RGBA-ABGR-RGB666

时间：2025-02-07 18:35:11

大小端详解：


小端：较高的有效字节存放在较高的的存储器地址，较低的有效字节存放在较低的存储器地址。
大端：较高的有效字节存放在较低的存储器地址，较低的有效字节存放在较高的存储器地址。
如果将一个16位的整数0x1234存放到一个短整型变量（short）中。这个短整型变量在内存中的存储在大小端模式由下表所示。 

 

地址偏移

大端模式

小端模式

0x00

12（OP0）

34（OP1）

0x01

34（OP1）

12（OP0）

 


YUV与RGB相互转换的公式如下（RGB取值范围均为0-255）：

Y = 0.299R + 0.587G + 0.114B
U = -0.147R - 0.289G + 0.436B
V = 0.615R - 0.515G - 0.100B
R = Y + 1.14V
G = Y - 0.39U - 0.58V
B = Y + 2.03U

//------------------------------------------------------------------------------------

在DirectShow中，常见的RGB格式有RGB1、RGB4、RGB8、RGB565、RGB555、RGB24、RGB32、ARGB32等；
常见的YUV格式有YUY2、YUYV、YVYU、UYVY、AYUV、Y41P、Y411、Y211、IF09、IYUV、YV12、YVU9、YUV411、YUV420等。
作为视频媒体类型的辅助说明类型（Subtype），它们对应的GUID见表2.3。
表2.3 常见的RGB和YUV格式
      GUID                                         格式描述
MEDIASUBTYPE_RGB1    2色，每个像素用1位表示，需要调色板
MEDIASUBTYPE_RGB4    16色，每个像素用4位表示，需要调色板
MEDIASUBTYPE_RGB8    256色，每个像素用8位表示，需要调色板
MEDIASUBTYPE_RGB565    每个像素用16位表示，RGB分量分别使用5位、6位、5位
MEDIASUBTYPE_RGB555    每个像素用16位表示，RGB分量都使用5位（剩下的1位不用）
MEDIASUBTYPE_RGB24    每个像素用24位表示，RGB分量各使用8位
MEDIASUBTYPE_RGB32    每个像素用32位表示，RGB分量各使用8位（剩下的8位不用）
MEDIASUBTYPE_ARGB32    每个像素用32位表示，RGB分量各使用8位（剩下的8位用于表示Alpha通道值）


下面分别介绍各种RGB格式。
¨ RGB1、RGB4、RGB8都是调色板类型的RGB格式，在描述这些媒体类型的格式细节时，通常会在BITMAPINFOHEADER数据结构后面跟着一个调色板（定义一系列颜色）。它们的图像数据并不是真正的颜色值，而是当前像素颜色值在调色板中的索引。以RGB1（2色位图）为例，比如它的调色板中定义的两种颜色值依次为0x000000（黑色）和0xFFFFFF（白色），那么图像数据001101010111…（每个像素用1位表示）表示对应各像素的颜色为：黑黑白白黑白黑白黑白白白…。
¨ RGB565使用16位表示一个像素，这16位中的5位用于R，6位用于G，5位用于B。程序中通常使用一个字（WORD，一个字等于两个字节）来操作一个像素。当读出一个像素后，这个字的各个位意义如下：
     高字节              低字节
R R R R R G G G     G G G B B B B B
可以组合使用屏蔽字和移位操作来得到RGB各分量的值：
#define RGB565_MASK_RED    0xF800
#define RGB565_MASK_GREEN  0x07E0
#define RGB565_MASK_BLUE   0x001F
R = (wPixel & RGB565_MASK_RED) >> 11;   // 取值范围0-31
G = (wPixel & RGB565_MASK_GREEN) >> 5;  // 取值范围0-63
B =  wPixel & RGB565_MASK_BLUE;         // 取值范围0-31
¨ RGB555是另一种16位的RGB格式，RGB分量都用5位表示（剩下的1位不用）。使用一个字读出一个像素后，这个字的各个位意义如下：
     高字节             低字节
X R R R R G G       G G G B B B B B       （X表示不用，可以忽略）
可以组合使用屏蔽字和移位操作来得到RGB各分量的值：
#define RGB555_MASK_RED    0x7C00
#define RGB555_MASK_GREEN  0x03E0
#define RGB555_MASK_BLUE   0x001F
R = (wPixel & RGB555_MASK_RED) >> 10;   // 取值范围0-31
G = (wPixel & RGB555_MASK_GREEN) >> 5;  // 取值范围0-31
B =  wPixel & RGB555_MASK_BLUE;         // 取值范围0-31
¨ RGB24使用24位来表示一个像素，RGB分量都用8位表示，取值范围为0-255。注意在内存中RGB各分量的排列顺序为：BGR BGR BGR…。通常可以使用RGBTRIPLE数据结构来操作一个像素，它的定义为：
typedef struct tagRGBTRIPLE { 
  BYTE rgbtBlue;    // 蓝色分量
  BYTE rgbtGreen;   // 绿色分量
  BYTE rgbtRed;     // 红色分量
} RGBTRIPLE;
¨ RGB32使用32位来表示一个像素，RGB分量各用去8位，剩下的8位用作Alpha通道或者不用。（ARGB32就是带Alpha通道的RGB32。）注意在内存中RGB各分量的排列顺序为：BGRA BGRA BGRA…。通常可以使用RGBQUAD数据结构来操作一个像素，它的定义为：
typedef struct tagRGBQUAD {
  BYTE    rgbBlue;      // 蓝色分量
  BYTE    rgbGreen;     // 绿色分量
  BYTE    rgbRed;       // 红色分量
  BYTE    rgbReserved;  // 保留字节（用作Alpha通道或忽略）
} RGBQUAD;


