CN111654705A - Mosaic image compression method based on novel color space conversion - Google Patents

Mosaic image compression method based on novel color space conversion Download PDF

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CN111654705A
CN111654705A CN202010505297.2A CN202010505297A CN111654705A CN 111654705 A CN111654705 A CN 111654705A CN 202010505297 A CN202010505297 A CN 202010505297A CN 111654705 A CN111654705 A CN 111654705A
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color space
space conversion
component
cfa
inverse
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CN111654705B (en
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朱树元
贺康
王忠荣
刘光辉
王正宁
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University of Electronic Science and Technology of China
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/186—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a colour or a chrominance component
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
    • H04N19/625—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding using discrete cosine transform [DCT]

Abstract

The invention belongs to the technical field of image processing, and particularly provides a mosaic image compression method based on novel color space conversion, which is used for solving the problems that the recovered image quality is low, the algorithm is complex and the method is only suitable for RGGB type CFA images with a certain specific format in the prior art. The invention uses G of CFA1RBG2The four-channel model is converted, then the novel color space conversion is used, the correlation among pixels is eliminated, and the bit number of the coded code stream is greatly reduced, so that the coded code stream can be directly coded through a traditional coding frame; moreover, the invention is suitable for RGGB type CFA with any format; meanwhile, the color space conversion matrix and the inverse conversion matrix are both true values, but not approximate results, so that rounding errors caused by the conversion matrix are eliminated; in addition, the invention updates the brightness component to be coded in a mathematical optimization mode, thereby bringing better recovery quality.

