音视频入门基础：像素格式专题（1）—

一、像素格式简介

像素格式（pixel format）指像素色彩按分量的大小和排列。这种格式以每个像素所使用的总位数以及用于存储像素色彩的红、绿、蓝和 alpha 分量的位数指定。在音视频领域，常用的像素格式包括RGB格式和YUV格式，本文对RGB格式进行简介。

二、RGB格式简介

三原色光模式（RGB color model，又称RGB表色系统、RGB颜色模型、红绿蓝颜色模型，是一种加色模型，将红（Red）、绿（Green）、蓝（Blue）三原色的色光以不同的比例相加混色，以合成产生各种色彩光。

RGB颜色模型的主要目的是在电子系统中检测，表示和显示图像，比如电视和电脑，利用大脑强制视觉生理模糊化（失焦），将红绿蓝三原色子像素合成为一色彩像素，产生感知色彩（其实此真彩色并非加色法所产生的合成色彩，原因为该三原色光从来没有重叠在一起，只是人类为了“想”看到色彩，大脑强制眼睛失焦而形成。情况其实就有点像看那些 autostereograms 的立体图时，大脑与眼睛扭曲才能看到“想”看的立体影像的情况）。红绿蓝三色模型在传统摄影中也有应用。在电子时代之前，基于人类对颜色的感知，RGB颜色模型已经有了坚实的理论支撑。

三、RGB24格式简介

RGB24是表明图像以RGB三原色，每个像素点3个字节表示的一种图像存储格式，即每像素24位编码的RGB值：使用三个8位无符号整数（0到255）表示红色、绿色和蓝色的强度。这是当前主流的标准表示方法，用于真彩色和JPEG或者TIFF等图像文件格式里的通用颜色交换。它可以产生一千六百万种颜色组合，比如：

(0, 0, 0)是黑色；(255, 255, 255)是白色；(255, 0, 0)是红色；(0, 255, 0)是绿色；(0, 0, 255)是蓝色；(255, 255, 0)是黄色；(0, 255, 255)是青色或水色；(255, 0, 255)是洋红色。

四、RGB24格式视频大小计算

由于RGB24格式每像素占3个字节。所以一张rgb图片的大小 = 图片的宽 * 图片的高 *3 byte。

一个rgb视频的大小 = 里面一张rgb图片的大小 * 该视频文件的总帧数。

下面我们用FFmpeg（我用的FFmpeg版本是5.0.3）来验证上述的公式。

首先查看FFmpeg将视频文件转为rgb文件的命令，执行命令：

ffmpeg -h full

打印ffmpeg的所有帮助信息

可以看到有一栏写道：-pix_fmts show available pixel formats

Print help / information / capabilities:
-L                  show license
-h topic            show help
-? topic            show help
-help topic         show help
--help topic        show help
-version            show version
-buildconf          show build configuration
-formats            show available formats
-muxers             show available muxers
-demuxers           show available demuxers
-devices            show available devices
-codecs             show available codecs
-decoders           show available decoders
-encoders           show available encoders
-bsfs               show available bit stream filters
-protocols          show available protocols
-filters            show available filters
-pix_fmts           show available pixel formats
-layouts            show standard channel layouts
-sample_fmts        show available audio sample formats
-dispositions       show available stream dispositions
-colors             show available color names
-sources device     list sources of the input device
-sinks device       list sinks of the output device
-hwaccels           show available HW acceleration methods

