JP4743884B2 - Image coding apparatus and control method thereof - Google Patents

Description

  The present invention relates to a technique for compressing and encoding image data.

  Conventionally, JPEG is known as a method for encoding an image. This JPEG encoding is performed in units of 8 × 8 pixel blocks in image data. All processes described below are executed in units of this block.

  First, orthogonal transform (DCT) is performed on the input block to obtain 8 × 8 orthogonal transform coefficients. Next, the orthogonal transform coefficient is quantized using a quantization table. Here, the value of the element of the quantization table is called a quantization step width. The quantization step width is larger as the orthogonal transform coefficient of the high frequency component. In the quantization, each orthogonal transform coefficient is divided by the corresponding quantization step width, and rounded off by rounding off after the decimal point. Therefore, the value after quantization becomes smaller as the high-frequency component, and the occurrence frequency of 0 becomes higher. Next, separate encoding is performed on the quantized values using one DC component (one) and one AC component (63).

  As for the DC component, since the correlation between the blocks is strong, the differential prediction coding is performed, and then the Huffman coding is performed. Differential prediction encoding is a method for encoding a difference from the immediately preceding value. Huffman coding is a coding method that shortens the overall code length by giving short codes to values with high occurrence frequency and long codes to values with low occurrence frequency.

  The AC component is subjected to Huffman coding after run-length coding. Run-length coding is a coding method in which the coding rate increases as the same value continues for a long time. By performing zigzag scanning on the quantized values, the encoding efficiency is improved by continuing the zero value of the high frequency component.

  JPEG encoding is a good compression method for natural images, as can be seen from the fact that it is installed in devices such as digital cameras. On the other hand, for images containing text, line drawings, etc., mosquito noise caused by the loss of high-frequency components of the image, block distortion caused by loss of quantization error of DC components and high-frequency components There is a problem that is likely to occur. Therefore, as a method of compressing while maintaining the resolution of characters and line drawings, the image is divided into blocks that are units of orthogonal transformation. Then, resolution information such as characters or line drawings of a color or density value specified in advance in block units is extracted, and gradation information obtained by performing replacement processing with appropriate values for the extracted area is subjected to orthogonal transformation or the like. Encode. On the other hand, a method has been proposed in which color information in block units is encoded by predictive encoding or the like.

  This assumes that the image is a combination of a natural image with relatively few high-frequency components and a character or line image that is binary information having the same density value locally. Then, in order to protect the character and line drawing information from signal distortion that occurs when the image is compressed / decompressed using orthogonal transform coding, binary information of the character / line drawing is extracted in advance, Lossless encoding is performed on binary information without causing deterioration.

For example, in Patent Document 1, the replacement process is performed with reference to an area other than the extraction area in the block in the replacement process for the extraction area. Specifically, the average value calculation unit calculates an average value of regions other than the extraction region, and performs a replacement process on the extraction region with the obtained average value. Alternatively, a method is shown in which the input pixel is delayed by one cycle, and the pixel in the extraction region is replaced with the delayed pixel. With such a method, it is possible to obtain an effect of concentrating the spectrum after orthogonal transformation in the low frequency region and suppressing deterioration in image quality after compression / decompression.
JP-A-4-326669

  However, in the above-described conventional image processing method, if the quantization step width used for quantization after orthogonal transformation is coarse, there is a high possibility that a quantization error will occur, and this quantization error causes block distortion of the image. The problem of being one of the causes remains.

  Further, when the DC component value of the coefficient after orthogonal transformation is encoded, the difference between the DC component value of the block of interest and the DC component value of the previous block is encoded. Since the DC components of the two blocks in the gradation image not including the character / line image have substantially the same value, the difference between them is small and it can be said that the encoding efficiency is high.

  However, attention is paid to two adjacent blocks in a region in which the character line drawing is included on the gradation image. In such a case, it is rather rare that the number of pixels of the character line drawing included in each block is the same, and the difference between the DC components of the two blocks is inevitably low. That is, the encoded data length of the DC component is not minimized, and there is still room for improvement.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a technique for efficiently encoding an image in which a gradation image and a character / line image are mixed while suppressing occurrence of block distortion.

