KR20150067708A - Image-data compressing circuit, image-data compressing method, and photographing apparatus - Google Patents

Abstract

Disclosed is an image-data compression circuit including a difference value calculating unit and an encoding unit. The difference value calculation unit: receives input of pixel values of two adjacent set bits in the same color; and outputs a value based on the difference value of the two pixel values in the set bits after calculating the value by referring to pixel values of optical black. The encoding unit which encodes the value, calculated by the difference value calculating unit based on the difference value, by using variable-length codes which can be instantaneously decoded.

Description

[0001] The present invention relates to an image-data compressing circuit, an image-data compressing method, and a photographing apparatus,

The present invention relates to an image-data compression circuit, an image-data compression method, and a photographing apparatus.

Recently, "live-view information of an image pickup device" is stored in a recording medium such as a nonvolatile memory card, and image processing software on a PC (personal computer) Processing. This "live-view information of the image pickup device" is referred to as a raw image.

Generally, in a camera body, it is preferable that a large-capacity image data is image-compressed with high efficiency and recorded on a recording medium such as a nonvolatile memory card. The video compression increases the number of recordable photographing times, thereby improving the convenience of the user. Therefore, various compression techniques have been studied for raw images as well as other image formats (for example, Japanese Patent Laid-Open No. 2009-194760).

Japanese Patent Application Laid-Open No. 2009-194760

Since the raw image is image-processed on the PC, the raw image compression is preferably a reversible compression method. Reversible compression is a compression method in which data is not deficient at all. When the code compressed by the reversible compression method is decoded, the data before compression can be completely restored. However, there is a problem that it is difficult to increase the compression ratio as compared with the irreversible compression method.

An embodiment of the present invention provides an image data compression circuit, an image data compression method, and a photographing device which can increase a data compression rate when a raw image is compressed, taking the above circumstances into consideration.

The image data compression circuit according to the first aspect of the present invention includes a difference value calculation unit and an encoding unit.

Wherein the difference value calculator receives the pixel values of two adjacent set bits of the same hue and refers to the pixel values of the optical black to determine a difference value between the two pixel values within the set bit, And outputs the calculated value.

The encoding unit encodes a value according to the difference value calculated by the difference value calculating unit, and encodes the value using variable-length codes capable of instant decoding.

Preferably, the encoding unit encodes a value according to the difference value calculated by the difference value calculating unit, and outputs a variable-length first code indicating a range of a value according to the difference value, Length second code representing the variable-length code.

Preferably, in the process in which the difference value calculating unit refers to the pixel values of the optical black, when the value corresponding to the difference value is calculated, the calculation result does not exceed the setting bit (overflow) Or subtracts the pixel value of the optical black from the set value to subtract.

The image data compression method according to the second aspect of the present invention includes a difference value calculation step and a coding step.

Wherein the difference value calculating step receives the pixel values of the adjacent two set bits as the same hue and refers to the pixel values of the optical black to determine a difference value of the two pixel values within the set bit And outputs the calculated value.

In the encoding step, a value according to the difference value calculated in the difference value calculating step is encoded and encoded using variable-length codes capable of instant decoding.

An image pickup apparatus according to a third aspect of the present invention includes an image pickup element, an analog-to-digital conversion section, a difference value calculation section, and an encoding section.

The imaging element has an effective pixel region and an optical black region.

The analog-to-digital converter converts the video signal from the imaging element into pixel values of a set bit.

Wherein the difference value calculator receives pixel values of two adjacent set bits of the same hue from the analog-to-digital converter and refers to pixel values of optical black, And outputs a value corresponding to the difference value of the pixel values.

The encoding unit encodes a value according to the difference value calculated by the difference value calculating unit, and encodes the value using variable-length codes capable of instant decoding.

According to the image-data compression circuit, the image-data compression method, and the image pickup apparatus of the embodiment of the present invention, the pixel value of the optical black pixel is referred to, and the difference of the two pixel values A value corresponding to the value is calculated. The pixel value of this optical black pixel is the dark current noise due to the heat (i.e., the offset component). That is, there is no valid pixel value in the range from the pixel value zero to the pixel value of the optical black pixel. Therefore, for example, by removing the pixel value component of the optical black pixel included in the pixel value, the value according to the difference value can be reduced as a whole. Therefore, by assigning a code having a smaller length to a value according to the difference value, it is possible to easily improve the compression rate.