意思是FFmpeg命令中可以通过-pix_fmts 选项显示可用的像素格式

所以执行命令：

ffmpeg -pix_fmts

得到输出如下：

Pixel formats:
I.... = Supported Input  format for conversion
.O... = Supported Output format for conversion
..H.. = Hardware accelerated format
...P. = Paletted format
....B = Bitstream format
FLAGS NAME            NB_COMPONENTS BITS_PER_PIXEL BIT_DEPTHS
-----
IO... yuv420p                3             12      8-8-8
IO... yuyv422                3             16      8-8-8
IO... rgb24                  3             24      8-8-8
IO... bgr24                  3             24      8-8-8
IO... yuv422p                3             16      8-8-8
IO... yuv444p                3             24      8-8-8
IO... yuv410p                3              9      8-8-8
IO... yuv411p                3             12      8-8-8
IO... gray                   1              8      8
IO..B monow                  1              1      1
IO..B monob                  1              1      1
I..P. pal8                   1              8      8
IO... yuvj420p               3             12      8-8-8
IO... yuvj422p               3             16      8-8-8
IO... yuvj444p               3             24      8-8-8
IO... uyvy422                3             16      8-8-8
..... uyyvyy411              3             12      8-8-8
IO... bgr8                   3              8      3-3-2
.O..B bgr4                   3              4      1-2-1
IO... bgr4_byte              3              4      1-2-1
IO... rgb8                   3              8      2-3-3
.O..B rgb4                   3              4      1-2-1
IO... rgb4_byte              3              4      1-2-1
IO... nv12                   3             12      8-8-8
IO... nv21                   3             12      8-8-8
IO... argb                   4             32      8-8-8-8
IO... rgba                   4             32      8-8-8-8
IO... abgr                   4             32      8-8-8-8
IO... bgra                   4             32      8-8-8-8
IO... gray16be               1             16      16
IO... gray16le               1             16      16
IO... yuv440p                3             16      8-8-8
IO... yuvj440p               3             16      8-8-8
IO... yuva420p               4             20      8-8-8-8
IO... rgb48be                3             48      16-16-16
IO... rgb48le                3             48      16-16-16
IO... rgb565be               3             16      5-6-5
IO... rgb565le               3             16      5-6-5
IO... rgb555be               3             15      5-5-5
IO... rgb555le               3             15      5-5-5
IO... bgr565be               3             16      5-6-5
IO... bgr565le               3             16      5-6-5
IO... bgr555be               3             15      5-5-5
IO... bgr555le               3             15      5-5-5
..H.. vaapi                  0              0      0
IO... yuv420p16le            3             24      16-16-16
IO... yuv420p16be            3             24      16-16-16
IO... yuv422p16le            3             32      16-16-16
IO... yuv422p16be            3             32      16-16-16
IO... yuv444p16le            3             48      16-16-16
IO... yuv444p16be            3             48      16-16-16
..H.. dxva2_vld              0              0      0
IO... rgb444le               3             12      4-4-4
IO... rgb444be               3             12      4-4-4
IO... bgr444le               3             12      4-4-4
IO... bgr444be               3             12      4-4-4
IO... ya8                    2             16      8-8
IO... bgr48be                3             48      16-16-16
IO... bgr48le                3             48      16-16-16
IO... yuv420p9be             3             13      9-9-9
IO... yuv420p9le             3             13      9-9-9
IO... yuv420p10be            3             15      10-10-10
IO... yuv420p10le            3             15      10-10-10
IO... yuv422p10be            3             20      10-10-10
IO... yuv422p10le            3             20      10-10-10
IO... yuv444p9be             3             27      9-9-9
IO... yuv444p9le             3             27      9-9-9
IO... yuv444p10be            3             30      10-10-10
IO... yuv444p10le            3             30      10-10-10
IO... yuv422p9be             3             18      9-9-9
IO... yuv422p9le             3             18      9-9-9
IO... gbrp                   3             24      8-8-8
IO... gbrp9be                3             27      9-9-9
IO... gbrp9le                3             27      9-9-9
IO... gbrp10be               3             30      10-10-10
IO... gbrp10le               3             30      10-10-10
IO... gbrp16be               3             48      16-16-16
IO... gbrp16le               3             48      16-16-16
IO... yuva422p               4             24      8-8-8-8
IO... yuva444p               4             32      8-8-8-8
IO... yuva420p9be            4             22      9-9-9-9
IO... yuva420p9le            4             22      9-9-9-9
IO... yuva422p9be            4             27      9-9-9-9
IO... yuva422p9le            4             27      9-9-9-9
IO... yuva444p9be            4             36      9-9-9-9
IO... yuva444p9le            4             36      9-9-9-9
IO... yuva420p10be           4             25      10-10-10-10
IO... yuva420p10le           4             25      10-10-10-10
IO... yuva422p10be           4             30      10-10-10-10
IO... yuva422p10le           4             30      10-10-10-10
IO... yuva444p10be           4             40      10-10-10-10
IO... yuva444p10le           4             40      10-10-10-10
IO... yuva420p16be           4             40      16-16-16-16
IO... yuva420p16le           4             40      16-16-16-16
IO... yuva422p16be           4             48      16-16-16-16
IO... yuva422p16le           4             48      16-16-16-16
IO... yuva444p16be           4             64      16-16-16-16
IO... yuva444p16le           4             64      16-16-16-16
..H.. vdpau                  0              0      0
IO... xyz12le                3             36      12-12-12
IO... xyz12be                3             36      12-12-12
..... nv16                   3             16      8-8-8
..... nv20le                 3             20      10-10-10
..... nv20be                 3             20      10-10-10
IO... rgba64be               4             64      16-16-16-16