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 for encoding image data in which a character line image and a gradation image are mixed,
Input means for inputting image data to be encoded in units of blocks composed of a plurality of pixels;
Extraction means for extracting character line drawing pixel data and identification information for identifying whether each pixel in the target block is character line drawing pixel data or gradation attribute pixel data from the inputted target block When,
First encoding means for encoding the pixel data of the character line drawing extracted by the extraction means;
Second encoding means for encoding the identification information extracted by the extraction means;
For each sub-block, which is a unit for performing orthogonal transform, included in the block of interest, pixel value extracted as a character line drawing in the sub-block is determined based on pixel data having a gradation attribute Replacing means for replacing with,
Third encoding means for orthogonally transforming and encoding the image data of each sub-block after being replaced by the replacement means;
Code string generation means for combining the encoded data generated by the first to third encoding means and generating encoded data of the block of interest;
The replacement means includes
DC component calculation means for calculating a DC component when it is assumed that the target sub-block in the target block is composed only of pixels having a gradation attribute;
It is determined whether or not a replacement candidate value determined based on a DC component value of a sub-block immediately before the target sub-block is within a value range of a pixel group having a gradation attribute in the target sub-block. Judgment means,
When the determination means determines that the replacement candidate value is within the range, a value that approximates the DC component value of the immediately preceding sub-block is determined as the replacement value of the target sub-block,
When the determination unit determines that the replacement candidate value is outside the range, a value approximated to the direct current component value of the target sub-block when it is assumed that the target sub-block has only gradation attribute pixels. , And a determining means for setting the replacement value ,
The first and second encoding means are lossless encoding means, and the third encoding means is an irreversible JPEG encoding means,
The target sub-block is composed of 8 × 8 pixels, and the number of pixels having a character / line drawing attribute is N, the sum of pixel data having gradation attributes, and the average values thereof are D and Dave. If a pixel is represented by 8 bits,
The direct current component calculation means obtains a direct current component DC calculated at the time of DCT conversion by the following equation:
DC = (D + Dave × N−128 × 64) / 8
The determining means determines the replacement value Z in the target sub-block according to any of the following equations, where DC _{−1 is} the DC component of the immediately preceding sub-block and DC DC of the _target sub-block is DC ₀ Do
Z = (8 × DC ₋₁ −D + 128 × 64) / N
Or
Z = (8 × DC ₀ −D + 128 × 64) / N
It is characterized by that.

  According to the present invention, when an image in which a gradation image and a character / line drawing are mixed is encoded in a block unit, a replacement value for replacing a pixel in which a character / line drawing pixel exists is generated as a DC component after frequency conversion. The block distortion that makes the value almost zero can be minimized. Further, as long as the properties of the gradation images of adjacent blocks are substantially equal, it is possible to obtain a high compression ratio by increasing the probability that the DC components of adjacent blocks are the same.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 5 is a block diagram of the image compression apparatus according to the embodiment.

  The blocking unit 501 inputs a block composed of n × m pixels from the image data to be encoded in units, divides it into 8 × 8 pixel sub-block units, and outputs the image data of the sub-blocks To do. Here, n and m are integer multiples of 8. In order to simplify the description, here, it is assumed that n = m = 16. That is, as shown in FIG. 6, the blocking unit 501 inputs a block of 16 × 16 pixels and outputs image data of four 8 × 8 pixel sub-blocks. Assume that the output order is the order of sub-blocks 0, 1, 2, and 3 as shown in FIG.

  The extraction unit 501 determines an extraction color from a block image (images of four sub-blocks) by a predetermined method and outputs it. Further, the extraction unit 501 outputs information indicating the position of the extracted color existing in the block as position information. This position information can also be said to be identification information indicating whether each pixel in the block is a pixel in the extracted color region or a pixel in the non-extracted color region. Therefore, this position information is 16 × 16 binary data, that is, 16 × 16 bit data.

  The replacement unit 503 performs replacement processing on the pixels in the extracted color area in units of sub-blocks based on the input position information, and outputs the result as gradation information.

  The first encoding unit 504 performs lossless compression on the extracted color information of the input block and outputs it. The second encoding unit 505 performs lossless compression on the position information of the input block and outputs it. The third encoding unit 506 performs irreversible compression (JPEG encoding in the embodiment) and outputs the gradation information of the four sub-blocks included in the input block. Then, the code string generation unit 507 combines the encoded data generated by each encoding unit to generate and output an encoded data string in block units.

  In order to simplify the description, it is assumed that the image data to be encoded in this embodiment is one pixel and one component, and one pixel is an 8-bit (256 gradation) image (grayscale image). Further, the value 255 is described as black and the value 0 is described as white. That is, description will be made assuming that the pixel value indicates the density. However, this is an example, and the present invention is not limited to this.

  First, the extraction unit 502 in the embodiment will be described.

  When black or a character line drawing close to black exists in a natural image or a CG gradation image, the density histograms of the blocks are as shown in FIGS. FIG. 7A shows a case where the density of the character / line image has a value sufficiently different from the density of the background (natural image, CG gradation image). This situation is just a situation in which the character line drawing can be visually distinguished from the background image. FIG. 5B shows a situation where the character line drawing and the background can be distinguished from each other, though not as much as FIG. In any case, since two frequency peaks appear, the position of the minimum value between the two peaks (in the case of FIG. 10 (a), an infrequent position between the two peaks may be used) is determined as a threshold value. To do.

  The extraction unit 502 determines the threshold value as described above, and determines a pixel having a value exceeding the threshold value as a pixel having the extracted color. Then, in order to specify the pixel position having the extracted color, the 16 × 16 pixels of the input block are binarized with the determined threshold value, and this is output as position information to the encoding unit 505 and the replacement unit 503. In the binarization, the pixel value exceeding the threshold is “1”, and the pixel value below the threshold is “0”. FIG. 8A shows this position information. The replacement unit 503 performs replacement processing in units of 8 × 8 pixel sub-blocks. Therefore, the extraction unit 502 supplies the replacement unit 503 with 8 × 8-bit position information of the position corresponding to the sub-block used in the replacement processing of the replacement unit 503 among the extracted 16 × 16-bit position information. . Note that this is not the case when the replacement unit 503 includes a buffer that stores and holds position information for one block.