Figs. 1A and 1B are block diagrams showing an example of the configuration of a photographing apparatus as an embodiment of the present invention. Fig.
2 is a block diagram for explaining a configuration example of the data compression unit shown in FIG.
3 is a flowchart for explaining the operation of the difference value calculating unit shown in FIG.
4 is an explanatory view for explaining the operation of the difference value calculating unit shown in FIG.
FIG. 5 is a diagram for explaining the operation of the entropy encoding unit shown in FIG. 2. FIG.
6 is a block diagram of a comparison technique cited when explaining the operation of the difference value calculating unit shown in Fig.
7 is a flowchart of a comparison technique cited when explaining the operation of the difference value calculating unit shown in FIG.
8 is an explanatory diagram of a comparison technique cited when explaining the operation of the difference value calculating unit shown in FIG.
FIG. 9 is a diagram for explaining the effect of the data compression unit shown in FIG. 2. FIG.

The following description and accompanying drawings are for understanding the operation according to the present invention, and parts that can be easily implemented by those skilled in the art can be omitted.

The present specification and drawings are not intended to limit the present invention, and the scope of the present invention should be determined by the claims. The terms used in the present specification should be construed to mean the meanings and concepts consistent with the technical idea of the present invention in order to best express the present invention.

In describing the present invention, when it is judged that the detailed description of the related art is blurred, the detailed description thereof will be omitted.

The present invention may be represented by functional block configurations and various processing steps. These functional blocks may be implemented with various hardware and / or software configurations that perform particular functions. For example, the invention may be implemented in a programming or scripting language such as C, C ++, Java, assembler, and the like.

In addition, the present invention may employ conventional techniques for electronic configuration, signal processing, and / or data processing.

In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, and redundant description will be omitted.

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

1A and 1B are block diagrams showing an example of the configuration of a photographing apparatus 1 which is an embodiment of the present invention. Fig. 1A shows the entire configuration of the photographing apparatus 1, and Fig. 1B shows a modification of some configurations shown in Fig. 1A.

1A, a photographing apparatus 1 includes an optical system 11, an image pickup device 12, an analog-to-digital conversion unit 13, an image processing apparatus 14, a system bus 15, a CPU , A frame memory 17, a recording medium interface (I / F) 18, and a recording medium 19. The photographing apparatus 1 is a device for photographing and recording images, such as a surveillance camera, a mobile phone having a camera function, a digital camera, and a digital video camera, or constitutes a part of the device.

The optical system 11 is composed of at least one lens and condenses or refracts an optical signal incident on the imaging element 12. [

The imaging element 12 is a semiconductor sensor that converts light signals incident through the optical system 11 into electrical signals using light receiving elements such as a plurality of photodiodes and includes an effective pixel region 121 and an optical black and an optical black region 122.

The effective pixel region 121 has a plurality of pixels to which an optical signal incident through the optical system 11 reaches. That is, the optical signal is converted into an electrical signal by the plurality of pixels in the effective pixel region 121. Each pixel of the effective pixel region 121 includes a light-receiving element such as a photodiode, a filter of any one of the three colors of RGB (red-green-blue), and a condensing microlens. Here, the filters of each color are arranged in an array called a Bayer array. The Bayer arrangement is an arrangement in which the R filter and the B filter are arranged in a straight line so that the G filters are arranged in a checkered pattern and filled in between the G filters. Therefore, the pixels of G are arranged in all the lines, and the pixels of R and B are arranged at intervals of one line.

The optical black region 122 is composed of a plurality of pixels to which the optical signal does not reach. These pixels are referred to as optical black pixels, ineffective pixels, and the like, which are used when determining the reference of the optical pixel value zero (i.e., black). The optical black pixel outputs an electrical signal in accordance with the charge caused by the dark current (i.e., of the offset component). The optical black region 122 is arranged so as to surround the periphery of the effective pixel region 121, for example. The arrangement of the optical black area 122 shown is an example, and the arrangement area of the optical black area 122 differs depending on the image pickup device.

The analog-to-digital converter 13 converts the analog video signal output from the imaging element 12 into a digital video signal. The image pickup device 12 and the analog-to-digital conversion section 13 may be integrally configured differently from those shown in Fig. The analog-to-digital converter 13 converts the analog video signal output from each pixel into a digital video signal (that is, a pixel value) of a predetermined bit for each pixel and writes it into the frame memory 17. The pixel values of the pixels output from the analog-to-digital converter 13 constitute the above-described raw image.

