CN100534206C - Image compression method - Google Patents

Abstract

The method includes following steps: (1) picking up RGB values of two adjacent pixels, and converting them to YUV values; (2) calculating U difference and V difference of two pixels; (3) if U difference exceeds first threshold value and/or V difference exceeds second threshold value, then executing step (4); if U difference is smaller than first threshold value and/or V difference is smaller than second threshold value, then executing step (5); (4) under RGB colority space, compressing the two pixels, then executing (6); (5) under YUV colority space, 2 times sampling chromatic component is carried out for the two pixels, and then executing (6); (6) output compressed code flow, then picking up RGB values of next two adjacent pixels, and executing (1). The invention can carry out image compression in YUV colority space and RGB colority space under selfadaption. Color of line in single pixel will not be lost in the process method.

Description

A kind of method for compressing image

Technical field

The present invention relates to a kind of method for compressing image, especially a kind of adaptive method for compressing image that carries out image compression at YUV color space or rgb color space.

Background technology

Along with the development of digital technology and information technology, multimedia application more and more widely.Not through the multi-medium data of overcompression, view data for example, video data and voice data all need jumbo memory space and transmission bandwidth.In order to save cost, utilize redundancy between data initial data is compressed and then to be stored and transmit, become the important content in information processing research field and chip manufacturing field.

Because human eye is more responsive more than colourity information UV for monochrome information Y, existing method for compressing image is that the RGB chrominance space is transformed into the YUV chrominance space, on brightness and chrominance information, carry out the compression of different compression ratios, can under the situation of not losing subjective visual quality do, utilize the characteristic of human eye to obtain certain image compression rate.Be exactly to carry out 2 times down-sampling (Downsampling) at the YUV chrominance space typically at the compression method of YUV chrominance space.

Because human eye is to brightness chrominance sensitivity, therefore the Downsapling method at yuv space is exactly to suppose that the colourity of neighbor is very approaching.The carrier chrominance signal that only keeps a pixel so in adjacent two pixels is given up the chrominance information of one other pixel.Two pixels are all used the same chrominance information during decoding.Because the color and the adjacent pixels color of single pixel lines are different, when color be not very near the time can produce the unacceptable line color of human eye and lose phenomenon.

This compression method at yuv space has obtained extensive use in the TV field.Yet on PC and monitor, the lines or the artificial various images of drawing of single pixel occur, as the versicolor single pixel lines that utilize mapping software such as CAD to draw through regular meeting.If adopting the method for the down-sampling under the YUV chrominance space this moment compresses, when single pixel lines are not in sampled point, the UV value of lines can be dropped, be used as its UV value during decompression with the UV value of adjacent sampled point, like this because the color of the color of lines itself and neighbor is different, can make the color of single pixel lines lose, this can not be accepted by the user in the application of PC and monitor.So non-natural figure of this compression and the defective compression method of picture are applied on PC and the monitor and have significant limitation.

Summary of the invention

The objective of the invention is to overcome the defective of conventional images compression method, a kind of method for compressing image is provided, adaptively carry out image compression at YUV color space and rgb color space, the color that single pixel lines can not occur is lost.

For achieving the above object, the invention provides a kind of method for compressing image, comprise the steps:

The rgb value of step 1, two neighbors of extraction, and be converted to the YUV value;

Step 2, the difference of U value of calculating two pixels and V value poor;

If the difference of step 3 U value surpasses first threshold, and/or the difference of V value surpasses then execution in step 4 of second threshold value; If less than second threshold value, then execution in step 5 less than the difference of first threshold and V value for the difference of U value;

Step 4, two pixels are carried out compression under the RGB chrominance space, execution in step 6;

Step 5, two pixels are carried out 2 times of down-samplings of chromatic component under the YUV chrominance space, execution in step 6;

Step 6, output compressed bit stream extract the rgb value of following two neighbors, execution in step 1 then.

When in the above-mentioned steps 4 two pixels being carried out the compression under the RGB chrominance space, give up the low-order bit of RGB component, and insert the identifier of rgb space.When in above-mentioned steps 5, two pixels being carried out 2 times of down-samplings of chromatic component under the YUV chrominance space, insert the fuzzy diagnosis symbol of yuv space.

Therefore, method for compressing image of the present invention, can adaptively carry out image compression at YUV color space and rgb color space, the color that single pixel lines can occur is not lost, make the down-sampling compression method under the YUV chrominance space can apply in the high-resolution display, go for display and the various monitor of PC.

Below by drawings and Examples, technical scheme of the present invention is described in further detail.

Description of drawings

Fig. 1 is the flow chart of method for compressing image of the present invention.