  The extraction unit 502 performs a raster scan of 16 × 16 for one block, sequentially extracts input pixel values whose position information is “1”, and arranges them in a one-dimensional manner as shown in FIG. The value is output to the encoding unit 504 as extracted color information.

  When the image data to be encoded is image data optically read by an image scanner or the like, each pixel of the character line drawing does not always have a single color due to reading accuracy and the like. Therefore, as shown in FIG. 8B, the value varies to some extent, but the variance is small. Accordingly, the encoding unit 504 in the present embodiment employs lossless encoding using, for example, predictive encoding.

  Note that when encoding image data directly output from a computer or an image rendered in accordance with print data from a computer without an image scanner, no error occurs, so a character line drawing has a single color ( Density). Accordingly, in such a case, all the pixels exceeding the threshold value have the same value, so the extracted color information is one piece of data. In this case, the encoding unit 504 may encode the one piece of data.

  Also, since the position information is 16 × 16 binary data as shown in FIG. 8A, the encoding unit 505 performs lossless encoding (predictive encoding, run length) specialized for binary data. Encoding etc. may be performed.

  Further, the extraction unit 502 sets all the bits of the position information to “0” when there is only one frequency peak for the density in the block, in other words, when there is no extracted color area in the block. Output. At this time, the extraction unit 502 outputs a signal indicating that there is no extracted color information in the block of interest to the code string generation unit 507. If the extracted color area exists in the block, a signal indicating the presence of extracted color information is output to the code string generation unit 507.

  On the other hand, the encoding unit 506 in the embodiment performs JPEG (lossy) encoding. JPEG encoding is an encoding technique mounted on a digital camera or the like, and is known to have a high compression rate for natural images. JPEG uses the fact that human vision is insensitive to high-frequency components, and removes the high-frequency components to some extent to reduce the amount of information before encoding. However, it is desirable that the edge of a character / line image or the like is clear. In other words, it can be said that the high-frequency component is desirably maintained in the character line drawing.

  In this respect, in this embodiment, the character / line drawing portion is losslessly encoded by the encoding units 505 and 504, and can be completely restored to the original value theoretically.

  In JPEG encoding, encoded data is generated through DCT transform (orthogonal transform), quantization, and entropy encoding of an 8 × 8 pixel sub-block. When a DCT transform is performed on a sub-block including a character / line image, the conversion coefficient obtained by the presence of the character / line image is affected. Further, in the quantization process, quantization is performed with a larger quantization step as the conversion coefficient of the high frequency component, but still the high frequency component cannot be completely cut. This is because if a quantization step that is large enough to completely cut off the high-frequency component is used, the image quality will deteriorate even if it is a natural image.

  Therefore, in this embodiment, when JPEG encoding is performed on a sub-block in which a character / line image exists, a block equivalent to that in which no character / line image exists is generated in the sub-block, and the encoding unit 506 performs JPEG encoding. To do. This is realized by the replacement unit 503 in FIG.

  Hereinafter, the replacement unit 503 in the embodiment will be described.

  FIG. 1 is a detailed block diagram of the replacement unit 503 in the embodiment. The replacement unit 503 includes a target value calculation unit 101, a coefficient value calculation unit 102, a representative value generation unit 103, two replacement candidate value calculation units 104 and 107, a replacement value generation unit 105, a representative value selection unit 106, and a candidate value. The selection unit 108 is configured. Hereinafter, each component which comprises the replacement part 503 in embodiment is demonstrated.

  The target value calculation unit 101 is based on 8 × 8 pixel data for one subblock and 8 × 8 bit position information of the corresponding subblock, and the total value D of the pixels excluding pixels having the extracted color Then, the average value Dave of the values of pixels excluding the pixels having the extracted color (pixels in the non-extracted color region) and the number N of pixels determined as the extracted color are calculated. Further, the target value calculation unit 101 obtains the maximum value Max and the minimum value Min of the pixel value in the non-extracted color region.

  As will be described in advance, when the number N of pixels determined to be the extracted color is 0, the processing described below is not performed, and the replacement generation unit 105 directly inputs the input pixel value to the encoding unit 906. Output. In the following, it should be noted that this is an explanation when N is other than 0. Further, the number of pixels determined as the non-extracted color may be used instead of the number of pixels determined as the extracted color. This is because there is a relationship of “the number of pixels determined to be an extracted color = 64−the number of pixels determined to be a non-extracted color”, and calculating one is equivalent to calculating the other.