The image processing apparatus 14 is constituted by a digital signal processor or the like and performs predetermined signal processing by executing a program stored in an internal nonvolatile memory device (not shown), for example. In the example shown in FIG. 1A, the image processing apparatus 14 has a data compression section 141, a video signal processing section 142, and a data compression-expansion section 143.

The data compressing section 141 reads the raw image written by the analog-to-digital conversion section 13 from the frame memory 17 to perform data compression processing and supplies the compressed image data to the frame memory 17, . The data compression section 141 is an example of the video data compression circuit of the present invention.

In general, there is a property that "adjacent pixel values of a natural image are similar" or "correlation of pixels ". This "correlation of pixels" can be used for a raw image compression technique. According to the correlation between pixels, the appearance frequency of the difference value between neighboring pixels of the same color is the highest, and the appearance frequency becomes lower as the difference value becomes larger. The frequency of appearance of difference values between adjacent pixels generally follows a Gaussian distribution centered on the difference value zero.

The data compression unit 141 compresses the difference value between adjacent pixels of the same color by a coding process that converts the difference value into a variable-length code according to the occurrence frequency of the difference value. That is, by performing a conversion process of assigning a code having a shorter code length to a value whose difference value is closer to zero, data representing the difference value is compressed.

Here, the data compression unit 141 of the present embodiment refers to the pixel values of the optical black pixel when calculating the difference value between adjacent pixels of the same color. This feature will be described later. The compressed image data can be written to the recording medium 19 directly from the data compressing unit 141 or through the frame memory 17.

The image signal processing unit 142 performs predetermined image processing on the image data read from the frame memory 17 or the recording medium 19 through the data compression-expansion unit 143, And writes it in the frame memory 17 or the recording medium 19 through the extension part 143. [ The image processing by the image signal processing unit 142 is not particularly limited, and examples thereof include optical correction, shading correction, defective pixel correction, RGB interpolation processing, gradation conversion processing, and edge emphasis processing.

The data compression-expansion unit 143 decompresses the compressed video data read out from the frame memory 17 or the recording medium 19 or decompresses the compressed video data read from the frame memory 17 or the recording medium 19 Compression processing is performed. The compression / decompression process by the data compression / decompression unit 143 can be performed by a plurality of compression methods including a reversible compression method and a non-reversible compression method. The data compression-decompression unit 143 decompresses image data representing a raw image compressed by, for example, the data compression unit 141 to generate uncompressed image data representing a raw image Processing and the like can be performed.

The system bus 15 includes an analog-to-digital conversion section 13, a data compression section 141, a data compression-expansion section 143, a CPU 16, a frame memory 17, And the like to transmit data such as image data. The CPU 16 controls the optical system 11, the image pickup element 12, the analog-digital conversion section 13, the image processing apparatus 14, and the like, for example, by executing a program stored in a predetermined nonvolatile memory device. , The frame memory 17, and the like.

The frame memory 17 includes, for example, a dynamic random access memory (DRAM), a static random access memory (SRAM), and the like.

The recording medium interface 18 controls data input / output when writing data to / from the recording medium 19 detachably connected or when reading data from the recording medium 19. The recording medium 19 is a nonvolatile recording medium configured to be removable.

In the photographing apparatus 1 shown in FIG. 1A, the connection relationship between the analog-digital conversion section 13, the data compression section 141, and the system bus 15 can be changed as shown in FIG. 1B, a configuration corresponding to the analog-to-digital conversion section 13 and the data compression section 141 in FIG. 1A is shown as an analog-digital conversion section 13a and a data compression section 141a. In the configuration example shown in Fig. 1B, the output of the analog-to-digital conversion section 13a can be input to the data compression section 141a, subjected to data compression processing, and then written to the frame memory 17 and the like.

1 shows the main components of the photographing apparatus 1. The photographing apparatus 1 includes components such as other power supply circuits, a display section, a control section, a communication device, a battery, and the like Some or all of them.

An exemplary configuration of the data compression unit 141 or 141a (hereinafter referred to as the data compression unit 141), which has been described with reference to FIG. 1, will be described with reference to FIGS. 2 to 5. FIG. In the lower part of FIG. 5, "NULL" means that the corresponding code does not exist.

The data compressing unit 141 shown in FIG. 2 includes a difference value calculating unit 21 and an entropy encoding unit 22.

The difference value calculating unit 21 calculates the difference value between the pixel values of the two pixels adjacent to each other in the effective pixel region 121 from the pixel values of the pre- The pixel values Pa and Pb are inputted and the pixel values OB of the pixels in the optical black region 122 are input.

