{"id":49,"date":"2007-11-15T17:18:51","date_gmt":"2007-11-16T01:18:51","guid":{"rendered":"http:\/\/multimedia.cx\/eggs\/yuv-and-rgb\/"},"modified":"2007-11-15T22:54:17","modified_gmt":"2007-11-16T06:54:17","slug":"yuv-and-rgb","status":"publish","type":"post","link":"https:\/\/multimedia.cx\/eggs\/yuv-and-rgb\/","title":{"rendered":"Video Coding Concepts: YUV and RGB Colorspaces And Pixel Formats"},"content":{"rendered":"

If you have any experience in programming computer graphics, you probably know all about red\/green\/blue (RGB) video modes and pixel formats. Guess what? It is all useless now that you are working on video codec technology!<\/p>\n

No, that’s not entirely true. Some video codecs operate on RGB video natively. A majority of modern codecs use some kind of YUV colorspace. We will get to that. Since many programmers are familiar with RGB pixel formats, let’s use that as a starting point.<\/p>\n

RGB Colors<\/strong><\/p>\n

To review, computers generally display RGB pixels. These pixels have red (R), green (G), and blue (B) components to them. Here are the various combinations of R, G, and B components at their minimum (0) and maximum (255\/0xFF) values:<\/p>\n\n\n\n\n\n\n\n\n\n\n
R<\/th>\nG<\/th>\nB<\/th>\ncolor<\/th>\nnotes:<\/th>\n<\/tr>\n
0x00<\/td>\n0x00<\/td>\n0x00<\/td>\n<\/td>\nabsence of R, G, and B = full black<\/em><\/td>\n<\/tr>\n
0x00<\/td>\n0x00<\/td>\n0xFF<\/td>\n<\/td>\nfull blue<\/em><\/td>\n<\/tr>\n
0x00<\/td>\n0xFF<\/td>\n0x00<\/td>\n<\/td>\nfull green<\/em><\/td>\n<\/tr>\n
0x00<\/td>\n0xFF<\/td>\n0xFF<\/td>\n<\/td>\n<\/tr>\n
0xFF<\/td>\n0x00<\/td>\n0x00<\/td>\n<\/td>\nfull red<\/em><\/td>\n<\/tr>\n
0xFF<\/td>\n0x00<\/td>\n0xFF<\/td>\n<\/td>\n<\/tr>\n
0xFF<\/td>\n0xFF<\/td>\n0x00<\/td>\n<\/td>\n<\/tr>\n
0xFF<\/td>\n0xFF<\/td>\n0xFF<\/td>\n<\/td>\nfull R, G, and B combine to make full white<\/em><\/td>\n<\/tr>\n<\/table>\n

YUV Colors<\/strong>
\nIf you are used to dealing with RGB colors, YUV will seem a bit unintuitive at first. What does YUV stand for? Nothing you would guess. It turns out Y stands for intensity. U stands for blue and V stands for red. U is also denoted as Cb<\/sub> and V is also denoted as Cr<\/sub>. So YUV is sometimes written as YCb<\/sub>Cr<\/sub>. <\/p>\n

Here are the various combinations of Y, U, and V components at their minimum (0) and maximum (255\/0xFF) values:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Y<\/th>\nU\/
Cb<\/sub><\/th>\n		V\/
Cr<\/sub><\/th>\n		color<\/th>\nnotes<\/th>\n<\/tr>\n
0x00<\/td>\n0x00<\/td>\n0x00<\/td>\n\"\"<\/td>\n<\/tr>\n
0x00<\/td>\n0x00<\/td>\n0xFF<\/td>\n\"\"<\/td>\n<\/tr>\n
0x00<\/td>\n0xFF<\/td>\n0x00<\/td>\n\"\"<\/td>\n<\/tr>\n
0x00<\/td>\n0xFF<\/td>\n0xFF<\/td>\n\"\"<\/td>\n<\/tr>\n
0xFF<\/td>\n0x00<\/td>\n0x00<\/td>\n\"\"<\/td>\nfull green<\/em><\/td>\n<\/tr>\n
0xFF<\/td>\n0x00<\/td>\n0xFF<\/td>\n\"\"<\/td>\n<\/tr>\n
0xFF<\/td>\n0xFF<\/td>\n0x00<\/td>\n\"\"<\/td>\n<\/tr>\n
0xFF<\/td>\n0xFF<\/td>\n0xFF<\/td>\n\"\"<\/td>\n<\/tr>\n
0x00<\/td>\n0x80<\/td>\n0x80<\/td>\n\"\"<\/td>\nfull black<\/em><\/td>\n<\/tr>\n
0x80<\/td>\n0x80<\/td>\n0x80<\/td>\n\"\"<\/td>\n<\/tr>\n
0xFF<\/td>\n0x80<\/td>\n0x80<\/td>\n\"\"<\/td>\nfull white<\/em><\/td>\n<\/tr>\n<\/table>\n

So, all minimum and all maximum components do not generate intuitive (read: similar to RGB) results. In fact, all 0s in the YUV colorspace result in a dull green rather than black. That last point is useful to understand when a video is displaying a lot of green block errors– that probably means that the decoder is skipping blocks of data completely and leaving the underlying YUV data as all 0.<\/p>\n

Further Reading:<\/strong><\/p>\n