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Video Coding Concepts: YUV and RGB Colorspaces And Pixel Formats

November 15th, 2007 by Multimedia Mike

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!

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.

RGB Colors

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:

R G B color notes:
0x00 0x00 0x00 absence of R, G, and B = full black
0x00 0x00 0xFF full blue
0x00 0xFF 0x00 full green
0x00 0xFF 0xFF
0xFF 0x00 0x00 full red
0xFF 0x00 0xFF
0xFF 0xFF 0x00
0xFF 0xFF 0xFF full R, G, and B combine to make full white

YUV Colors
If 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 and V is also denoted as Cr. So YUV is sometimes written as YCbCr.

Here are the various combinations of Y, U, and V components at their minimum (0) and maximum (255/0xFF) values:

Y U/
Cb
V/
Cr
color notes
0x00 0x00 0x00
0x00 0x00 0xFF
0x00 0xFF 0x00
0x00 0xFF 0xFF
0xFF 0x00 0x00 full green
0xFF 0x00 0xFF
0xFF 0xFF 0x00
0xFF 0xFF 0xFF
0x00 0x80 0x80 full black
0x80 0x80 0x80
0xFF 0x80 0x80 full white

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.

Further Reading:

Posted in Codec Technology, Video Codecs | 4 Comments »

4 Responses

  1. Kostya Says:

    Hey, U and V are actually signed! So really we talk about 0xFF 0 0 as white and 0 0 0 as black, and green would be (X -128 -128).

  2. pengvado Says:

    Color gamut warrants a mention. YUV=0,-128,-128 doesn’t correspond to any RGB color. It should have negative R and B values, i.e. green colored but with total brightness 0, if that makes any sense :). Dull green is just an approximation you get by clipping the negative values to 0. It is the conventional approximation, but I think that has more to do with computational efficiency than correctness.

  3. Multimedia Mike Says:

    This is eerily reminiscent of when I posted about Sound Blaster frequencies (and here is the correction).

    However, the formulas I was working from are:

    R = Y + C1 * (V – 128)
    G = Y – C2 * (V – 128) – C3 * (U – 128)
    B = Y + C4 * (U – 128)

    So that accounts for the scale.

    @Pengvado: Indeed, there is a lot of theory behind this. Then there is the difference between regular YUV and full-scale/JPEG YUV in FFmpeg (PIX_FMT_YUV420P vs. PIX_FMT_YUVJ420P).

  4. Chris Bruce Says:

    I produce LCD TVs for bathrooms.

    When feeding the TV via HDMI from an IP STB the screen colours are ok. When I place the TV into stand-by and then turn the TV on again from stand-by the colours are bright green.

    The main board manufacturer has asked if the STB is transmitting in RGB, YUV 422 or 444.

    Any ideas please anyone..?

    Chris