Here, the pixel values Pa and Pb are pixel values of two adjacent pixels among the same color pixels in the same frame. Further, the pixel values OB of the optical black pixels can be replaced with the minimum pixel value among the pixel values of the optical black pixels in the same frame, for example. These pixel values OB can be stored in the header area of the image data file including the compressed image data and can be referred to in the decoding process.

Next, the operation of the difference value calculating unit 21 will be described with reference to Figs. 3 and 4. Fig. Hereinafter, the pixel value Pa and the pixel value Pb will be described as 8 bits. Of course, the number of bits of the pixel value Pa and the pixel value Pb is not limited to 8 bits but may be 7 bits or less and 9 bits or more.

First, the difference value calculation unit 21 inputs the pixel values Pa and Pb of neighboring pixels and the pixel value OB of the optical black pixel (the minimum value among the pixel values of all the optical black pixels) do.

Step 2: Difference value calculation unit 21 performs a subtraction process (Pb-Pa).

Step 3: There are three difference value calculation processing methods by the difference value calculating unit 21, and it is determined which processing type to select according to the subtraction processing result in the second step. The difference value calculating unit 21 sets the processing type as the first type when Pb-Pa <-128 as the subtraction processing in the second step. Further, the difference value calculating section 21 sets the processing type as the second type when Pb-Pa> 127 as the subtraction processing of the second step. Then, the difference value calculating unit 21 sets the processing type as the third type in the case of the others.

Step 4: The difference value calculating unit 21 calculates a value (Sabun) according to the difference value as the processing type determined in the third step. Here, the value (Sabun) according to the difference value is a value corresponding to the difference value (Pa-Pb), which is the same value as the difference value (Pa-Pb) . In the case of the first type, the difference value calculating unit 21 calculates the value Sabun according to the difference value by the calculation process of Sabun = (255-Pa) + (Pb-OB). In the case of the second type, the difference value calculating unit 21 calculates the value Sabun according to the difference value by the calculation process of Sabun = - (255-Pb) - (Pa-OB). Then, in the case of the third type, the difference value calculating unit 21 calculates the value Sabun corresponding to the difference value by the calculation process of Sabun = Pb - Pa. Here, the difference value calculating unit 21 stores type information (Case) indicating a value (Sabun) according to the obtained difference value and a value (Sabun) depending on which type as the difference value, to the entropy encoding unit 22 .

Actually, when the value Sabun according to the difference value is obtained as the third type, the value Sabun according to the difference value is not a value indicating the difference value (Pb-Pa) between the pixel values Pb and Pa of the adjacent two pixels Is a value that can represent the difference value (Pb-Pa) by adding a predetermined calculation process. For example, when the value (Sabun) according to the difference value is calculated as the first type, the difference value (Pb-Pa) can be calculated by performing the calculation processing of Sabun + OB-255. Further, when the value (Sabun) according to the difference value is calculated as the second type, the difference value (Pb-Pa) can be calculated by the calculation process of Sabun-OB + 255.

If the input pixel values Pa and Pb are represented as unsigned 8-bit integers, the pixel value can take a value from 0 to 255. [ The minimum value is -255 when Pa = 0 and Pb = 255, +255 when the maximum value is Pa = 255 and Pb = 0 in the difference value (Pb-Pa) between the two pixel values Pa and Pb do.

On the other hand, the value of the signed 8-bit integer is in the range of -128 to +127. Therefore, when the difference value (Pb-Pa) between the two pixel values Pa and Pb is simply obtained by the subtraction formula "Pb-Pa ", the calculation result (i.e., difference value) of the subtraction formula can not be displayed as an 8- . An increase in the number of bits leads to an increase in the amount of data. In addition, an increase in the number of bits may be linked to an increase in processing time. Thus, in the present embodiment, the calculation method of the difference value is divided into three types according to the magnitude of the difference value so that the range of the difference value is within the number of bits of the input pixel value. In addition, classification in the case of the magnitude of the difference value in the third step can be performed, for example, by checking whether or not an increase in the number of digits has occurred or overflow has occurred in the subtraction process in the second step have. Therefore, the operation "Pb-Pa" in the second step can be performed by an 8-bit subtraction process.