The value of the pixel at coordinates (i, j) in the 8 × 8 pixels of the sub-block is represented as X _{i, j,} and 8 × 8 pieces of position information is represented as T _{i, j} . Here, T _{i, j} is a value of “0” or “1”. When T _{i, j} = 1, the pixel X _{i, j} means a pixel having an extracted color, that is, a pixel determined as a character line drawing. When T _{i, j} = 0, the pixel X _{i, j} means a pixel having a non-extracted color, that is, a pixel determined as a natural image (or background image) (see FIG. 8A). Accordingly, the total value D, the average value Dave, and the number N are calculated as follows. I and j are integers of 0 or more and 7 or less.
N = ΣΣT _{i, j}
D = ΣΣX _{i, j} × (1−T _{i, j} )
Dave = D / (64-N)
Here, ΣΣ indicates the sum of i, j = 0, 1, 2,...

  FIG. 2 is a flowchart showing a specific processing procedure of the target value calculation unit 101. The processing procedure will be described below with reference to FIG.

  First, in step S201, a variable D for obtaining the total value and a variable N for obtaining the number are each initialized to 0. Further, “255” is substituted for the minimum value MIN as an initial value, and “0” is substituted for the maximum value MAX as an initial value.

  Next, in step S202, it is determined based on the position information whether or not the target pixel is a pixel in the extracted color region, that is, whether or not the target pixel is a pixel having the extracted color. If it is determined that the target pixel is a pixel in the non-extracted color region, the process proceeds to step S203, and the value of the target pixel data is added to the variable D. Then, the process proceeds to step S209.

  On the other hand, if it is determined that the target pixel is a pixel in the extracted color area, the process proceeds to step S204, and the variable N is increased by “1”.

  Thereafter, the process proceeds to step S205, and it is determined whether or not the value of the target pixel is smaller than the value of the variable MIN. If it is determined that the target pixel value is smaller than the value of the variable MIN, the variable MIN is updated by substituting the value of the target pixel for the variable MIN in step S206.

  In step S207, it is determined whether the value of the target pixel is larger than the value of the variable MAX. If it is determined that the target pixel value is larger than the value of the variable MAX, the variable MAX is updated by substituting the value of the target pixel for the variable MAX in step S208.

  In step S209, it is determined whether processing for all pixels (64 pixels) in the sub-block has been performed. If not, the process from step S202 is repeated to process the next pixel.

  If it is determined in step S209 that the processing for all the pixels in the sub-block has been performed, the process proceeds to step S210, and the pixels in the non-extracted color area are based on D and N obtained as described above. The average value Dave is obtained.

  As a result of the above, the target value calculation unit 101, for each subblock, the total value D and the average value Dave of the pixel values in the non-extracted color area, the number N of pixels in the extracted color area, and the maximum in the non-extracted area The value MAX and the minimum value MIN can be obtained.

  Next, the coefficient value calculation unit 102 will be described. The coefficient value calculation unit 102 calculates a DC coefficient, which is a direct current component obtained by DCT conversion, based on the total value D, average value Dave, and number N output from the target value calculation unit 101.

In general, in the DCT transformation, when the pixel value is X _{i, j} and the coefficient obtained by the transformation is Yu _{, v} , the following calculation is performed.
Y _{u, v} = (1/4) α (u) α (v) ΣΣ (X _{i, j} -128) cos ((2i + 1) uπ / 16) cos ((2j + 1) vπ / 16)

Here, only when obtaining the DC component DC, u and v are both “0”, and α (0) is 1 / √2. Therefore, the DC component DC after DCT conversion is expressed by the following equation (1).
DC = Y _0,0 = (1/8) ΣΣ (x _{i, j} −128) (1)

Basically, the replacement unit 903 of the present embodiment replaces the pixel value determined to have the extracted color in the sub-block so as to have the average value Dave of the pixels determined to be the non-extracted color. Therefore, the coefficient value calculation unit 102 transforms the above equation (1) into the following equation (2) using D, Dave, and N calculated by the target value calculation unit 101, and converts the temporary DC component to Calculate and output to the representative value generation unit 103.
DC = {total value of pixels in non-extracted color area + (average value of pixels in non-extracted color area * number of pixels in extracted color area) −128 × 64} / 8
= (D + Dave × N−128 × 64) / 8 (2)

  Next, the representative value generation unit 103 in the embodiment will be described.

  In JPEG, as described above, each coefficient obtained by DCT conversion is quantized. Here, it is assumed that the quantization step value of the DC component performed by the encoding unit 506 in the embodiment is “16”. Usually, when quantization is performed, a value after the decimal point is generated, so that the value after the decimal point is rounded off.

  Therefore, the representative value generation unit 103 of the present embodiment adds “8” (half the value of the quantization step) to the DC component value calculated by the coefficient value calculation unit 102, and then sets the lower 4 bits to 0. . As a result, the DC component value calculated by the coefficient value calculation unit 102 can be an integral multiple of the quantization step, and errors generated in the quantization process performed by the encoding unit 506 are substantially reduced to zero. That is, the representative value generation unit 103 converts the DC component value calculated by the coefficient calculation unit 102 into a value that approximates the DC component value by an integer multiple of the quantization step, and the result is the quantized representative value Dp. To the representative value selection unit 106.

