Method to process raster images

FIELD: information technologies.

SUBSTANCE: in the method of processing of raster images, including compression of an image by the method of "cut block coding" or its modifications, before the procedure of compressing coding they perform digital filtration, which increases sharpness of the compressed image, and after the decoding procedure they perform smoothing digital filtration of the decoded image.

EFFECT: improved quality of decoded raster images when methods used for their compressing coding on the basis of cut block coding or their modifications, improved current formalised criteria.

3 cl, 8 dwg

 

The invention relates to the field of processing of raster images, and in particular to methods compressive coding and decoding for communications systems and storage of raster images.

The invention can be used in transmission systems and data storage, computing, instrumentation, telemetry systems.

A method of processing a raster image, in which the encoding of the image in order compression is performed on the basis JPG (Cm. application for U.S. patent US 2007/0036451 A1).

The disadvantage of this method is that it does not provide a guaranteed, constant and known in advance, the compression ratio for the whole compressed image as a whole, and for each fragment of a given size. Also a disadvantage is that such data do not have a high resistance to damage and distortion during transmission. The distortion of the compressed data corresponding to a specific area of the image leads to distortions in other areas. As with compression of images using the JPEG method, and when you restore, the required number of elementary operations per pixel is large enough. While it is impossible to effectively parallelize the compression process, as all the blocks are not independent.

There is also known a method of processing a raster image, in which codiovan the image with the aim of compression is performed on the basis of a truncated block coding (BTC - "block truncation coding" in the English-language literature). (See application for U.S. patent US 2010/0067812 A1). It assumes a partitioning of the image into blocks of a given size. While the blocks can be of arbitrary size and shape, not necessarily square, and not necessarily they must have the same size for the entire image.

This method is used as a prototype of the proposed invention.

The disadvantage of this method is that the quality of the reconstructed after compression of the image commonly used for formalized criteria such as peak signal-to-noise ratio (PASS or PSNR in English literature), standard deviation (MSE or MSE in English literature), structural similarity (SSim English literature) and others, when using blocks larger than 4×4 pixels, is much worse than when compressing the same image using the JPEG method with the same compression ratio. Reducing the block size improves the quality of the reconstructed images, but at the same time dramatically lowers the compression ratio.

In the proposed method is the technical problem of improving the quality of the decoded bitmap images when used for the compressive encoding method based on a truncated block coding or modifications.

This technical problem is solved by the CSOs, a method for processing raster images, including image compression method "truncated block coding or modifications, before the procedure compressive coding perform digital filtering, which sharpens the compressed image, and after the decoding procedure perform smoothing digital filtering of the decoded image. Filtering can be applied to all pixels of the compressed image, or selectively, for example, increase the sharpness filtering for low-contrast blocks, a smoothing filter for blocks with high contrast. Filtering can also be adaptive, directly or indirectly takes into account the distribution of pixels for brightness in the vicinity of the filtered pixel.

This method is illustrated with the help of the attached drawings for example, selection of the parameters agreed filters with window size 3×3, taking into account the fact that increasing the sharpness (phase prefilter), and smoothing filters (stage postfilter) must be symmetric, i.e. the multipliers for the pixels located at a specified distance from the current (filtered)should be the same regardless of the direction. 1 shows a block 3×3 with the prefilter. Figure 2 shows a block 3×3 when postfilter. Figure 3 in the form of a three-dimensional graph shown resultativity model program for the selection of the filter parameters for the conventional etalonnage image LENA with kodirovanie method truncated block coding with block size 3×3 and the number of quantization levels is equal to two.

Figure 4 shows the original block of 3×3 in another example, the use of agreed filters applied to the pixel block of 3×3 when the procedure of the prefilter and postfilter also use the pixels of the environment beyond the boundaries of the block. Figure 5 shows a decoded block after application of the method of the truncated block coding without filtering. Figure 6 shows the original block after the prefilter. Figure 7 shows the decoded block after the method of a truncated block coding with prefiltration. On Fig shows the decoded block after postfilter.

In the classical implementation of the method of the truncated block coding for each block of the image are computed average brightnessuand standard deviation of brightness σ, then formed the bit matrix, in which each pixel block, the brightness of which exceeds threshold ofu, is mapped to one bit (for definiteness - 0, if the brightness of the pixel is less than theu, 1 otherwise). For each block, the encoder stores u, and σ is constructed by the method described above the bit matrix actually contains quantized to two levels of brightness values of pixels of the block. When restoring the pixels in the bit matrix correspond to the values 1 and 0, are replaced with

u+σm-qqandu-σqm-q

(here m is the number of pixels in the block, and q is the number of pixels whose brightness exceeds the threshold value, i.e., those that bit in the matrix corresponds to 1). This approach allows for each block of the reconstructed image to maintain constant average brightness and variance.

Recovered after compression method of a truncated block coding of images contain a number of visual artifacts associated with a significant (up to 2, 4 or 8 depending on the number of quantization levels) reduction in the number of gradation of brightness of the image within each of the coded block and as a consequence with shlian who eat vnutriporovogo relief.

The compression ratio when using the truncated block encoding is defined only by the size of the block that does not depend on features of the image, and at 8 quantization levels is usually from 2 (block of 4×4 pixels) to 2.5 (block 8×8).