However, in the first type of condition (Pb-Pa <-128), the result of the subtraction (Pb-Pa) is a value from the minimum value of -255 to -129 (a value smaller than -128 by 1). When the integer 255 is added to the result of the subtraction processing, for example, the value obtained by adding 255 is 0 to 126. [ This range is within the range of the signed 8-bit integer value. That is, for example, by adding a process of adding the integer "255", a value according to the difference value in the case of (Pb-Pa <-128) can be represented by a signed 8-bit integer.

On the other hand, in the second type of condition (Pb-Pa &gt; 127), the result of the subtraction (Pb-Pa) is a value from the maximum value +255 to +128 (one greater than +127). For example, when subtracting the integer "-255" from the result of the subtraction processing, the value after subtracting "-255" becomes a value from 0 to -127. This range is within the range of the signed 8-bit integer value. That is, for example, by adding a process of subtracting an integer 255, a value according to a difference value in the case of (Pb-Pa &gt; 127) can be displayed as a signed 8-bit integer.

The difference value calculating unit 21 calculates a value Sabun according to the difference value of the two pixel values within the number of bits of the pixel values Pa and Pb with reference to the pixel value OB of the optical black pixel Output.

In the process in which the difference value calculating unit 21 refers to the pixel value OB of the optical black pixel, when calculating the value Sabun according to the difference value, the calculation result of the difference value indicates the number of bits of the pixel values Pa and Pb The pixel value OB of the optical black pixel is subtracted from the set value (255 in the case of 8 bits) to be added or subtracted so as not to overflow. Hereinafter, this point will be described.

As described above, the image pickup device 12 is provided with effective pixels for receiving charges by virtue of photoelectric effect and optical black pixels for detecting offset of charges including a dark current And pixels). When the imaging element 12 receives heat, it has a property of generating charges even in a light shielding state, and is called dark current noise.

The effective pixel value is the sum of the " original charge amount due to light reception "and the" charge amount including dark current ". The optical black pixels are usually disposed around the effective pixels and are shielded from light, so that no charge is generated due to light reception. That is, only the offset information of the dark current noise from the optical black pixels can be detected. There is no effective pixel value in the range from 0 to the pixel value of the optical black pixel among the pixel values of the effective pixels. Therefore, even if the pixel value OB of the optical black pixel is removed from the pixel values Pa and Pb, the dynamic range of the pixel value (i.e., the size of the range obtained by subtracting the noise value from the maximum value of the pixel value) It does not.

In addition, a plurality of optical black pixels are typically provided in the optical black region 122 described with reference to FIG. Thus, pixel values of optical black pixels are often a plurality of different values. In the case of this embodiment, one optical black pixel value OB is masked. Here, the dynamic range changes when a large pixel value OB is used or when a small pixel value OB is used among a plurality of pixel values. Therefore, in the present embodiment, the minimum value OB of the optical black pixel values is referred to from the viewpoint of not lowering the dynamic range of the pixel value. However, the optical black pixel value is not limited thereto, and may be a predetermined fixed value predicted as a minimum value according to, for example, use conditions. Alternatively, for example, an average value of pixel values of a plurality of optical black pixels may be referred to. In this case, although the dynamic range slightly deteriorates, an effect of further increasing the compression ratio can be expected.

On the other hand, in the process of calculating the difference value, the smaller the two pixel values are, the smaller the maximum value of the difference values becomes and the larger the minimum value becomes. That is, when a difference value between two pixel values is obtained, if the two pixel values are decreased by subtracting a constant offset value from each pixel value, the maximum value of the difference values becomes smaller and the minimum value becomes larger.

For example, when the pixel value OB of the optical black pixel is subtracted in advance from the two pixel values Pa and Pb, the difference value Pb-Pa is the difference value (Pb-OB) - (Pa-OB)). Since the maximum value of the difference value in this case is (Pb-OB) is "255-OB" and (Pa-OB) is "0" do. On the other hand, the minimum value of the difference value ((Pb-OB) - (Pa-OB)) is "-255 + OB" since (Pb-OB) is "0" and (Pa-OB) is "255-OB".

