CN1610377A - Method for color space transformation for compressing 16-bit color images - Google Patents

Abstract

The invention discloses a color space conversion method for compressing 16-bit color images. On the basis of traditional image or video compression technologies, such as JPEG series, MPEG series, and H.26X series, the conversion of 24-bit RGB format to 16-bit RGB format is added at the encoding end, and the color space conversion method can be used to remove the input redundant information and simplify the operation steps of the decoding end, thereby reducing the decoding complexity and improving the video compression quality.

Description

Method for color space transformation for compressing 16-bit color images

technical field

The invention relates to the field of multimedia technologies related to portable mobile devices, in particular to a color space conversion method for compressing 16-bit color images.

Background technique

With the popularization of portable mobile devices, such as personal digital assistants (PDAs), mobile phones, etc., a large number of multimedia applications based on portable mobile devices have emerged. Portable mobile devices have problems such as weak computing power, short battery life, and 16-bit color LCD panels; and in multimedia, the computational complexity of video decoding is relatively high, so traditional video encoding methods for portable mobile devices cannot meet the requirements. Users' requirements for multimedia, especially video technology-related applications. According to the inherent characteristics of portable devices, the transformation of general video codec technology can improve the processing speed of video decoding and the performance of video compression algorithm.

The so-called color space transformation for compressing images refers to performing linear transformation on the input digital image and mapping it to the color space defined by the corresponding compression algorithm. This color space is suitable for the compression of digital images and videos. The digital images here include the usual digital images and frames in digital video sequences.

Digital image and video coding technologies generally use YCrCb as the color space for compression. When the input is an RGB image, the color space divides the RGB color information into a luminance component Y and two chrominance components CrCb. Since the human eye is much more sensitive to luminance than chroma, the luminance is sampled 1:1 during the compression process, while the chroma is sampled less than or equal to 1:1, so as to achieve a certain degree of compression. Currently, traditional compression techniques transform 24-bit RGB images into YCrCb format.

As mentioned above, the liquid crystal display panel of the portable mobile device does not support images in 24-bit RGB format, but supports images in 16-bit RGB format, including two specific formats of RGB565 and RGB555. Using traditional compression technology, the display terminal must first convert the decoded image from YCrCb to 24-bit RGB format, and then into a displayable 16-bit RGB format. Such a method will inevitably increase the amount of calculation, and the RGB24 format contains more color information than the RGB16 format, and this information is redundant on display devices that only support the RGB16 format.

If a color space mapping from RGB16 to YCrCb can be defined according to the characteristics of the RGB16 color space, the compression efficiency can be improved and the decoding complexity can be reduced.

Contents of the invention

The object of the present invention is to provide a color space conversion method for compressing 16-bit color images.

The technical scheme that the present invention solves its technical problem adopts is as follows:

1) When encoding, the color image in 16-bit RGB format is transformed into a new color space according to formula (1), named rYCrCb, the nature of this color space is the same as that of YcrCb, RGB color information is divided into a brightness component Y _r and two The chrominance components Cr _r and Cb _r can be used for compression; the subsequent compression process, including sampling, DCT transformation, quantization, run-length coding, and variable length coding, can be based on existing image or video compression standards on the obtained transformation results , including JPEG series, MPEG series, and H.26X series, use the encoding algorithm steps to proceed;

Y _r ＝R+G+B

Cb _r =4B-Y _r =-R-G+3B (1)

Cr _r =4R-Y _r =3R-GB

Among them: R, G, B respectively represent the value of R, G, B three components of the pixel in the 16-bit RGB image, Y _r represents the brightness component in the rYCrCb space, Cb _r and Cr _r represent the two values in the rYCrCb space Chromaticity component; The following formula (2) is the matrix form expression of formula (1);

$\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\\ {\mathrm{<span\; class="notranslate">\; Cb</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\\ {\mathrm{<span\; class="notranslate">\; Cr</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}\end{array}\right]<span\; class="notranslate">\; =</span>\left[\begin{array}{ccc}\mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; 1</span>}\\ <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}& <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; 3</span>}\\ \mathrm{<span\; class="notranslate">\; 3</span>}& <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}& <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}\end{array}\right]<span\; class="notranslate">\; \&Center\; Dot;</span>\left[\begin{array}{c}\mathrm{<span\; class="notranslate">\; R</span>}\\ \mathrm{<span\; class="notranslate">\; G</span>}\\ \mathrm{<span\; class="notranslate">\; B</span>}\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Where: the symbols are the same as those of formula (1);

2) During decoding, according to existing image or video compression standards, including JPEG series, MPEG series, and H.26X series, the data on the rYCrCb space is obtained by using its encoding algorithm steps, and 16 is obtained according to the inverse transformation method of formula (3). Bit RGB format data, directly output to portable devices for display;

G＝(2Y _r -Cb _r -Cr _r )//4

R＝(Y _r +Cr _r )//4 (3)

B＝(Y _r +Cb _r )//4

Among them: the letter symbols are the same as formula (1), and "//" means rounding.

