CN111614962B - Perceptual image compression method based on region block level JND prediction - Google Patents
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Abstract
The invention relates to a perceptual image compression method based on region block level JND prediction, which comprises the following steps: 1) generating a JND value of a region block level by using a Otsu threshold method according to the images in the data set and the corresponding JND information; 2) establishing a CNN-based region block level JND prediction model according to the generated region block level JND value; 3) compressing the test image under a plurality of fixed QF values to obtain a plurality of corresponding distorted images, dividing all the distorted images into a plurality of non-overlapping area blocks, predicting a JND label of each area block, and finally acquiring a final JND value of each area block by adopting a label processing method; 4) and preprocessing the test image according to the target compressed QF value and the final JND value of each area block, selecting the largest area block sensing QF value as a compression parameter, and compressing the preprocessed test image by JPEG. Compared with the prior art, the method has the advantages of self-adaptive prediction, good compression quality, high compression efficiency and the like.
Description
Technical Field
The invention relates to the field of image compression, in particular to a perceptual image compression method based on region block level JND prediction.
Background
With the development of social networks and multimedia technologies, a great deal of picture information is generated on the internet. Based on recent statistics, Instagram users upload approximately 9000 ten thousand pictures per day. Therefore, how to store and transmit the images is a very challenging task, and existing image compression standards such as JPEG, h.264 and HEVC all use PSNR and MSE as standards for measuring distortion, however, PSNR considers each pixel point as important in the calculation process and is inconsistent with the human eye visual system, and therefore, it is very important to research an image compression algorithm oriented to the human eye visual system.
Various approaches have been proposed to address this challenge, including JND-based approaches, attention model-based approaches, and the like. Currently, image/video perceptual compression methods based on JND (Just Noticeable distortion) are the focus of research. The existing JND models are mainly divided into two types: pixel-based domain and dct (discrete Cosine transform) based domain. The pixel domain-based method mainly considers the brightness masking effect and the contrast masking effect in a human eye visual system; the JND model of the DCT domain is added with a spatial contrast function on the basis of a pixel domain model. Although the existing perceptual coding model can reduce perceptual redundant information in coding to a certain extent, only limited visual characteristics are considered and the perceptual redundant information is not changed along with the change of quantization parameters, and the latest perceptual experiment shows that the perception of a human visual system on image quality presents a staircase shape and is not continuously changed, and each mutation point can be regarded as a JND value. However, for a single image, a large amount of subjective experiments are required to obtain a final JND value, and the final JND value cannot be applied in reality.
Disclosure of Invention
The present invention is directed to overcoming the above-mentioned drawbacks of the prior art and providing a perceptual image compression method based on region block level JND prediction.
The purpose of the invention can be realized by the following technical scheme:
a perceptual image compression method based on region block level JND prediction comprises the following steps:
1) generating a JND value of a region block level by using a Otsu threshold method according to the images in the data set and the corresponding JND information;
2) establishing a CNN-based region block level JND prediction model according to the generated region block level JND value;
3) compressing the test image under a plurality of fixed QF values to obtain a plurality of corresponding distortion images, dividing all the distortion images into a plurality of non-overlapping area blocks, predicting a JND label of each area block, and finally acquiring a final JND value of each area block by adopting a label processing method;
4) and preprocessing the test image according to the target compressed QF value and the final JND value of each area block, selecting the largest area block sensing QF value as a compression parameter, and compressing the preprocessed test image by JPEG.
The step 1) specifically comprises the following steps:
11) for a smooth region, setting the region block level JND value in the smooth region to be consistent with the image level JND value, there are:
wherein S isITo test the picture level JND value of picture I,for the ith area block b in the test image IiThe compression parameters of (1);
12) for the region with complex texture, the SSIM value of each region block is obtained under the image level JND value;
13) taking the quality difference delta SSIM of each region block under the continuous JND value as the intensity of each region block, and adaptively judging a distortion region under the current image level JND value by using the region block as a basic unit by using a greater fluid threshold method;
14) and circularly executing the steps 12) -13) until all image levels JND of each image are completely executed, and generating a final region block level JND value.