  Next, one replacement candidate value calculation unit 104 will be described.

Here, the quantization representative value of the target sub-block is temporarily represented as Dp. When the value Z of the pixel in the extraction color region is taken, Dp and Z need to satisfy the following equation (3) with reference to equation (2).
Dp = (D + Z × N−128 × 64) / 8 (3)

Here, it is expressed as a quantization representative value Dp ₋₁ of the sub-block immediately before the target sub-block. In order to make the quantized representative value Dp of the target sub-block equal to the quantized representative value Dp of the immediately preceding sub-block, Expression (3) becomes the following Expression (3 ′).
Dp ₋₁ = (D + Z × N−128 × 64) / 8 (3 ′)
The replacement candidate value calculation unit 104 selects the number N of pixels in the extracted color region of the target sub-block from the target value calculation unit 101, the total value D of the pixels in the non-extracted color region, and the representative value selection unit 106 described later. A representative value Dp ₋₁ of the immediately preceding sub-block is input, and a candidate value Z and a remainder value R are generated.
Z = (8 × Dp ₋₁ −D + 128 × 64) / N (4)
= (8 × (quantization representative value of immediately preceding sub-block) − (total value of pixels in non-extracted color area) −128 × 64} / (number of pixels determined as extracted color area)
R = (8 × Dp ₋₁ −D + 128 × 64)% N (5)
Here, “A% B” is a function that returns the remainder when the integer A is divided by the integer B.

  That is, if the value of each pixel determined to have the extracted color is set to the value obtained by Expression (4), no error occurs in the DC component when quantized by the encoding unit 506 in the subsequent stage. . However, the pixel value must be an integer. That is, in this state, the remainder value (remainder) according to the equation (4) is ignored. Therefore, the replacement candidate value calculation unit 104 calculates the remainder value R using Equation (5). Although the details will be described later, the remainder value R is distributed to the pixel group of the extracted color area.

  The replacement candidate value calculation unit 104 outputs the candidate value Z calculated as described above to the representative value selection unit 106 as a quantized representative value, and outputs the remainder value R to the candidate value selection unit 108.

  In the embodiment, it is assumed that one block includes four sub blocks. Therefore, when the first sub-block 0 is the target sub-block, there is no sub-block immediately before that. That is, when the target sub-block is the first sub-block 0, the replacement candidate value calculation unit 104 performs a meaningless process. However, since the representative value selection unit 106 described below selects the representative value Dp from the representative value generation unit 103 when selecting the representative value for the first sub-block 0, no problem occurs.

Hereinafter, the representative value selection unit 106 will be described. The representative value selection unit 106 selects and outputs one of the quantized representative values output from the representative value generation unit 103 and the replacement candidate value calculation unit 104. The selection conditions are as follows.
i) When selecting a quantized representative value for sub-block 0 The representative value selecting unit 106 selects the quantized representative value Dp generated by the representative value generating unit 103, the replacement candidate value calculating unit 107, and the replacement candidate Output to value 104. At this time, the selected quantized representative value is stored in an internal memory (not shown), and a control signal indicating that the quantized representative value is selected from the representative value generating unit 103 is notified to the candidate value selecting unit 108.

Note that the quantized representative value Dp output from the representative value selection unit 106 to the replacement candidate value calculation unit 104 is the quantization of the immediately preceding subblock when the replacement candidate value calculation unit 104 processes the next subblock. It is a representative value.
ii) When selecting quantized representative values of sub-blocks 1, 2, and 3 other than sub-block 0 The representative value selector 106 outputs the candidate value Z output from the replacement candidate value calculator 104 and the target value calculator 101. It is determined whether the relationship between MIN and MAX calculated in step 1 satisfies the following condition.

MIN ≦ Z <MAX (6)
The fact that the above inequality (6) is not satisfied means that the color of each pixel in the non-extracted color area included in the target sub-block is different from that of the immediately preceding sub-block. Therefore, if the candidate value calculated by the replacement candidate value calculation unit 104 is selected as the quantized representative value of the target sub-block, the range of gradation values that can be taken in the sub-block after replacement increases. As a result, the high frequency component AC after DCT conversion in the encoding unit 506 is likely to contain many values other than 0, and the encoding efficiency deteriorates. Therefore, if the representative value selection unit 106 determines that the inequality (6) is not satisfied, the representative value selection unit 106 selects the quantized representative value Dp generated by the representative value generation unit 103, and the replacement candidate value calculation unit 107, Output to the replacement candidate value 104.

  On the other hand, when it is determined that the inequality (6) is satisfied, the representative value selecting unit 106 selects the candidate value from the replacement candidate value calculating unit 104 as the quantized representative value Dp, the replacement candidate value calculating unit 107, and Output to the replacement candidate value 104. As a result, since the quantization representative value of the target subblock and the quantization representative value of the immediately preceding subblock are the same value, the code length when the differential DC component is encoded can be minimized.

  Regardless of which is selected and output, the representative value selection unit 106 stores the selected quantized representative value in the internal memory, and sends a control signal indicating which is selected to the candidate value selection unit 108. Notice.

