant bit to the least significant bit is detected as the effective bit number, and each effective bit number Counting means for counting the number of occurrences of
Boundary bit position determining means for determining the boundary bit position B based on the distribution of the number of occurrences of the number of effective bits counted by the counting means;
The coefficients are separated into an upper bit part higher than the boundary bit position B and a lower bit part below the boundary bit position B, and the upper bit part of each coefficient is run-length encoded, and the lower bit A first coefficient encoding means for encoding a bit plane in units;
Redundancy determining means for determining whether the bit plane at the boundary bit position B of each coefficient has a large or small redundancy by using a preset threshold;
When the redundancy determining means determines that the redundancy is high, the coefficients are separated into an upper bit part higher than the boundary bit position B-1 and a lower bit part below the boundary bit position B-1. A second coefficient encoding means for performing run length encoding on the upper bit part of each coefficient and encoding the lower bit part on a bit plane basis;
If it is determined by the redundancy determining means that the redundancy is low, the encoded data obtained by the first coefficient encoding means is selected and output as the encoded data of the block of interest;
When the redundancy determining means determines that there is a lot of redundancy, the code amount of the encoded data obtained by the first coefficient encoding means and the second coefficient encoding means An image encoding apparatus comprising: a selection unit that compares a code amount of encoded data and selects and outputs encoded data with a small code amount as encoded data of the block of interest. The boundary bit position determining means includes
The number of occurrences of each valid bit number b counted by the counting means is defined as N (b), the maximum number of valid bits whose appearance number is 1 or more is defined as bmax, and the variable b = 0, 1, 2,. The summation function of
S (bmax−b) = ΣN (bmax−i) (i = 0, 1,..., B)
In relation to a preset threshold value Th,
S (bmax−b)> Th
The image encoding apparatus according to claim 1, wherein a maximum integer b that satisfies the following is obtained, and the maximum integer b is determined as the boundary bit position B. Performs frequency conversion and quantization processing in units of pixel blocks, separates each quantized coefficient into an upper bit part and a lower bit part, run-length encoding for the upper bit part, and bit for the lower bit part An image encoding device that re-encodes encoded image data encoded in units of planes,
Input means for inputting encoded data in units of pixel blocks from the encoded image data;
By analyzing the input encoded image data, a boundary bit position B for separating each coefficient in the pixel block of interest into the upper bit part and the lower bit part is extracted;
Redundancy determining means for determining whether the bit plane at the boundary bit position B extracted by the extracting means has a large or small redundancy using a preset threshold;
A coefficient decoding unit that decodes up to each coefficient of the pixel block of interest when the redundancy determining unit determines that the redundancy is high;
Each coefficient decoded by the coefficient decoding means is separated into an upper bit part and a lower bit part according to a boundary bit position B-1 obtained by subtracting 1 from the boundary bit position B, and the upper bit part is run-length encoded. Coefficient encoding means for encoding the lower bit part in bit plane units,
If it is determined by the redundancy determining means that the redundancy is low, the encoded data of the pixel block input by the input means is selected and output as the encoded data of the target pixel block,
When the redundancy determining means determines that there is a lot of redundancy, the code amount of the encoded data obtained by the coefficient encoding means is compared with the code amount of the encoded data input by the input means. And selecting means for selecting and outputting encoded data with a small code amount as encoded data of the pixel block of interest. Performs frequency conversion and quantization processing in units of pixel blocks, separates each quantized coefficient into an upper bit part and a lower bit part, run-length encoding for the upper bit part, and bit for the lower bit part An image encoding device that re-encodes encoded image data encoded in units of planes,
Input means for inputting encoded data in units of pixel blocks from the encoded image data;
By analyzing the input encoded image data, extraction means for extracting a boundary bit position B1 for separating the coefficients in the pixel block of interest into the upper bit part and the lower bit part;
Coefficient decoding means for decoding up to each coefficient based on the encoded data of the pixel block of interest input by the input means;
For each coefficient obtained by decoding by the coefficient decoding means, the number of bits from the bit position first becoming “1” from the most significant bit to the least significant bit to the least significant bit is detected as the effective bit number, Counting means for counting the number of occurrences of each effective bit number;
Boundary bit position determining means for determining the boundary bit position B2 based on the distribution of the number of occurrences of the number of effective bits counted by the counting means;
Comparing means for comparing the boundary bit position B1 and the boundary bit position B2;
When the comparison means indicate that the comparison results are different from each other, each coefficient decoded by the coefficient decoding means is separated into an upper bit part and a lower bit part according to the boundary bit position B2, and the run length of the upper bit part is separated. Coding, coefficient encoding means for encoding the lower bit part in bit plane units,