When the difference value between adjacent two pixels is obtained after subtracting the pixel value OB of the optical black pixel from the pixel values of the adjacent two pixels, the first type condition (Pb-Pa <-128) , The result of the subtraction (Pb-Pa) (= (Pb-OB) - (Pa-OB)) is a value from -128 to -128 which is smaller than -128 in the above minimum value "-255 + OB". When the integer "255-OB" is added to the result of the subtraction processing, the value after adding "255-OB" becomes a value from 0 to (-129 + 255-OB) = (126-OB). This range is within the range (-128 to +127) of the signed 8-bit integer if OB is a normal value. That is, by adding a process of adding the value obtained by subtracting OB from the integer 255 (Pb-Pa <-128), a value according to the difference value can be displayed as a signed 8-bit integer. That is, in the first type, the Sabun value can be represented by an 8-bit signed integer by obtaining the Sabun value by arithmetic processing of (Pb-OB) - (Pa-OB) + (255-OB). The calculation formula of (Pb-OB) - (Pa-OB) + (255-OB) becomes Pb-Pa + 255-OB in summary and Sabun = (255- (Pb-OB). In addition, the expression of Sabun = (255-Pa) + (Pb-OB) expresses the order of operations. That is, by obtaining the first (255-Pa), then (Pb-OB), and then ((255-Pa) The result of the processing can be within the range of signed 8-bit integers. (255-Pa) and (Pb-OB) can be reversed.

On the other hand, in the case of the second type of condition (Pb-Pa &gt; 127), the result of the subtraction Pb-Pa (= (Pb- OB) - (Pa- Which is greater than +128. For example, when the integer 255-OB is subtracted from the result of the subtraction processing, the value after subtracting 255-OB is from 0 to +128- (255-OB) = (-127 + OB) . This range is within the range (-128 to +127) of the signed 8-bit integer if OB is a normal value. That is, for example, by adding a process of subtracting the value obtained by subtracting OB from the integer 255, a value according to the difference value in the case of (Pb-Pa> 127) can be expressed as a signed 8-bit integer. That is, in the second type, the Sabun value can be displayed as a signed 8-bit integer by calculating the Sabun value by an operation of (Pb-OB) - (Pa-OB) - (255-OB). Since the formula of (Pb-OB) - (Pa-OB) - (255-OB) is summarized as Pb-Pa-255 + OB, Sabun = - (255-Pb) - Pa-OB). In addition, the expression of Sabun = - (255-Pb) - (Pa-OB) expresses the order of operations. Namely, (255-Pb) is obtained first, then the complement of the result of (255-Pb) is obtained, then (Pa-OB) is obtained, - ((Pa-OB)), the result of each process can be made within the range of signed 8-bit integers. (255-Pb) or (255-Pb) and the operation of obtaining (Pa-OB) can be reversed.

FIG. 4 shows the relationship between the Sabun values obtained by the first to third types of processing methods and the pixel values OB of the optical black pixels, and the pixel values Pa and Pb. Fig. 4 is a diagram showing values of pixel values (levels of 0 to 255) in the vertical direction.

In the first type, the Sabun value is not the "Pa-Pb" shown by the solid line arrows but the difference values between the pixel value Pa and the pixel value Pb shown by the dotted arrows, the maximum value 255 and the value of OB .

In the second type, not the "Pa-Pb" itself shown by solid line arrows but the difference values between the pixel value Pa and the pixel value Pb shown by the dotted arrows and the value of OB and the maximum value 255 are used .

In the third type, the value of Sabun is set to the value of "Pa-Pb" shown by a solid line arrow.

Next, the entropy encoding unit 22 of Fig. 2 will be described. The entropy encoding unit 22 is an example of the encoding unit of the present invention. The entropy encoding unit 22 converts the value Sabun according to the difference value calculated by the difference value calculation unit 21 by using a variable-length code A capable of instantaneous decoding. Here, the entropy encoding unit 22 of the present embodiment converts the value Sabun according to the difference value calculated by the difference value calculating unit 21 into a variable-length code A (the first code And a variable-length code B (second code) indicating the value Sabun. The variable-length code A capable of instantaneous decoding can be generated by entropy coding, which allocates a variable-length code based on the probability of occurrence of information, for example, Huffman coding. Huffman coding is a technique based on the idea that a short code (number of bits) is assigned to a value with a high appearance frequency and a long code (number of bits) is assigned to a value with a low appearance frequency. In addition, the fact that instantaneous decoding is possible means that decoding is possible within the range of the code.

The entropy encoding unit 22 generates codes A and B based on the input Sabun value with reference to a conversion table prepared in advance indicating a conversion method. Here, the correspondence relationship between the Sabun value and the sign A and the sign B will be described with reference to FIG. 5 shows the length of a sign obtained by dividing the entire range of the Sabun value into 8 ranges and adding the Sabun value corresponding to each range, the code A, the code B, and the code A and the code B. FIG.