  Another replacement candidate value calculation unit 107 is substantially the same as the processing of the replacement candidate value calculation unit 104. However, the replacement candidate value calculation unit 107 refers to the quantized representative value output from the representative value selection unit 106.

  Next, the candidate value selection unit 108 will be described. The candidate value selection unit 108, in accordance with a control signal from the representative value selection unit 106, sets of candidate values and remainder values calculated by the replacement value candidate calculation unit 104 and candidate values calculated by the replacement value candidate calculation unit 107. And one of the set of remainder values is selected and output to the replacement value generation unit 105.

  Specifically, when the control signal from the representative value selection unit 106 indicates that the quantized representative value from the representative value generation unit 103 has been selected, the DC component of the target sub-block is converted to the quantized representative value generation unit 103. It means to match the quantized representative value. Therefore, in this case, the representative value selection unit 106 selects and outputs the candidate value and replacement value of the replacement candidate value calculation unit 107. On the other hand, when the control signal from the representative value selection unit 106 indicates that the quantized representative value of the immediately preceding subblock has been selected, the DC component of the target subblock is matched with the quantized representative value of the immediately preceding subblock. Therefore, the representative value selection unit 106 selects and outputs the candidate value and replacement value from the replacement candidate value calculation unit 104.

  Next, the replacement value generation unit 105 will be described. As described above, the replacement value generation unit 105, when the position information in the target subblock is all “0”, that is, when the extracted color area does not exist in the target subblock, the pixel of the input subblock The value is output to the encoding unit 506 as it is.

  Here, a description will be given assuming that an extracted color region exists in the target sub-block. The processing of the replacement value generation unit 105 in this case will be described with reference to the flowchart of FIG. This process replaces the pixel value in the extracted color area in the target sub-block with the candidate value Z output from the candidate value selection unit 108, and further adds “1” to the R pixels to be replaced. By adding, the remainder value R is distributed.

  In the flowchart of FIG. 3, the update direction of the position of the target pixel will be described as a raster scan direction.

  First, in step S301, the remainder value R is set as an initial value in the counter C.

  Next, in step S302, it is determined whether or not the target pixel is in the extracted color area. If it is determined that the target pixel is in the non-extraction area, the process proceeds to step S306, and the input pixel data is output as it is.

  If it is determined in step S302 that the target pixel is in the extracted color area, the process proceeds to step S303, and it is determined whether or not the counter C is “0”. If not, the process proceeds to step S304. In step S305, “Z + 1” is output instead of the input target pixel value. Then, the counter C is decremented by “1”.

  In step S307, it is determined whether the output of 64 pieces of pixel data has been completed. If not, the process returns to step S302.

  As a result of the above processing, when it is determined that the pixel is within the extracted color area and the counter C is other than “0”, the processing of step S304 is performed, but the counter C becomes “0”. If so, the process proceeds to step S305. In step S305, the candidate value Z is output instead of the input target pixel value.

  The processing content by the replacement value generation unit 105 will be described with reference to FIGS.

  FIGS. 4A to 4D show gradation image data after replacement processing of four sub-blocks included in one block. In the drawing, the shaded area indicates the extracted color area of each sub-block. In the case shown in the figure, the DC components of each sub-block are all “−800”. As a result, the difference values of the DC components after quantization of the sub-blocks 1, 2, and 3 subsequent to the first sub-block 0 in the encoding unit 506 are 0. If the quantization step width is 16, no quantization error has occurred during quantization.

  When receiving a signal indicating the presence of extracted color information from the extraction unit 502, the code string generation unit 507 generates a header having an identification bit (for example, “1”) indicating the presence of the extracted color area. The code string generation unit 507 combines the header and the extracted color information encoded data, the position information encoded data, and the gradation encoded data of the four sub-blocks output from the encoding units 504 to 506. And output. FIG. 9A shows the structure of the encoded data of one block in this case.

  In addition, when receiving a signal indicating that there is no extracted color information from the extraction unit 902, the code string generation unit 507 generates a header having an identification bit (eg, “0”) indicating that there is no extracted color area. Then, the code string generation unit 507 combines the header and the gradation encoded data of the four sub-blocks output from the encoding unit 506 and outputs the combined data. That is, the encoded data of position information and extracted color information is ignored. FIG. 9B shows the structure of the encoded data of one block in this case.

  FIG. 10 shows a block configuration diagram of an apparatus that decodes the encoded data, and the flow of the decoding process will be described for the time being. As illustrated, the apparatus includes an encoding data buffer 601, an analysis / separation unit 602, an extraction color information decoding unit 603, a position information decoding unit 604, a gradation image decoding unit 605, and an image block generation unit 606. Composed.

  When the encoded data for one block is stored in the encoded data buffer 601, the analysis / separation unit 602 analyzes the header of the encoded data and determines an identification bit therein. When the identification bit is “1”, it means that the encoded data has the structure shown in FIG. 9A. Therefore, the analysis / separation unit 602 stores each encoded data stored in the encoded data buffer 601. Is distributed to each of the decoding units 604 to 606. At this time, the analysis / separation unit 602 outputs identification bit information to the image block generation unit 606. The gradation image decoding unit 605 decodes the input encoded data of the four sub-blocks, generates image data for one block, that is, 16 × 16 pixels, and outputs the image data to the image block generation unit 607.