When the comparison results of the comparison means indicate that they are equal to each other, the encoded data of the pixel block input by the input means is selected and output as the encoded data of the target pixel block,
When indicating that the comparison results of the comparison means are different from each other, the code amount of the encoded data obtained by the coefficient encoding means is compared with the code amount of the encoded data input by the input means, An image encoding apparatus comprising: selection means for selecting and outputting encoded data with a small code amount as encoded data of the pixel block of interest. The boundary bit position determining means includes
The number of occurrences of each valid bit number b counted by the counting means is defined as N (b), the maximum number of valid bits whose appearance number is 1 or more is defined as bmax, and the variable b = 0, 1, 2,. The summation function of
S (bmax−b) = ΣN (bmax−i) (i = 0, 1,..., B)
In relation to a preset threshold value Th,
S (bmax−b)> Th
The image encoding apparatus according to claim 4, wherein a maximum integer b that satisfies the following is obtained, and the maximum integer b is determined as the boundary bit position B <b> 2. A control method for an image encoding device that performs frequency conversion and quantization processing in units of pixel blocks from multi-value image data, and encodes each multi-value coefficient obtained by the quantization processing,
For each coefficient obtained from the pixel block of interest, the number of bits from the bit position that first becomes “1” from the most significant bit to the least significant bit to the least significant bit is detected as the effective bit number, and each effective bit number A counting process for counting the number of occurrences of
A boundary bit position determining step for determining the boundary bit position B based on the distribution of the number of occurrences of the number of effective bits counted in the counting step;
The coefficients are separated into an upper bit part higher than the boundary bit position B and a lower bit part below the boundary bit position B, and the upper bit part of each coefficient is run-length encoded, and the lower bit A first coefficient encoding step for encoding a bit plane in units;
A redundancy determination step of determining whether or not the bit plane at the boundary bit position B of each coefficient has a large or small redundancy by using a preset threshold;
When it is determined that the redundancy is high in the redundancy determination step, the coefficients are separated into an upper bit part higher than the boundary bit position B-1 and a lower bit part below the boundary bit position B-1. A second coefficient encoding step of performing run length encoding on the upper bit part of each coefficient and encoding the lower bit part on a bit plane basis;
If it is determined that the redundancy is low in the redundancy determination step, the encoded data obtained in the first coefficient encoding step is selected and output as the encoded data of the block of interest;
When it is determined that the redundancy is high in the redundancy determination step, the code amount of the encoded data obtained in the first coefficient encoding step and the code amount obtained in the second coefficient encoding step A control method for an image encoding device, comprising: a selection step of comparing a code amount of encoded data and selecting and outputting encoded data having a small code amount as encoded data of the block of interest. The boundary bit position determining step includes:
The number of occurrences of each effective bit number b counted in the counting step is defined as N (b), the maximum number of effective bits with an appearance number of 1 or more is defined as bmax, and the variable b = 0, 1, 2,. The summation function of
S (bmax−b) = ΣN (bmax−i) (i = 0, 1,..., B)
In relation to a preset threshold value Th,
S (bmax−b)> Th
The control method for an image encoding device according to claim 6, wherein a maximum integer b that satisfies the following is obtained, and the maximum integer b is determined as the boundary bit position B. Performs frequency conversion and quantization processing in units of pixel blocks, separates each quantized coefficient into an upper bit part and a lower bit part, run-length encoding for the upper bit part, and bit for the lower bit part A control method for an image encoding device that re-encodes encoded image data encoded in units of planes,
An input step of inputting encoded data in units of the pixel blocks from the encoded image data;
Analyzing the input encoded image data to extract a boundary bit position B for separating the coefficients in the pixel block of interest into the upper bit part and the lower bit part; and
A redundancy determination step of determining whether the bit plane at the boundary bit position B extracted in the extraction step has a high or low redundancy by using a preset threshold;
A coefficient decoding step of decoding up to each coefficient of the pixel block of interest when it is determined that the redundancy is high in the redundancy determination step;
Each coefficient decoded in the coefficient decoding step is separated into an upper bit part and a lower bit part according to a boundary bit position B-1 obtained by subtracting 1 from the boundary bit position B, and the upper bit part is run-length encoded. A coefficient encoding process for encoding the lower bit part in units of bit planes;
When it is determined that the redundancy is low in the redundancy determination step, the encoded data of the pixel block input in the input step is selected and output as the encoded data of the pixel block of interest.