下面介绍各种YUV格式。YUV格式通常有两大类：打包（packed）格式和平面（planar）格式。前者将YUV分量存放在同一个数组中，
通常是几个相邻的像素组成一个宏像素（macro-pixel）；而后者使用三个数组分开存放YUV三个分量，就像是一个三维平面一样。
表2.3中的YUY2到Y211都是打包格式，而IF09到YVU9都是平面格式。（注意：在介绍各种具体格式时，YUV各分量都会带有下标，
如Y0、U0、V0表示第一个像素的YUV分量，Y1、U1、V1表示第二个像素的YUV分量，以此类推。）

/downloads/v4l-dvb-apis/


Table 2.18. Packed RGB Image Formats

Identifier Code   Byte 0 in memory   Byte 1   Byte 2   Byte 3
    Bit 7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0
V4L2_PIX_FMT_RGB332 'RGB1'   r2 r1 r0 g2 g1 g0 b1 b0                                                    
V4L2_PIX_FMT_ARGB444 'AR12'   g3 g2 g1 g0 b3 b2 b1 b0   a3 a2 a1 a0 r3 r2 r1 r0                                  
V4L2_PIX_FMT_XRGB444 'XR12'   g3 g2 g1 g0 b3 b2 b1 b0   - - - - r3 r2 r1 r0                                  
V4L2_PIX_FMT_ARGB555 'AR15'   g2 g1 g0 b4 b3 b2 b1 b0   a r4 r3 r2 r1 r0 g4 g3                                  
V4L2_PIX_FMT_XRGB555 'XR15'   g2 g1 g0 b4 b3 b2 b1 b0   - r4 r3 r2 r1 r0 g4 g3                                  
V4L2_PIX_FMT_RGB565 'RGBP'   g2 g1 g0 b4 b3 b2 b1 b0   r4 r3 r2 r1 r0 g5 g4 g3                                  
V4L2_PIX_FMT_ARGB555X 'AR15' | (1 << 31)   a r4 r3 r2 r1 r0 g4 g3   g2 g1 g0 b4 b3 b2 b1 b0                                  
V4L2_PIX_FMT_XRGB555X 'XR15' | (1 << 31)   - r4 r3 r2 r1 r0 g4 g3   g2 g1 g0 b4 b3 b2 b1 b0                                  
V4L2_PIX_FMT_RGB565X 'RGBR'   r4 r3 r2 r1 r0 g5 g4 g3   g2 g1 g0 b4 b3 b2 b1 b0                                  
V4L2_PIX_FMT_BGR24 'BGR3'   b7 b6 b5 b4 b3 b2 b1 b0   g7 g6 g5 g4 g3 g2 g1 g0   r7 r6 r5 r4 r3 r2 r1 r0                
V4L2_PIX_FMT_RGB24 'RGB3'   r7 r6 r5 r4 r3 r2 r1 r0   g7 g6 g5 g4 g3 g2 g1 g0   b7 b6 b5 b4 b3 b2 b1 b0                
V4L2_PIX_FMT_BGR666 'BGRH'   b5 b4 b3 b2 b1 b0 g5 g4   g3 g2 g1 g0 r5 r4 r3 r2   r1 r0 - - - - - -   - - - - - - - -
V4L2_PIX_FMT_ABGR32 'AR24'   b7 b6 b5 b4 b3 b2 b1 b0   g7 g6 g5 g4 g3 g2 g1 g0   r7 r6 r5 r4 r3 r2 r1 r0   a7 a6 a5 a4 a3 a2 a1 a0
V4L2_PIX_FMT_XBGR32 'XR24'   b7 b6 b5 b4 b3 b2 b1 b0   g7 g6 g5 g4 g3 g2 g1 g0   r7 r6 r5 r4 r3 r2 r1 r0   - - - - - - - -
V4L2_PIX_FMT_ARGB32 'BA24'   a7 a6 a5 a4 a3 a2 a1 a0   r7 r6 r5 r4 r3 r2 r1 r0   g7 g6 g5 g4 g3 g2 g1 g0   b7 b6 b5 b4 b3 b2 b1 b0
V4L2_PIX_FMT_XRGB32 'BX24'   - - - - - - - -   r7 r6 r5 r4 r3 r2 r1 r0   g7 g6 g5 g4 g3 g2 g1 g0   b7 b6 b5 b4 b3 b2 b1 b0