In the proposed method of processing a raster image before compression encoding perform digital filtering, which sharpens the compressed image, and after the decoding procedure perform smoothing digital filtering of the decoded image.

Examples of the method.

Example 1.

(Pair agreed filters applied to the entire image, to each pixel, regardless of its brightness and the brightness of the pixels of the environment).

Consider the example of selection of the parameters agreed filters with window size 3×3, taking into account the fact that increasing the sharpness and smoothing filters are made symmetric, i.e. the multipliers for the pixels located at a specified distance from the current (filtered)should be the same regardless of the direction.

In the first example: m0 (w0) is a multiplier applied to the current filtered pixel, m1and m2(w1and w2) - multipliers that are applied to pixels distant from the current one position in vertical, horizontal and diagonal directions. the since the average brightness of the image using filters should not be changed, to increase the sharpness and smoothing filter conditions are respectively

m0-4*(m1+m2)=1

and

w0+4*(w1+w2)=1.

Thus, each filter is fully described only by a couple of factors, and experimental evaluation of the filter parameters is reduced to the selection of pairs (m1, m2) and (w1, w2), providing a minimum standard deviation (MSE), defined as

MSE=1N×Mj=1Ni=1M(Ii,j-Ii,j)2

where Ii,jandIi,jaccordingly, the pixels of the original and restored images with coordinates i and j, and N and M - the length of the pixel rows and the number of rows of the image.

When using the simulator to search couples agreed filters DL which each increase the sharpness of the filter, used for pre-filtering, select appropriate (i.e. providing a minimum for a given increase sharpness filter is the standard deviation) filter.

Figure 3 in the form of a three-dimensional graph shows the results of model programs for the selection of the filter parameters for LENA image with kodirovanie method truncated block coding with block size 3×3 and the number of quantization levels equal to two. Horizontal plane corresponds to the different values of the pairs (m1, m2), vertically corresponding to each pair of the smallest value of the standard deviation. The graph is a concave surface with a minimum at the point with coordinates m1=17/64, m2=3/64.

The use of increasing the sharpness of the filter with the specified parameters with correlated smoothing filter with w1=26/256, w2=11/256 allows for the compression method of a truncated block coding with block size 3×3 and the number of quantization levels 2 to get for the test image LENA is the standard deviation MSE=8,464. Without filtering, the standard deviation for the same image encoded using a block of the same size and the same number of quantization levels is 18,196.

p> Thus, the use of matched filtering ensured the win for MSE in 2.15 times that corresponds to the increase in peak ratio of signal to noise on 3.324 dB without increasing the amount of transmitted data decoder.

Example 2.

In the second example, the use of agreed filters applied to the pixel block of 3×3 and the procedure of the prefilter and postfilter also use the pixels of the environment beyond the boundaries of the block.

When using the truncated block coding without filtering the standard deviation for a given decoded block (not the entire image, as in example 1) is 65,78, when using filtration rate 18.89.

The claimed method of processing a raster image can be carried out in industry with the use of developed technologies, materials and computational processes and can be used in transmission systems and data storage.

The proposed method of processing a raster image has been pilot-tested in LLC VOCORD Element". Experimental verification of this method was shown to improve the quality of the decoded bitmap images when used for the compressive encoding method based on a truncated block coding or modifications.

The experiments conducted on the test series depicts the th, using the increasing sharpness and smoothing filter with a window of constant size 3×3 pixel has demonstrated that this approach allows to reduce the deviation in 2-4 times. The parameters of this filter type is determined by the size of the block and the number of quantization levels and not depend on the nature of the compressed image. This allows you to use the same pair of filters for compression of different images, which is confirmed by experiments. The application of the described approach is possible using any modification of the method of the truncated block encoding.

It was found that the features of the implementation of filters used for pre-filtering the encoded images and postfilter restored images, not of crucial importance. Essential for a filter to be applied to the pixels of the original image, is its ability to emphasize the relief elements of the block image, i.e. to increase the difference between the minimum and maximum brightness values of the pixels of the encoding block, and the filter used on the stage postfilter, the ability to mitigate these elements, i.e. to reduce the difference between the minimum and maximum brightness values of pixels of the decoded block. Filters can romanatica not all pixels of the image, and selectively, to meet the established criteria, for example, in low-contrast blocks. Filters can also be adaptive, directly or indirectly takes into account the distribution of pixels for brightness in the vicinity of the filtered pixel.

The proposed method of processing bitmaps recommended for use in transmission systems and data storage, computing, instrumentation, telemetry systems.

1. The method of processing raster images, including image compression method "truncated block coding" or its modification, characterized in that before the procedure compressive coding perform digital filtering, which sharpens the compressed image, and after the decoding procedure perform smoothing digital filtering of the decoded image.

2. The method of processing a raster image according to claim 1, wherein increasing the sharpness filtering and smoothing filter can not be applied to all pixels of the image.

3. The method of processing a raster image according to claim 1, wherein increasing the sharpness filter and a smoothing filter may be adaptive, directly or indirectly takes into account the distribution of pixels for brightness in the vicinity of the filtered pixel.



 

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