In the step 13), the expression of the quality difference Δ SSIM of each region block under the continuous JND values is:
wherein the content of the first and second substances,when the compression parameter isThen, the ith area block b in the test image IiSSIM value of.
The step 2) specifically comprises the following steps:
21) sorting the generated JND values of the region block level from small to large, and marking training label values after classification to form a data set;
22) 90% of the area blocks in the data set were used for training and 10% for testing;
23) and training a region block level JND prediction model by adopting an AlexNet network.
In the step 23), in the training process of the region block level JND prediction model, the size of the image block is set to 64 × 64, the initial learning rate is set to 0.001, the maximum iteration number is set to 250000, and the size of the batch size is set to 64.
The step 3) specifically comprises the following steps:
31) compressing the test image under a plurality of fixed QF values to obtain a plurality of distorted images;
32) dividing all distorted images into a plurality of non-overlapping area blocks, and predicting a JND label of each area block by adopting an area block level JND prediction model;
33) when the prediction JND labels of the area blocks at the same position of a plurality of distorted images meet the judgment formulaStep 34) is performed, if not, step 35) is performed, wherein q is performedi、qjQF values, b area blocks and L (-) prediction JND labels respectively;
34) the QF value corresponding to the JND label is the JND value of the current area block;
35) sorting the JND label values from small to large to enable the JND label values to meet a judgment formula in 33), and acquiring a corresponding QF value as the JND value of the current area block.
The fixed QF values are 9 in total, 15, 20, 25, 30, 35, 40, 45, 50 and 55, and the non-overlapping area blocks are 64 × 64 in size.
The step 4) specifically comprises the following steps:
wherein the content of the first and second substances,for the k-th JND value,is the total number of predicted JND values;
42) the target compressed QF value is preset toThe final adopted perceived QF valueComprises the following steps:
wherein the content of the first and second substances,is the 1 st JND value and is,is as followsA JND value;
43) selecting the largest region block perception QF value as the compression parameter of the image levelThe expression is as follows:
wherein, NBIThe number of the area blocks in the test image I;
44) if the JND value of the area block is smaller than the compression parameter of the image level, preprocessing the image;
45) after all DCT coefficients are preprocessed, inverse DCT transform operation is carried out to generate a preprocessed test image, and the image-level compression parameters in the step 43) are adoptedAnd (5) compressing by adopting standard JPEG to obtain a compressed image.
In the step 43), if the predicted JND value of the partial region block is small, the following steps are performed:
in the step 44), the preprocessing the image specifically includes:
wherein the content of the first and second substances,is the DCT coefficient at the quantized position (m, n).
Compared with the prior art, the invention has the following advantages:
firstly, adaptive prediction: the method and the device do not need to carry out subjective experiments, and adaptively predict the JND information of the block level of the region according to the content of the input test image, and are used for perceptual coding.
Secondly, the compression quality is good: the method avoids the condition that subjective quality is reduced due to the fact that the JND value of individual region prediction is low, protects the quality of the region with simple texture, improves compression efficiency, and obtains subjective quality similar to JPEG.
Thirdly, the compression efficiency is high: the method can adaptively predict JND information according to the content of the region block of the test image, improves the compression efficiency of the image, selects 10 images in a Kodak data set as tests, respectively tests 3 QF values which are 75, 50 and 30 from high to low, respectively, and compared with a JPEG algorithm, under the condition of similar subjective perception quality, the code rate is respectively saved by 43.91 percent, 18.76 percent and 13.11 percent, and the compression efficiency exceeds that of other similar models.
Drawings
Fig. 1 is a flow chart of model-based training and perceptual coding in the present invention, where fig. 1a is a flow chart of model-based training and fig. 1b is a flow chart of perceptual coding.
FIG. 2 is a schematic diagram of a selected test image.