Bit 7 is the most significant bit.

The usage and value of the alpha bits (a) in the ARGB and ABGR formats (collectively referred to as alpha formats) depend on the device type and hardware operation.Capture devices (including capture queues of mem-to-mem devices) fill the alpha component in memory. When the device outputs an alpha channel the alpha component will have a meaningful value. Otherwise, when the device doesn't output an alpha channel but can set the alpha bit to a user-configurable value, the V4L2_CID_ALPHA_COMPONENTcontrol is used to specify that alpha value, and the alpha component of all pixels will be set to the value specified by that control. Otherwise a corresponding format without an alpha component (XRGB or XBGR) must be used instead of an alpha format.

Output devices (including output queues of mem-to-mem devices and video output overlay devices) read the alpha component from memory. When the device processes the alpha channel the alpha component must be filled with meaningful values by applications. Otherwise a corresponding format without an alpha component (XRGB or XBGR) must be used instead of an alpha format.

The XRGB and XBGR formats contain undefined bits (-). Applications, devices and drivers must ignore those bits, for both capture and output devices.

Example 2.1. V4L2_PIX_FMT_BGR24 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0: B00 G00 R00 B01 G01 R01 B02 G02 R02 B03 G03 R03
start + 12: B10 G10 R10 B11 G11 R11 B12 G12 R12 B13 G13 R13
start + 24: B20 G20 R20 B21 G21 R21 B22 G22 R22 B23 G23 R23
start + 36: B30 G30 R30 B31 G31 R31 B32 G32 R32 B33 G33 R33


Formats defined in Table 2.19, “Deprecated Packed RGB Image Formats” are deprecated and must not be used by new drivers. They are documented here for reference. The meaning of their alpha bits (a) is ill-defined and interpreted as in either the corresponding ARGB or XRGB format, depending on the driver.

Table 2.19. Deprecated Packed RGB Image Formats

Identifier Code   Byte 0 in memory   Byte 1   Byte 2   Byte 3
    Bit 7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0   7 6 5 4 3 2 1 0
V4L2_PIX_FMT_RGB444 'R444'   g3 g2 g1 g0 b3 b2 b1 b0   a3 a2 a1 a0 r3 r2 r1 r0                                  
V4L2_PIX_FMT_RGB555 'RGBO'   g2 g1 g0 b4 b3 b2 b1 b0   a r4 r3 r2 r1 r0 g4 g3                                  
V4L2_PIX_FMT_RGB555X 'RGBQ'   a r4 r3 r2 r1 r0 g4 g3   g2 g1 g0 b4 b3 b2 b1 b0                                  
V4L2_PIX_FMT_BGR32 'BGR4'   b7 b6 b5 b4 b3 b2 b1 b0   g7 g6 g5 g4 g3 g2 g1 g0   r7 r6 r5 r4 r3 r2 r1 r0   a7 a6 a5 a4 a3 a2 a1 a0
V4L2_PIX_FMT_RGB32 'RGB4'   a7 a6 a5 a4 a3 a2 a1 a0   r7 r6 r5 r4 r3 r2 r1 r0   g7 g6 g5 g4 g3 g2 g1 g0   b7 b6 b5 b4 b3 b2 b1 b0

A test utility to determine which RGB formats a driver actually supports is available from the LinuxTV v4l-dvb repository. See /repo/ for access instructions.