In the example shown in FIG. 5, the range from the minimum value to the maximum value of the Sabun value is divided into eight ranges, and the shorter code A and the code B are assigned to the range of the closer Sabun value to zero. Here, each range and each code A correspond to each other. Further, the number of data of the Sabun value corresponding to each range is variable, and the number of data included in the range "8" of the closest Sabun value to zero is set to 1, The number of data included in the range "7 " is further reduced to the number of data including the range of the Sabun value closer to zero, such as 2, ... and so on.

According to the correspondence relationship shown in FIG. 5, for example, when the Sabun value is "0", the range where the Sabun value is located is "8" The length of the code including the sign A and the sign B is 2 bits. For example, when the Sabun value is "+1", the range where the Sabun value is located is "7", so that the sign A is "010" and the sign B is "0" The length of the combined code is 4 bits t. For example, when the Sabun value is "-1", the range where the Sabun value is located is "7", so that the sign A is "010", the sign B is "0" or "1" Quot;), the length of the code obtained by adding the code A and the code B is 4 bits.

The entropy encoding unit 22 converts the inputted Sabun value into a code A and a code B, for example, by referring to the table of correspondence relations shown in Fig. Here, the entropy encoding unit 22 uses the information (Case) of the calculation type of the Sabun value for the code consisting of the code A and the code B. [ That is, the content of the Sabun value in the entropy encoding unit 22 is changed by the Case value. The entropy encoding unit 22 generates a sign corresponding to the difference value (Pb-Pa) by this additional processing.

The entropy encoding unit 22 adds, for example, a fixed-length or variable-length predetermined code capable of instantaneous decoding corresponding to the Case value to the code consisting of the code A and the code B, or by extending the code A It is possible to add information indicating the Case value by making it indicate the Case value in addition to the range of the Sabun value. The entropy encoding unit 22 sequentially generates code corresponding to each set of the Sabun value and the Case value, and outputs it as a compressed code.

As described with reference to FIGS. 2 to 4, the difference value calculating unit 21 of the present embodiment calculates the difference value (Pb-Pa) between the pixel value Pa and the pixel value Pb by using the optical black ) Pixel of the pixel value (OB). Next, the effect of the configuration in which the pixel value OB of the optical black pixel is referred to is compared with the case where the pixel value OB of the optical black pixel is not referred to.

The configuration in the case where the pixel value OB of the optical black pixel is not referred to is the same as that of the optical black pixel in the input in comparison with the difference value calculating unit 21 shown in Fig. the difference value calculating unit 31 in which the pixel value OB of the black pixel is omitted, and the entropy encoding unit 22 shown in Fig. 2 are combined. The difference value calculator 31 shown in FIG. 6 inputs pixel values Pa and Pb of adjacent pixels as shown in FIG. 7 (step 1a). Next, the difference value calculating section 31 carries out a subtraction process of Pb-Pa (Step 2a). Next, the difference value calculating unit 31 performs the difference value processing type determination processing (step 3a). There are three difference value processing types by the difference value calculating unit 31, and the processing type is changed according to the subtraction processing result in step 2a. That is, when Pb-Pa <-128, the processing type is set to the first type. If Pb-Pa &gt; 127, the process type is set to the second type. In other cases, the processing type is set to the third type.

Next, the difference value calculating unit 31 calculates a value Sabun according to the difference value as the processing type determined in the step 3a. The difference value calculating unit 31 performs an operation process of Sabun = (255-Pa) + Pb in the case of the first type and an operation process of Sabun = - (255-Pb) -Pa in the case of the second type And in the case of the third type, calculation processing of Sabun = Pb-Pa is performed. The difference value calculator 31 then outputs the obtained Sabun value and Case value to the entropy encoder 22. The Sabun value in this case is a value shown by the arrows in the dotted line (for the first type and the second type) or the solid line (for the third type) in Fig.

Next, with reference to Fig. 9, when the pixel value OB of the optical black pixel is referred to (this embodiment) and the pixel value OB of the optical black pixel is not referred to (Fig. Comparison technique described with reference to Fig. 8) will be described.

The pixel values Pa and Pb of the adjacent pixels are 8 bits, and the OB value is 32. [ The entropy encoding unit 22 performs encoding processing using the table described with reference to Fig.