Embodiment

In image processing and Video processing, need carry out the storage of frame.For conserve storage and satisfy certain data bus bandwidth requirement, can take method for compressing image to carry out image compression.Compression method must possess the characteristics that operand is low, subjective visual quality do is lost in the resource consumption less and not.The down-sampling that carries out chromatic component at the YUV chrominance space is extensively applied in various images and the video compression.Yet because PC display and monitor often show the lines or the artificial various images of drawing of single pixel, the unacceptable single pixel of human eye can appear in the compression method of this yuv space and line color is lost, or the image border color phenomenon of losing.For under the prerequisite of not losing subjective visual quality do, the lines or the various figure of single pixel compressed, the present invention can the adaptive YUV of being chosen in or rgb color space carry out image compression, remedy the defective of YUV colourity down-sampling.

Referring to shown in Figure 1, be the flow chart of method for compressing image of the present invention, concrete steps are as follows:

The rgb value of step 101, two neighbors of extraction, and be converted to the YUV value;

Promptly the value of the RGB of two pixels is carried out the conversion of rgb space to yuv space, the fundamental formular of rgb space and yuv space conversion is:

$\left\{\begin{array}{c}Y=0.299R+0.587G+0.114B\\ U=-0.172R-0.339G+0.511B+128\\ V=0.511R-0.428G-0.083B+128\end{array}\right.;$

In order to cancel the operation of floating number, round after each coefficient in the above-mentioned formula can being multiply by 2 n power, last result carries out moving to right for n time again, in order to guarantee the counting accuracy of RGB when the YUV color space is changed, result of calculation is taked the method that rounds up, and the big more precision of the value of n is high more, the conversion formula when present embodiment is n=8:

$\left\{\begin{array}{c}Y=((77R+150G+29B)+128)>>8\\ U=(((-44R-87G+131B)+128)>>8)+128\\ V=(((131R-110G-21B)+128)>>8)+128\end{array}\right.\};$

Step 102, the difference of U value of calculating two pixels and V value poor;

If the original YUV value of two pixels is respectively Y1, U1, V1 and Y2, U2, V2 is the inputs of 8 bits, asks for the difference of the U component and the V component of two pixels so by following formula:

ΔU＝|U1-U2|

ΔV＝|V1-V2|；

If the difference of step 103 U value surpasses first threshold, and/or the difference of V value surpasses then execution in step 104 of second threshold value; If less than second threshold value, then execution in step 105 less than the difference of first threshold and V value for the difference of U value;

This step compares the difference of the U value of current two pixels will compressing and the difference of V value, if the difference of two chromatic components thinks so that all less than separately threshold value two color of pixel are more approaching, can adopt the method for this colourity down-sampling; If the difference of arbitrary chromatic component is greater than separately threshold value, just think that these two color of pixel keep off, if at this moment still use the method for colourity down-sampling, can cause image impairment excessive, therefore automatically switch to the compression of rgb space, do not have very big information loss to guarantee two pixels;

The first threshold that makes U is Uth, and second threshold value of V is Vth,

When Δ U＜Uth and Δ V＜Vth execution in step 105 then;

As Δ U 〉=Uth or Δ V 〉=Vth execution in step 104 then;

Step 104, two pixels are carried out compression under the RGB chrominance space, execution in step 106;

Because if the difference that a chromatic component arranged is then compressed at rgb space greater than separately threshold value, directly intercept the low-order bit of RGB component, and insert the identifier of rgb space;

Compression under the RGB chrominance space, abandon the compression ratio that the low bit of RGB component obtains to require, be the input of 24 bits for each pixel just, obtain the output of average 16 bits of each pixel, because human eye is in three color components of RGB, susceptibility to the G component is the highest, adopt following intercept method: the original rgb value of establishing two pixels is R1, G1, B1 and R2, G2, B2 is the inputs of 8 bits, is output as high 5 bit R1 after the rgb space compression so, high 6 bit G1, high 5 bit B1, high 5 bit R2, high 5 bit G2, high 5 bit B2 and 1 bit space identifier " 0 ", the average number of bits of such two pixels is 16 bits; What judge that when compression adopt by space identifier " 0 " during decoding is the rgb space compression method;

Step 105, two pixels are carried out 2 times of down-samplings of chromatic component under the YUV chrominance space, execution in step 106;

Because the difference of U component and V component is then carried out 2 times of down-samplings of YUV chromatic component all less than separately threshold value, insert the pattern recognition symbol of yuv space;

2 times of down-samplings of YUV chrominance space can be selected the UV value of the UV value of any pixel in two pixels as one other pixel, also can select the UV value of the UV value of that smaller pixel of brightness value in two pixels, and the lowest bit position of using the V value is as the compression stroke identifier as one other pixel;