  When the identification bit is “0”, it means that the encoded data of the block has the structure of FIG. 9B. Therefore, the analysis / separation unit 602 distributes the encoded data stored in the encoded data buffer 601 only to the gradation image decoding unit 606. At this time, the analysis / separation unit 602 also outputs the identification bit information to the image block generation unit 606. The gradation image decoding unit 606 decodes the encoded data of the four sub-blocks, generates image data for one block, that is, 16 × 16 pixels, and outputs the image data to the image block generation unit 607.

  When the identification bit from the analysis / separation unit 602 is “0”, the image block generation unit 607 outputs the image data of the 16 × 16 pixel block output from the gradation image decoding unit 605 as it is.

  Further, when the identification bit from the analysis / separation unit 602 is “1”, the image block generation unit 607 outputs the pixel whose position information is “0” from the gradation image decoding unit 605. Output pixel data. For the pixels whose position information is “1”, the pixel data decoded by the extracted color information generation unit 603 is output in order.

  As described above, according to the present embodiment, the extracted color area is included in each sub-block included in the block, and between the maximum value and the minimum value of the non-extracted color area in the target sub-block. Suppose that there is a DC component of the immediately preceding sub-block. In this case, according to the present embodiment, the replacement value of the pixel value in the extracted color region in the target subblock is determined so that the quantized representative value of the target subblock matches the previous quantized representative value. . As a result, the high-frequency component is reduced in the gradation pixel value in each sub-block, and the difference between the DC components of the target sub-block and the immediately preceding sub-block can be made zero. That is, the occurrence of sub-block distortion can be suppressed and the amount of generated codes can be reduced.

  On the other hand, when there is no DC component of the immediately preceding sub-block between the maximum value and the minimum value of the non-extracted color region in the target sub-block, so that it matches the quantization representative value of the target sub-block itself, A replacement value for the pixel value in the extracted color area in the target sub-block is determined. As a result, although the DC component difference between the sub-blocks does not become zero, the high-frequency component is reduced in the gradation pixel value in the target sub-block, and the occurrence of sub-block distortion is suppressed and generated. The amount of code can also be reduced.

  In the above-described embodiment, it has been described that the quantized values of the DC components of consecutive sub blocks are matched as much as possible. However, the present invention is not limited to this. A configuration in which the value is made as small as possible may be used. In this case, the replacement candidate value calculation unit 104 selects a plurality of candidates from the quantized representative value located between the quantized representative value of the immediately preceding sub-block and the quantized representative value generated by the quantized representative value generating unit 103. Generate a set of values and remainder values. Then, among the candidate values satisfying the inequality (6), a value close to the quantization representative value of the immediately preceding sub-block is selected. With this configuration, it is possible to minimize the code amount of the DC component while suppressing the code amount of the AC component.

  Although the embodiment according to the present invention has been described above, the functions of the configuration illustrated in FIG. 5 may be realized by a processor such as a CPU or MPU executing a computer program. That is, the present invention also includes computer programs. In general, the computer program is stored in a computer-readable storage medium such as a CD-ROM. The computer-readable storage medium is set in a reading device (CD-ROM drive), and can be executed by copying or installing in the system. Therefore, it is obvious that such a computer-readable storage medium is also within the scope of the present invention.

It is a block block diagram of the replacement part of embodiment. It is a flowchart for demonstrating the processing content of the target value calculation part of embodiment. It is a flowchart for demonstrating the processing content of the replacement value production | generation part of embodiment. It is a figure which shows an example of the process result of a substitution value production | generation part. It is a block block diagram of the image coding apparatus of embodiment. It is a figure which shows the correspondence of the block of an encoding unit, and a subblock. It is a figure for demonstrating the processing content of an extraction part. It is a figure which shows an example of the positional information and extraction color information which an extraction part produces | generates. It is a figure which shows the structure of the coding data for 1 block which a code sequence production | generation part produces | generates. It is a block block diagram of the image decoding apparatus in embodiment.

Claims (6)