Description

Mosaic image compression method based on novel color space conversion
Technical Field
The invention belongs to the technical field of image processing, particularly relates to the field of RGGB type CFA compression coding, and particularly provides a mosaic image compression method based on novel color space conversion.
Background
To produce color images, most digital cameras use a single CMOS board with several different color filters and use interpolation techniques to produce full color images. RGGB type Color Filter Array (CFA), refers to the generic name of a CFA having two green filters, one red Filter and one blue Filter in a 2 × 2 region; among them, Bayer-CFA is most commonly used, and it has green filters at the upper left and lower right, red filters at the upper right, and filters at the lower left in each 2 × 2 region. If a full-color image is interpolated, there is only one color component in each pixel of the CFA, so the other two color components of a given pixel must be interpolated using neighboring pixel information; although there are several possible interpolation algorithms, it is clear that they all lead to an increase in redundancy from an information-theoretic point of view.
At present, most of compression methods of CMOS images on a civil digital camera carry out image compression after interpolation of the images, the compression data volume is three times of CFA, and the image compression real-time performance of a space camera is not facilitated. The existing compression methods of RGGB type CFA, which either convert two green pixels into one, reduce the quality of the restored image; or a complex structural transformation or numerical filling is designed for the Y matrix, the algorithm is complex and is only suitable for RGGB type CFA of a specific format.
Based on the method, the invention provides a mosaic image compression method based on novel color space conversion.
Disclosure of Invention
The invention aims to solve the problems that the prior art is low in restored image quality, complex in algorithm and only suitable for RGGB type CFA images in a certain specific format, and provides a mosaic image compression method based on novel color space conversion, which is simple to implement, can bring higher restored quality under the condition that the bit number of coded code streams is the same, and is suitable for RGGB type CFA in any format.
In order to achieve the purpose, the invention adopts the technical scheme that:
the mosaic image compression method based on the novel color space conversion comprises the following steps:
step 1, G in RGGB type CFA image1、R、B、G2The components are respectively and independently spliced to form a channel, and a four-channel model is generated;
step 2, performing color space conversion on the four-channel model generated in the step 1, and converting the four-channel model into an YDgCoCg model to obtain a brightness component Y, chrominance components Dg, Co and Cg;
step 3, performing DCT and JPEG chroma quantization on the chroma components Dg, Co and Cg in the step 2 to generate chroma components Dg ', Co ' and Cg ';
step 4, inverse quantization is carried out on the chrominance component Cg 'in the step 3, IDCT is carried out to generate chrominance component Cg', mathematical optimization is carried out, and the luminance component is generated through calculation
Figure BDA0002526325480000021
Step 5, comparing the brightness component in the step 4
Figure BDA0002526325480000022
DCT and JPEG brightness quantization are performed to generate brightness component
Figure BDA0002526325480000023
Step 6, JPEG chroma coding is carried out on the chroma components Dg ', Co ' and Cg ' in the step 3, and the brightness component in the step 5 is subjected to
Figure BDA0002526325480000024
JPEG brightness coding is carried out, and the code stream is sent;
and 7, receiving the code stream, performing inverse coding, inverse quantization, IDCT and inverse color space conversion, and recovering the CFA image.
Further, the color space in step 2 is converted into:
Figure BDA0002526325480000025
further, in step 4, the mathematical optimization is as follows:
Figure BDA0002526325480000026
further, the step 7 specifically includes:
s71, receiving data, and performing inverse coding, inverse quantization and IDCT to generate component
Figure BDA0002526325480000027
Dg″、Co″、Cg″；
S72, component pair
Figure BDA0002526325480000028
Dg ', Co ', Cg ' are inverse color space converted to obtain a component G1″、R″、B″′、G″2：
Figure BDA0002526325480000029
S73, component G1″、R″、B″′、G″2The reverse operation of step 1 is performed to recover the RGGB type CFA image.
The working principle of the invention is as follows:
the invention provides a four-channel model, the component size of the brightness code is changed into 1/4 of the size of CFA through color space conversion, and the bit number of the code stream generated by the chroma code is very small, so that the bit number of the code stream is effectively reduced;
the color space conversion method comprises the following steps:
Figure BDA0002526325480000031
wherein, Y is a brightness component, Dg, Co, Cg are chrominance components, Dg represents green color difference: dg ═ G2-G1；
Assuming that the Y, Dg, Co and Cg components are DCT, quantized and encoded to obtain Y ', Dg ', Co ' and Cg ' respectively, then inverse coding, inverse quantization and IDCT are restored to obtain Y ', Dg ', Co ', Cg ', and Cg ', which are subjected to inverse color space conversion:
Figure BDA0002526325480000032
the distortion generated at this time with respect to the original CFA is:
Figure BDA0002526325480000033
to minimize distortion:
Figure BDA0002526325480000034
finding a singular point, and according to the practical significance, the point is the minimum value point:
Figure BDA0002526325480000035
it can be seen that the compression coding causes severe distortion of the chrominance channels, resulting in the inverse quantization of the luminance component at the decoding end to be closer
Figure BDA0002526325480000036
(instead of Y), it can bring less overall distortion, i.e. higher recovery quality; therefore, the luminance component processed at the encoding end by the present invention is
Figure BDA0002526325480000037
The components are obtained by DCT, quantization and coding
Figure BDA0002526325480000038
Then, the code is subjected to inverse coding, inverse quantization and IDCT to obtain the code
Figure BDA0002526325480000039
And
Figure BDA00025263254800000310
there is still a gap, but this gap isMuch smaller than the sum of Y' generated by directly encoding Y and decoding
Figure BDA00025263254800000311
The difference in (a).
In conclusion, the beneficial effects of the invention are as follows:
the invention provides a mosaic image compression method based on novel color space conversion, which is used for compressing G of CFA1RBG2The four-channel model is converted, then the novel color space conversion is used, the correlation among pixels is eliminated, and the bit number of the coded code stream is greatly reduced, so that the coded code stream can be directly coded through a traditional coding frame; moreover, the invention is suitable for RGGB type CFA with any format; meanwhile, the color space conversion matrix and the inverse conversion matrix are both true values, but not approximate results, so that rounding errors caused by the conversion matrix are eliminated; in addition, the invention updates the brightness component to be coded in a mathematical optimization mode, thereby bringing better recovery quality.
Drawings
Fig. 1 is a schematic flow chart of a mosaic image compression method based on novel color space conversion according to the present invention.
Fig. 2 is a schematic diagram of colors of each pixel point of the Bayer-format CFA according to an embodiment of the present invention.
FIG. 3 is a schematic diagram of color space conversion according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The embodiment provides a mosaic image compression method based on novel color space conversion, and the flow of the method is shown in fig. 1; this embodiment takes Bayer-CFA as an example, and specifically includes the following steps:
step S1, G in RGGB type CFA image1RBG2The components are respectively and independently spliced into a channel and are changed into a four-channel model, so that each component independently forms a matrix, and the complex structure of CFA is eliminated;
specifically, the Bayer-CFA is shown in FIG. 2, which divides the Bayer-CFA into 2 × 2 pixel blocks3, selecting all the elements in the upper left corner of the 2 × 2 pixel block to be combined into G1Component, choosing all the elements in the upper right corner of the 2 × 2 pixel block to combine into an R component, choosing all the elements in the lower left corner of the 2 × 2 pixel block to combine into a B component, choosing all the elements in the lower right corner of the 2 × 2 pixel block to combine into a G component2A component;
step S2, converting the four channels G1RBG2The model is switched to the four-channel YDgCoCg model, shown in FIG. 3, eliminating G1RBG2The correlation among the four channels reduces the bit number required by coding;
Figure BDA0002526325480000041
step S3, DCT and JPEG chroma quantization are carried out on the Dg, Co and Cg components in the step S2 to generate Dg ', Co ' and Cg ';
step S4, IDCT, reverse quantization Cg 'generation Cg', mathematical optimization, calculation generation
Figure BDA0002526325480000051
The recovered luminance component is closer to
Figure BDA0002526325480000052
Will introduce less distortion;
specifically, IDCT, inverse quantization Cg' generates Cg ″, and a new component is calculated
Figure BDA0002526325480000053
Figure BDA0002526325480000054
Step S5, pair
Figure BDA0002526325480000055
DCT and JPEG luminance quantization are performed to generate
Figure BDA0002526325480000056
Step S6, JPEG-chroma coding the Dg ', Co' and Cg 'generated in S3, and JPEG-chroma coding the Dg', Co 'and Cg' produced in S5
Figure BDA0002526325480000057
JPEG brightness coding is carried out, and the code stream is sent;
step S7, receiving the code stream, performing inverse coding, inverse quantization, IDCT and inverse color space conversion, and recovering the CFA image; the method specifically comprises the following steps:
s71, receiving data, and sequentially performing inverse coding, inverse quantization and IDCT generation
Figure BDA0002526325480000058
Dg″、Co″、Cg″；
S72, mixing
Figure BDA0002526325480000059
Dg ", Co", Cg "for the inverse color-space conversion:
Figure BDA00025263254800000510
s73, the reverse operation of S1 is performed to recover Bayer-CFA.
The present invention has higher restoration quality by calculating the restoration quality of the compressed image by restoring the CFA image. In the invention, the size of the component needing brightness coding is 1/4 of the size of CFA, so the bit number of the coding code stream is less; the traditional coding frame can be directly used for direct coding, and the robustness is high; finally, it should be noted that, although the Bayer format CFA is taken as an example in the present embodiment, the present invention is also effective for any RGGB type CFA.
While the invention has been described with reference to specific embodiments, any feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise; all of the disclosed features, or all of the method or process steps, may be combined in any combination, except mutually exclusive features and/or steps.