FIG. 3 is a flow chart of the method of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
As shown in fig. 1, the present invention provides a perceptual image compression method based on region block level JND prediction, comprising the steps of:
1) the method comprises the following steps of calculating a JND value of a region block level by using a Otsu threshold method according to images in a data set and corresponding JND information:
11) suppose picture I, its corresponding picture level JND value is:
12) for smooth region smooth, we consider its region block biIndicates that the JND value of (1) is consistent with the image level JND valueComprises the following steps:
13) for complex regions, the quality difference under consecutive JND values is calculated for each region block, expressed as:
wherein the content of the first and second substances,when the compression parameter isIn time, the ith area block biAn SSIM value of;
14) taking the quality difference in the step 13) as the strength of each region block, selecting an optimal threshold by utilizing an Otsu threshold algorithm, and adaptively dividing the image into two categories, namely a distortion region and a non-distortion region;
15) circularly executing the operation of the step 14), until all image levels JND of each image are executed, generating final region block level JND information, namely a region block level JND value;
2) establishing a CNN-based region block level JND prediction model according to the generated region block level JND information;
21) sorting the generated JND values of the region block level from small to large, dividing the JND values into 43 classes in total, and setting the training label value to be 0 to 42;
22) 90% of the area blocks in the data set were used for training and 10% for testing;
23) the method comprises the steps that a classical AlexNet network is utilized to train a region block level JND prediction model, in the training process of the region block level JND prediction model, the size of an image block is 64 x 64, the initial learning rate is set to be 0.001, the maximum iteration number is 250000, the size of a batch size is 64, and the test accuracy is 89.52%.
3) The test image was compressed at 9 fixed QF values (quality factors) to obtain 9 distorted images. Then dividing the 9 distorted images into 64 multiplied by 64 non-overlapping area blocks, predicting a JND label of each area block, and finally solving a final JND value of each area block by using a designed label processing method;
31) compressing the test image under 9 fixed QF values to obtain 9 distorted images, wherein the QF values are respectively 15, 20, 25, 30, 35, 40, 45, 50 and 55;
32) dividing 9 distorted images into 64 multiplied by 64 non-overlapping area blocks, and predicting a JND label of each area block;
33) if the prediction JND tags of the co-located area blocks of the 9 distorted images satisfy the following formula,
the QF value corresponding to the JND label is the JND value of the current area block;
34) if the predicted JND labels of the area blocks of the 9 distorted images at the same position do not meet the formula in the step 33), sorting the JND label values from small to large to enable the JND label values to meet the formula in the step 33), and then solving the corresponding QF value;
4) according to the target compressed QF value and the JND value of each area block, preprocessing the test image, compressing the preprocessed test image by JPEG, and selecting the maximum JND value of the area block as a compression parameter, wherein the method specifically comprises the following steps:
41) assuming that the test image is I, the predicted JND value of the ith area block is:
wherein the content of the first and second substances,represents the kth JND value and,represents the number of JND values;
42) assume a predetermined compressed QF value of qtarThe final adopted perceptual QF can then be expressed as:
43) selecting the maximum perception QF value of the region block as the compressed QF of the image level, and expressing as follows:
wherein, NBIFor testing the number of the area blocks in the image, if the predicted JND value of a part of the area blocks is smaller, the following formula is adopted for processing:
in an actual compression process, in order to guarantee the quality of the smooth region, a predicted JND value of the smooth region is set as a compressed QF value at an image level.
44) If the JND value of the region block is smaller than the compression parameter at the image level, the image is preprocessed using the following formula,
wherein the content of the first and second substances,represents the DCT coefficients at the (m, n) positions after quantization;
45) after all DCT coefficients are preprocessed, inverse DCT transform operation is carried out to generate a preprocessed test image, and then standard JPEG is used for compression, wherein the compression parameter is the image level QF value in 43).
To verify the performance of the method of the present application, the following experiment was designed.
Randomly selecting 10 test images from the Kodak data set, predicting the JND value of the area block in 10 test images according to the method of the present invention as shown in fig. 2, then compressing 10 test images at given QF of 75, 50 and 30 respectively, covering high, medium and low quality, JPEG being the comparison algorithm of the present invention, the comparison results with JPEG being shown in table 1, table 2 and table 3,
DMOS=MOSΩ-MOSori
wherein Ω represents different compression methods, BPP represents the number of bits consumed by each pixel, MOS represents the subjective score, and ori represents the original JPEG algorithm.