If the original YUV value of two pixels is respectively Y1, U1, V1 and Y2, U2, V2 is the inputs of 8 bits, total bit number of two pixels is 48 bits so, average each pixel is 24 bits, becomes behind 2 times of down-samplings of yuv space: 8 bit Y1,8 bit U1, high 7 bit V1,8 bit Y2 and 1 bit space identifier " 1 ", total bit number of two pixels is 32 bits, on average each pixel is 16 bits; What adopt when judging compression by space identifier " 1 " during decoding is 2 times of Downsapling methods of YUV colourity;

Step 106, output compressed bit stream extract the rgb value of following two neighbors then, execution in step 1, and the compressed bit stream of output be average each pixel 16 bit, the compression ratio of obtaining with the method for traditional 2 times of down-samplings of YUV carrier chrominance signal is the same.

Below the pixel data in the above-mentioned steps is judged and compression is given an example and specified:

Example 1: as shown in table 1, be the original RGB data of pixel 0 and pixel 1,

Table 1, the original RGB data of pixel 0 and pixel 1

Data after two pixel RGB change to the YUV color space are as shown in table 2,

Table 2, the data after two pixel RGB change to the YUV color space

Therefore: Δ U=|162-91|=71; Δ V=|171-81|=90;

If Uth=10, Vth=10.This moment Δ U Uth, Δ V〉Vth, therefore adopt the compression of rgb space, for pixel 0, give up R0 low 3, low 2 of G0, low 3 of B0; For pixel 1, that gives up R1 hangs down 3, and G1 hangs down 3, and B1 hangs down 3; The last compression stroke identifier " 0 " that adds a bit again.

Final output bit is: 01100 (R0), 000000 (G0), 01100 (B0), 00000 (R1), 01101 (G1), 00000 (B1) 0.

Example 2:

As shown in table 3, be the original RGB data of pixel 0 and pixel 1,

Table 3, the original RGB data of pixel 0 and pixel 1

Data after two pixel RGB change to the YUV color space are as shown in table 4,

Table 4, the data after two pixel RGB change to the YUV color space

ΔU＝|112-116|＝4；

ΔV＝|162-157|＝5；

If Uth=10, Vth=10, this moment Δ U＜Uth, therefore Δ V＜Vth adopts 2 times of Downsapling methods of YUV colourity, abandons the value of the U1 and the V1 of pixel 1, abandons the lowest bit of the V0 of pixel 0, adds the compression stroke identifier " 1 " of 1 bit at last again.

Final output bit is: 01001110 (Y0), 01110000 (U0), 1010001 (V0), 01010001 (Y1) 1.

For most natural picture, when the threshold value of UV is provided with suitablely, overwhelming majority pixel all is to adopt the YUV Downsapling method, and this is that this also is the reason place that YUV chrominance space Downsapling method is able to extensive use because of the very strong correlation of color between the neighbor of natural picture.And for some special pictures, such as the figure of single pixel lines or drafting, can automatically switch to rgb color space at lines and pattern edge compresses, and can not cause the subjective vision loss of picture like this.

It should be noted last that, above embodiment is only unrestricted in order to technical scheme of the present invention to be described, although the present invention is had been described in detail with reference to preferred embodiment, those of ordinary skill in the art is to be understood that, can make amendment or be equal to replacement technical scheme of the present invention, and not break away from the spirit and scope of technical solution of the present invention.

Claims (3)

1, a kind of method for compressing image, comprising following steps:

The rgb value of step 1, two neighbors of extraction, and be converted to the YUV value;

Step 2, the difference of U value of calculating two pixels and V value poor;

If the difference of step 3 U value surpasses first threshold, and/or the difference of V value surpasses then execution in step 4 of second threshold value; If less than second threshold value, then execution in step 5 less than the difference of first threshold and V value for the difference of U value;

Step 4, two pixels are carried out compression under the RGB chrominance space, execution in step 6;

Step 5, two pixels are carried out 2 times of down-samplings of chromatic component under the YUV chrominance space, execution in step 6;

Step 6, output compressed bit stream extract the rgb value of following two neighbors, execution in step 1 then.

2, method for compressing image according to claim 1 when in the wherein said step 4 two pixels being carried out the compression under the RGB chrominance space, is given up the low-order bit of RGB component, and inserts the identifier of rgb space.

3, method for compressing image according to claim 1 and 2 when two pixels being carried out 2 times of down-samplings of chromatic component under the YUV chrominance space in the wherein said step 5, inserts the fuzzy diagnosis symbol of yuv space.

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