Claims (4)

1. The mosaic image compression method based on the novel color space conversion comprises the following steps:
step 1, G in RGGB type CFA image1、R、B、G2The components are respectively and independently spliced to form a channel, and a four-channel model is generated;
step 2, performing color space conversion on the four-channel model generated in the step 1, and converting the four-channel model into an YDgCoCg model to obtain a brightness component Y, chrominance components Dg, Co and Cg;
step 3, performing DCT and JPEG chroma quantization on the chroma components Dg, Co and Cg in the step 2 to generate chroma components Dg ', Co ' and Cg ';
step 4, inverse quantization is carried out on the chrominance component Cg 'in the step 3, IDCT is carried out to generate chrominance component Cg', mathematical optimization is carried out, and the luminance component is generated through calculation
Figure FDA0002526325470000011
Step 5, comparing the brightness component in the step 4
Figure FDA0002526325470000012
DCT and JPEG brightness quantization are performed to generate brightness component
Figure FDA0002526325470000013
Step 6, JPEG chroma coding is carried out on the chroma components Dg ', Co ' and Cg ' in the step 3, and the brightness component in the step 5 is subjected to
Figure FDA0002526325470000014
JPEG brightness coding is carried out, and the code stream is sent;
and 7, receiving the code stream, performing inverse coding, inverse quantization, IDCT and inverse color space conversion, and recovering the CFA image.
2. The method for compressing a mosaic image based on a novel color space conversion as claimed in claim 1, wherein said step 2 color space conversion comprises:
Figure FDA0002526325470000015
3. the method for compressing a mosaic image based on novel color space conversion according to claim 1, wherein in said step 4, mathematical optimization is:
Figure FDA0002526325470000016
4. the method for compressing a mosaic image based on novel color space conversion according to claim 1, wherein said step 7 specifically comprises:
s71, receiving data, and performing inverse coding, inverse quantization and IDCT to generate component
Figure FDA0002526325470000017
Dg″、Co″、Cg″；
S72, component pair
Figure FDA0002526325470000018
Dg ', Co ', Cg ' are inverse color space converted to obtain a component G1″、R″、B″′、G″2：
Figure FDA0002526325470000021
S73, component G1″、R″、B″′、G″2The reverse operation of step 1 is performed to recover the RGGB type CFA image.
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