Fig. 9 is a diagram showing a relationship between a plurality of combinations of adjacent pixel values Pa and Pb, a Sabun value calculated by a comparative technique in each example, a length of a sign obtained by adding a sign A and a sign B corresponding to the Sabun value, And the length of the sign including the sign A and the sign B corresponding to the Sabun value are shown together. For example, in the case of Pa = 240 and Pb = 80, in the comparison technique, the Sabun value is 95 and the sign length is 12 bits. In this embodiment, the Sabun value is 63 and the code length is 10 bits. For example, in the case of Pa = 254 and Pb = 33, the Sabun value is 34 and the length of the code is 10 bits in the comparative technique. In this embodiment, the Sabun value is 2 and the code length is 5 bits.

As shown in FIG. 9, according to the present embodiment for calculating the value Sabun according to the difference value with reference to the pixel value OB of the optical black pixel, compared with the case of not referring to the OB, The effect of shortening the length can be obtained. As described above, according to the present embodiment, the value Sabun corresponding to the difference value surely decreases. Therefore, since the image compression efficiency is improved, for example, the recording photographing time storable in the recording medium 19 can be increased.

Meanwhile, the present invention can be embodied in computer readable code on a computer readable recording medium.

Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device and the like, and also a carrier wave (for example, transmission via the Internet) . In addition, the computer-readable recording medium may be distributed over network-connected computer systems so that the codes can be stored and executed in a distributed manner. In addition, functional programs, codes, and code segments for implementing the present invention can be easily deduced by programmers skilled in the art to which the present invention belongs.

As described above, according to the image-data compression circuit, the image-data compression method, and the image pickup apparatus of the embodiment of the present invention, the pixel value of the optical black pixel is referred to, A value corresponding to the difference value of the pixel values is calculated. The pixel value of this optical black pixel is the dark current noise due to the heat (i.e., the offset component). That is, there is no valid pixel value in the range from the pixel value zero to the pixel value of the optical black pixel. Therefore, for example, by removing the pixel value component of the optical black pixel included in the pixel value, the value according to the difference value can be reduced as a whole. Therefore, by assigning a code having a smaller length to a value according to the difference value, it is possible to easily improve the compression rate.

The present invention has been described above with reference to preferred embodiments. It will be understood by those skilled in the art that the present invention may be embodied in various other forms without departing from the spirit or essential characteristics thereof.

Therefore, the above-described embodiments should be considered in a descriptive sense rather than a restrictive sense. The scope of the present invention is defined by the appended claims rather than by the foregoing description, and the inventions claimed by the claims and the inventions equivalent to the claimed invention are to be construed as being included in the present invention.

There is a possibility that data other than video data is also used.

1: photographing apparatus, 11: optical system,
12: image pickup element, 121: effective pixel area,
122: an optical black region,
13, 13a: an analog-to-digital converter,
14: Image processing apparatus, 141, 141a: Data compression unit,
142: video signal processing unit, 143: data compression-expansion unit,
15: system bus, 16: CPU,
17: frame memory, 18: recording medium interface,
19: recording medium, 21, 31: difference value calculating section,
22: Entropy coding section.

Claims (5)

The pixel values of two adjoining set bits as the same color are inputted and the value corresponding to the difference value of the two pixel values within the set bit is calculated and outputted by referring to the pixel values of optical black A difference value calculation unit; And
And a coding unit for coding a value according to the difference value calculated by the difference value calculating unit, using variable-length codes capable of instantaneous decoding. 2. The apparatus of claim 1,
A first code of variable-length indicating a range of a value according to the difference value, and a second variable-length code indicating a value according to the difference value, encoding a value according to the difference value calculated by the difference value calculating unit, Into a code. 3. The image processing method according to claim 1 or 2, wherein in the processing in which the difference value calculating section refers to the pixel values of the optical black,
And subtracts the pixel value of the optical black from a set value to be added or subtracted so that the calculated result does not exceed the set bit (overflow) when calculating the value according to the difference value. The pixel values of two adjoining set bits as the same color are inputted and the value corresponding to the difference value of the two pixel values within the set bit is calculated and outputted by referring to the pixel values of optical black A difference value calculating step; And
And encoding the value according to the difference value calculated in the difference value calculation step using variable-length codes capable of instantaneous decoding. An imaging device having an effective pixel area and an optical black area;
An analog-to-digital converter for converting a video signal from the image pickup element into pixel values of a set bit;
Wherein the pixel value of two adjacent set bits of the same color is input from the analog-to-digital conversion unit, and the pixel value of the optical black is referred to, and the difference value of the two pixel values And outputs the calculated value; And
And a coding unit for coding a value according to the difference value calculated by the difference value calculating unit, using variable-length codes capable of instantaneous decoding.

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