IO... rgba64le               4             64      16-16-16-16
IO... bgra64be               4             64      16-16-16-16
IO... bgra64le               4             64      16-16-16-16
IO... yvyu422                3             16      8-8-8
IO... ya16be                 2             32      16-16
IO... ya16le                 2             32      16-16
IO... gbrap                  4             32      8-8-8-8
IO... gbrap16be              4             64      16-16-16-16
IO... gbrap16le              4             64      16-16-16-16
..H.. qsv                    0              0      0
..H.. mmal                   0              0      0
..H.. d3d11va_vld            0              0      0
..H.. cuda                   0              0      0
IO... 0rgb                   3             24      8-8-8
IO... rgb0                   3             24      8-8-8
IO... 0bgr                   3             24      8-8-8
IO... bgr0                   3             24      8-8-8
IO... yuv420p12be            3             18      12-12-12
IO... yuv420p12le            3             18      12-12-12
IO... yuv420p14be            3             21      14-14-14
IO... yuv420p14le            3             21      14-14-14
IO... yuv422p12be            3             24      12-12-12
IO... yuv422p12le            3             24      12-12-12
IO... yuv422p14be            3             28      14-14-14
IO... yuv422p14le            3             28      14-14-14
IO... yuv444p12be            3             36      12-12-12
IO... yuv444p12le            3             36      12-12-12
IO... yuv444p14be            3             42      14-14-14
IO... yuv444p14le            3             42      14-14-14
IO... gbrp12be               3             36      12-12-12
IO... gbrp12le               3             36      12-12-12
IO... gbrp14be               3             42      14-14-14
IO... gbrp14le               3             42      14-14-14
IO... yuvj411p               3             12      8-8-8
I.... bayer_bggr8            3              8      2-4-2
I.... bayer_rggb8            3              8      2-4-2
I.... bayer_gbrg8            3              8      2-4-2
I.... bayer_grbg8            3              8      2-4-2
I.... bayer_bggr16le         3             16      4-8-4
I.... bayer_bggr16be         3             16      4-8-4
I.... bayer_rggb16le         3             16      4-8-4
I.... bayer_rggb16be         3             16      4-8-4
I.... bayer_gbrg16le         3             16      4-8-4
I.... bayer_gbrg16be         3             16      4-8-4
I.... bayer_grbg16le         3             16      4-8-4
I.... bayer_grbg16be         3             16      4-8-4
..H.. xvmc                   0              0      0
IO... yuv440p10le            3             20      10-10-10
IO... yuv440p10be            3             20      10-10-10
IO... yuv440p12le            3             24      12-12-12
IO... yuv440p12be            3             24      12-12-12
IO... ayuv64le               4             64      16-16-16-16
..... ayuv64be               4             64      16-16-16-16
..H.. videotoolbox_vld       0              0      0
IO... p010le                 3             15      10-10-10
IO... p010be                 3             15      10-10-10
IO... gbrap12be              4             48      12-12-12-12
IO... gbrap12le              4             48      12-12-12-12
IO... gbrap10be              4             40      10-10-10-10
IO... gbrap10le              4             40      10-10-10-10
..H.. mediacodec             0              0      0
IO... gray12be               1             12      12
IO... gray12le               1             12      12
IO... gray10be               1             10      10
IO... gray10le               1             10      10
IO... p016le                 3             24      16-16-16
IO... p016be                 3             24      16-16-16
..H.. d3d11                  0              0      0
IO... gray9be                1              9      9
IO... gray9le                1              9      9
IO... gbrpf32be              3             96      32-32-32
IO... gbrpf32le              3             96      32-32-32
IO... gbrapf32be             4            128      32-32-32-32
IO... gbrapf32le             4            128      32-32-32-32
..H.. drm_prime              0              0      0
..H.. opencl                 0              0      0
IO... gray14be               1             14      14
IO... gray14le               1             14      14
IO... grayf32be              1             32      32
IO... grayf32le              1             32      32
IO... yuva422p12be           4             36      12-12-12-12
IO... yuva422p12le           4             36      12-12-12-12
IO... yuva444p12be           4             48      12-12-12-12
IO... yuva444p12le           4             48      12-12-12-12
IO... nv24                   3             24      8-8-8
IO... nv42                   3             24      8-8-8
..H.. vulkan                 0              0      0
..... y210be                 3             20      10-10-10
I.... y210le                 3             20      10-10-10
IO... x2rgb10le              3             30      10-10-10
..... x2rgb10be              3             30      10-10-10
IO... x2bgr10le              3             30      10-10-10
..... x2bgr10be              3             30      10-10-10
IO... p210be                 3             20      10-10-10
IO... p210le                 3             20      10-10-10
IO... p410be                 3             30      10-10-10
IO... p410le                 3             30      10-10-10
IO... p216be                 3             32      16-16-16
IO... p216le                 3             32      16-16-16
IO... p416be                 3             48      16-16-16
IO... p416le                 3             48      16-16-16

可以看到ffmpeg是支持rgb24的像素格式转换的。

我们用下面的这个“视频素材_天空中的云.mp4”视频作为演示：

该视频的大小为4.57 MB (4,794,032 字节)

分辨率为：1280 * 720

通过命令：

ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 视频素材_天空中的云.mp4

可以得到该视频总共有387帧

将“视频素材_天空中的云.mp4”转换为像素格式为rgb24的文件：output.rgb

ffmpeg -i 视频素材_天空中的云.mp4 -pix_fmt rgb24 output.rgb

该文件即包含原视频“视频素材_天空中的云.mp4” 里面解压并像素格式转换为rgb24后的所有数据

可以看到它的大小为0.99 GB = 1,069,977,600 byte = 1280 * 720 * 3 * 387 byte。所以可以证明“RGB24格式每像素占3个字节” 以及相关公式是正确的。

五、播放RGB24格式视频

常用的播放器无法播放裸rgb24格式的视频，但可以通过YUView 来播放，该播放器支持播放裸RGB 文件。

打开YUView，"File" -> "Open File"

设置视频宽度和高度，选择RGB Format为RGB 8bit（因为rgb24每个像素占8位），点击播放按钮就能播放了