/wiki/RGBA_color_space#cite_note-1


Representation[edit]

In computer graphics, pixels encoding the RGBA color space information must be stored in computer memory (or in files on disk), in well defined formats. There are several ways to encode RGBA colors, which can lead to confusion when image data is exchanged. These encodings are often denoted by the four letters in some order (. RGBA, ARGB, etc.). Unfortunately, the interpretation of these 4-letter mnemonics is not well established, leading to further confusion. There are two typical ways to understand a mnemonic such as "RGBA":

  • In the byte-order scheme, "RGBA" is understood to mean a byte R, followed by a byte G, followed by a byte B, and followed by a byte A. This scheme is commonly used for describing file formats or network protocols, which are both byte-oriented.
  • In the word-order scheme, "RGBA" is understood to represent a complete 32-bit word, where R is more significant than G, which is more significant than B, which is more significant than A. This scheme can be used to describe the memory layout on a particular system. Its meaning varies depending on the endianness of the system.

In a [big-endian] system, the two schemes are equivalent. This is not the case for a [little-endian] system, where the two mnemonics are reverses of each other. Therefore, to be unambiguous, it is important to state which ordering is used when referring to the encoding.

Format As byte-order As word-order
Little-endian Big-endian Little-endian Big-endian
RGBA (byte-order) RGBA8888 RGBA8888 ABGR32 RGBA32
ARGB (word-order) BGRA8888 ARGB8888 ARGB32 ARGB32
RGBA (word-order) ABGR8888 RGBA8888 RGBA32 RGBA32

RGBA (byte-order)[edit]

In OpenGL and Portable Network Graphics (PNG), the RGBA (byte-order) is used, where the colors are stored in memory such that R is at the lowest address, G after it, B after that, and A last. On a little endian architecture this is equivalent to ABGR (word-order).[1]

Even when there are more than 8 bits per channel (such as 16 bits or floating-point), the channels are still stored in RGBA order. In PNG, the channels are stored as 16-bit integers in network order (big-endian).

ARGB (word-order)[edit]

In the ARGB (word-order) encoding the intensity of each channel sample is defined by 8 bits, and are arranged in memory in such manner that a single 32-bit unsigned integer has the alpha sample in the highest 8 bits, followed by the red sample, green sample and finally the blue sample in the lowest 8 bits:

ARGB values are typically expressed using 8 hexadecimal digits, with each pair of the hexadecimal digits representing the values of the Alpha, Red, Green and Blue channel, respectively. For example, 80FFFF00 represents 50.2% opaque (non-premultiplied) yellow. The 80 hex value, which is 128 in decimal, represents a 50.2% alpha value because 128 is approximately 50.2% of the maximum value of 255 (FF hex); to continue to decipher the 80FFFF00 value, the first FFrepresents the maximum value red can have; the second FF is like the previous but for green; the final 00 represents the minimum value blue can have (effectively – no blue). Consequently, red + green yields yellow. In cases where the alpha is not used this can be shortened to 6 digits RRGGBB, this is why it was chosen to put the alpha in the top bits. Depending on the context a 0x or a number sign (#)[2] is put before the hex digits.

On little-endian systems, this is equivalent to BGRA (byte-order). On big-endian systems, this is equivalent to ARGB (byte-order).

RGBA hexadecimal (word-order)[edit]

This section's factual accuracy is disputed. Please help to ensure that disputed statements are reliably sourced. See the relevant discussion on the talk page. (December 2015)

In some software originating on big-endian machines such as Silicon Graphics, RGBA (word-order) means color is specified similar to ARGB (word-order) but with the alpha in the bottom 8 bits rather than the top. For example, 808000FF would be Red and Green:50.2%, Blue:0% and Alpha:100%, a brown. This is used in, .Portable Arbitrary Map (PAM).

The bytes are stored in memory on a little-endian machine in the order ABGR (byte-order).


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