TABLE 1 Performance of the invention at a QF of 75
TABLE 2 Performance of the invention at a QF of 50
TABLE 3 Performance of the invention at a QF of 30
Claims (5)
1. A perceptual image compression method based on region block level JND prediction is characterized by comprising the following steps:
1) generating a JND value of a region block level by using an Otsu threshold method according to the images in the data set and the corresponding JND information, and specifically comprising the following steps of:
11) for a smooth region, setting the region block level JND value in the smooth region to be consistent with the image level JND value, there are:
wherein S isITo test the picture level JND values of picture I,for the ith area block b in the test image IiThe compression parameters of (2);
12) for the region with complex texture, the SSIM value of each region block is obtained under the image level JND value;
13) taking the mass difference delta SSIM of each area block under the continuous JND value as the intensity of each area block, and adaptively judging a distortion area under the current image level JND value by using the Otsu threshold method and taking the area block as a basic unit;
14) circularly executing the step 12) -the step 13) until all image levels JND of each image are executed, and generating a final area block level JND value;
2) establishing a CNN-based region block level JND prediction model according to the generated region block level JND value;
3) compressing the test image under a plurality of fixed QF values to obtain a plurality of corresponding distorted images, dividing all the distorted images into a plurality of non-overlapping area blocks, predicting a JND label of each area block, and finally acquiring a final JND value of each area block by adopting a label processing method;
the step 3) specifically comprises the following steps:
31) compressing the test image under a plurality of fixed QF values to obtain a plurality of distorted images;
32) dividing all distorted images into a plurality of non-overlapping area blocks, and predicting a JND label of each area block by adopting an area block level JND prediction model;
33) when the prediction JND label of the area block at the same position of the multiple distorted images meets the judgment formulaStep 34) is performed, if not, step 35) is performed, wherein q is performedi、qjQF values, b area blocks and L (-) prediction JND labels respectively;
34) the QF value corresponding to the JND label is the JND value of the current area block;
35) sorting the JND label values from small to large to enable the JND label values to meet a judgment formula in 33), and then acquiring a corresponding QF value as a JND value of the current area block;
4) according to the target compressed QF value and the final JND value of each area block, preprocessing the test image, selecting the largest of the perceived QF values of the area blocks as a compression parameter, and compressing the preprocessed test image by JPEG (joint photographic experts group), wherein the method specifically comprises the following steps:
wherein the content of the first and second substances,for the k-th JND value,is the total number of predicted JND values;
42) presetting a target compressed QF value ofThe perceived QF value ultimately adoptedComprises the following steps:
wherein the content of the first and second substances,is the 1 st JND value and is,is as followsA JND value;
43) selecting the largest region block perception QF value as the compression parameter of the image levelThe expression is as follows:
wherein, NBIThe number of the area blocks in the test image I;
if the predicted JND value of the partial area block is smaller, the following method is adopted:
44) if the JND value of the area block is smaller than the compression parameter of the image level, preprocessing the image;
45) after all DCT coefficients are preprocessed, inverse DCT transform operation is carried out to generate a preprocessed test image, and the image-level compression parameters in the step 43) are adoptedThe method comprises the following steps of compressing by adopting standard JPEG to obtain a compressed image, and specifically preprocessing the image:
2. The method as claimed in claim 1, wherein in step 13), the expression of the quality difference Δ SSIM of each region block at consecutive JND values is:
3. The method as claimed in claim 1, wherein the step 2) comprises the following steps:
21) sorting the generated JND values of the region block level from small to large, and marking training label values after classification to form a data set;
22) 90% of the area blocks in the data set were used for training and 10% for testing;
23) and training a region block level JND prediction model by adopting an AlexNet network.
4. The method as claimed in claim 3, wherein in step 23), in the training process of the JND prediction model, the image block size is set to 64 × 64, the initial learning rate is set to 0.001, the maximum iteration number is set to 250000, and the batch size is set to 64.
5. The method as claimed in claim 1, wherein the fixed QF values are 9 in total, 15, 20, 25, 30, 35, 40, 45, 50 and 55, and the non-overlapping region blocks are 64 × 64 in size.
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