Compared with the background technology, the present invention has the beneficial effects that:

The invention is a brand-new method for color space transformation. It is suitable for compression of 16-bit RGB color images or videos. On the basis of traditional image or video compression technologies, such as JPEG series, MPEG series, and H.26X series, the conversion of 24-bit RGB format to 16-bit RGB format is added at the encoding end, and the color space conversion method can be used to remove the input redundant information and simplify the operation steps of the decoding end, thereby reducing the decoding complexity and improving the video compression quality.

Detailed ways

When implementing digital image or video compression, all algorithms must be performed in a specific color space. For example, in the currently widely used compression standards, JPEG series, MPEG series, and H.26X series, when the format of the input digital image or video is RGB24, use the recommended transformation formula to transform to YCrCb space, and then perform sampling, Compression steps such as DCT transformation, quantization, run-length coding, and variable-length coding; if the format is RGB16, the RGB16 format is converted to the RGB24 format by complementing bits, and then the above operation steps are performed. The decoding process is reversed, and finally an image or video in RGB24 format can be obtained; if RGB16 format needs to be displayed, the RGB24 format is converted to RGB16 format by truncation. In this process, the compressed space is YCrCb space.

On the portable mobile device, if the image or video in RGB16 format is compressed, the compression space rYCrCb space designed by the present invention is adopted, and only the color space conversion module in the above process needs to be replaced. Specific steps are as follows:

When encoding, when the format of the input digital image or video is RGB16, use the formula (1) to transform the data into the rYCrCb space; if the format is RGB24, convert the RGB24 format to the RGB16 format by truncation, and then use the formula ( 1). In the compressed space rYCrCb, the compression steps such as traditional sampling, DCT transformation, quantization, run length coding and variable length coding can be used for compression. This step can eliminate redundant information and improve compression efficiency.

When decoding, the traditional decompression step is performed first to obtain the data in the rYCrCb space. At this time, the image or video in RGB16 format can be obtained by using formula (3), and can be directly output to a display device supporting RGB16 format. Therefore, the decoding complexity of this method is low, and the efficiency is higher.

Claims (1)

1. method that is used to compress the color notation conversion space of 16 coloured images is characterized in that:

1) when coding transforms to new color space with the coloured image of 16 rgb formats according to formula (1), called after rYCrCb, and the character of this color space is the same with YcrCb, and the RGB colouring information is divided into a luminance component Y _rWith two chromatic component Cr _r, Cb _r, can be used for compression; Follow-up compression process comprises sampling, dct transform, quantification, Run-Length Coding, variable-length encoding, can be on the gained transformation results, according to existing image or video compression standard, comprise JPEG series, MPEG series, H.26X serial, adopt its encryption algorithm step, get final product;

Y _r＝R+G+B

Cb _r＝4B-Y _r＝-R-G+3B (1)

Cr _r＝4R-Y _r＝3R-G-B

Wherein: R, G, B represent the R of picture element in 16 RGB images respectively, G, the numerical value of three components of B, Y _rRepresent luminance component in the rYCrCb space, Cb _r, Cr _rRepresent two chromatic components in the rYCrCb space respectively; Following formula (2) is that the matrix form of formula (1) is expressed;

$\left[\begin{array}{c}{Y}_r\\ {\mathrm{Cb}}_r\\ {\mathrm{Cr}}_r\end{array}\right]=\left[\begin{array}{ccc}1& 1& 1\\ -1& -1& 3\\ 3& -1& -1\end{array}\right]\&CenterDot;\left[\begin{array}{c}R\\ G\\ B\end{array}\right]---\left(2\right)$

Wherein: symbol is identical with formula (1);

When 2) decoding, according to existing image or video compression standard, comprise JPEG series, MPEG series, H.26X serial, adopt its encryption algorithm step to obtain data on the rYCrCb space, obtain the data of 16 rgb formats according to the inverse transform method of formula (3), directly output to portable equipment and show;

G＝(2Y _r-Cb _r-Cr _r)//4

R＝(Y _r+Cr _r)//4 (3)

B＝(Y _r+Cb _r)//4

Wherein: letter character is identical with formula (1), and " // " expression rounds up.