An image encoding device for encoding image data in which a character line image and a gradation image are mixed,
Input means for inputting image data to be encoded in units of blocks composed of a plurality of pixels;
Extraction means for extracting character line drawing pixel data and identification information for identifying whether each pixel in the target block is character line drawing pixel data or gradation attribute pixel data from the inputted target block When,
First encoding means for encoding the pixel data of the character line drawing extracted by the extraction means;
Second encoding means for encoding the identification information extracted by the extraction means;
For each sub-block, which is a unit for performing orthogonal transform, included in the block of interest, pixel value extracted as a character line drawing in the sub-block is determined based on pixel data having a gradation attribute Replacing means for replacing with,
Third encoding means for orthogonally transforming and encoding the image data of each sub-block after being replaced by the replacement means;
Code string generation means for combining the encoded data generated by the first to third encoding means and generating encoded data of the block of interest;
The replacement means includes
DC component calculation means for calculating a DC component when it is assumed that the target sub-block in the target block is composed only of pixels having a gradation attribute;
It is determined whether or not a replacement candidate value determined based on a DC component value of a sub-block immediately before the target sub-block is within a value range of a pixel group having a gradation attribute in the target sub-block. Judgment means,
When the determination means determines that the replacement candidate value is within the range, a value that approximates the DC component value of the immediately preceding sub-block is determined as the replacement value of the target sub-block,
When the determination unit determines that the replacement candidate value is outside the range, a value approximated to the direct current component value of the target sub-block when it is assumed that the target sub-block has only gradation attribute pixels. , And a determining means for setting the replacement value ,
The first and second encoding means are lossless encoding means, and the third encoding means is an irreversible JPEG encoding means,
The target sub-block is composed of 8 × 8 pixels, and the number of pixels having a character / line drawing attribute is N, the sum of pixel data having gradation attributes, and the average values thereof are D and Dave. If a pixel is represented by 8 bits,
The direct current component calculation means obtains a direct current component DC calculated at the time of DCT conversion by the following equation:
DC = (D + Dave × N−128 × 64) / 8
The determining means determines the replacement value Z in the target sub-block according to any of the following equations, where DC _{−1 is} the DC component of the immediately preceding sub-block and DC DC of the _target sub-block is DC ₀ Do
Z = (8 × DC ₋₁ −D + 128 × 64) / N
Or
Z = (8 × DC ₀ −D + 128 × 64) / N
An image encoding apparatus characterized by that. Further, the replacement means determines the remainder value R generated when determining the replacement value Z according to the following equation:
R = (8 × DC ₋₁ −D + 128 × 64)% N
Or
R = (8 × DC ₀ −D + 128 × 64)% N
(A% B is a function that returns the remainder when integer A is divided by integer B)
The data of “N−R” pixels determined as the character / line image is replaced with a replacement value Z,
The image encoding apparatus according to claim 1, wherein data of R pixels determined as the character / line image is replaced with “Z + 1”.   When the target sub-block is the first sub-block of one block, the determining means sets a value that approximates a direct current component value when it is assumed that the target sub-block has only pixels with gradation attributes to the replacement value The image encoding apparatus according to claim 1, wherein: A control method of an image encoding device that encodes image data in which a character line image and a gradation image are mixed,
An input process for inputting image data to be encoded in units of blocks each composed of a plurality of pixels;
Extraction step of extracting character line drawing pixel data and identification information for identifying whether each pixel in the target block is character line drawing pixel data or gradation attribute pixel data from the inputted attention block When,
A first encoding step for encoding the pixel data of the character line drawing extracted in the extraction step;
A second encoding step for encoding the identification information extracted in the extraction step;
For each sub-block, which is a unit for performing orthogonal transform, included in the block of interest, pixel value extracted as a character line drawing in the sub-block is determined based on pixel data having a gradation attribute A replacement step of replacing with,
A third encoding step of orthogonally transforming and encoding the image data of each sub-block after being replaced in the replacement step;
A code string generation step of combining the encoded data generated in the first to third encoding steps and generating encoded data of the block of interest,
The replacement step includes
A direct current component calculation step for calculating a direct current component when it is assumed that the target sub-block in the target block includes only pixels having a gradation attribute;
It is determined whether or not a replacement candidate value determined based on a DC component value of a sub-block immediately before the target sub-block is within a value range of a pixel group having a gradation attribute in the target sub-block. A decision process;
In the determination step, when it is determined that the replacement candidate value is within the range, a value that approximates the DC component value of the immediately preceding sub-block is determined as the replacement value of the target sub-block,
In the determination step, when it is determined that the replacement candidate value is outside the range, a value approximated to the DC component value of the target sub-block when it is assumed that the target sub-block has only gradation attribute pixels And a determination step for setting the replacement value ,
The first and second encoding steps are lossless encoding steps, and the third encoding step is an irreversible JPEG encoding step,
The target sub-block is composed of 8 × 8 pixels, and the number of pixels having a character / line drawing attribute is N, the sum of pixel data having gradation attributes, and the average values thereof are D and Dave. If a pixel is represented by 8 bits,
In the direct current component calculation step, the direct current component DC calculated at the time of DCT conversion is obtained by the following equation:
DC = (D + Dave × N−128 × 64) / 8
In the determining step, when the DC component of the immediately preceding sub-block is DC ₋₁ and the DC component of the target sub-block is DC ₀ , the replacement value Z in the target sub-block is determined according to any of the following equations Do
Z = (8 × DC ₋₁ −D + 128 × 64) / N
Or
Z = (8 × DC ₀ −D + 128 × 64) / N
A control method for an image encoding device. Computer by executing reading, computer program characterized the computer function as an image encoding apparatus according to any one of claims 1 to 3. A computer-readable storage medium storing the computer program according to claim 5 .

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