When it is determined that the redundancy is high in the redundancy determination step, the code amount of the encoded data obtained in the coefficient encoding step is compared with the code amount of the encoded data input in the input step And a selection step of selecting and outputting encoded data with a small amount of code as encoded data of the pixel block of interest, and a control method for the image encoding apparatus. Performs frequency conversion and quantization processing in units of pixel blocks, separates each quantized coefficient into an upper bit part and a lower bit part, run-length encoding for the upper bit part, and bit for the lower bit part A control method for an image encoding device that re-encodes encoded image data encoded in units of planes,
An input step of inputting encoded data in units of pixel blocks from the encoded image data;
Analyzing the input encoded image data to extract a boundary bit position B1 for separating the coefficients in the pixel block of interest into the upper bit part and the lower bit part; and
A coefficient decoding step of decoding up to each coefficient based on the encoded data of the pixel block of interest input in the input step;
For each coefficient obtained by decoding in the coefficient decoding step, the number of bits from the bit position that first becomes “1” from the most significant bit to the least significant bit to the least significant bit is detected as the effective bit number, A counting step for counting the number of occurrences of each effective bit number;
A boundary bit position determining step for determining a boundary bit position B2 based on the distribution of the number of occurrences of the number of effective bits counted in the counting step;
A comparison step of comparing the boundary bit position B1 and the boundary bit position B2.
When the comparison results indicate that the comparison results are different from each other, each coefficient decoded in the coefficient decoding step is separated into an upper bit part and a lower bit part according to the boundary bit position B2, and the run length is set for the upper bit part. Coding, a coefficient coding process for coding the lower bit part in bit plane units,
When the comparison result of the comparison step indicates that they are equal to each other, the encoded data of the pixel block input in the input step is selected and output as the encoded data of the pixel block of interest,
When the comparison result of the comparison step indicates that they are different from each other, the code amount of the encoded data obtained in the coefficient encoding step is compared with the code amount of the encoded data input in the input step, And a selection step of selecting and outputting encoded data with a small amount of code as encoded data of the pixel block of interest, and a control method for the image encoding apparatus. The boundary bit position determining step includes:
The number of occurrences of each effective bit number b counted in the counting step is defined as N (b), the maximum number of effective bits with an appearance number of 1 or more is defined as bmax, and the variable b = 0, 1, 2,. The summation function of
S (bmax−b) = ΣN (bmax−b) (i = 0, 1,..., B)
In relation to a preset threshold value Th,
S (bmax−b)> Th
10. The control method for an image encoding device according to claim 9, wherein a maximum integer b that satisfies the above is obtained, and the maximum integer b is determined as the boundary bit position B2.   A computer program that causes a computer to function as the image encoding device according to claim 1 by being read and executed by a computer.   A computer-readable storage medium storing the computer program according to claim 11.

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