chroma subsampling

http://en.wikipedia.org/wiki/Chroma_subsampling:

Sampling systems and ratios

The subsampling scheme is commonly expressed as a three part ratio (e.g. 4:2:2), although sometimes expressed as four parts (e.g. 4:2:2:4). The parts are (in their respective order):

• Luma horizontal sampling reference (originally, as a multiple of 3.579 MHz in the NTSC television system)
• Cr horizontal factor (relative to first digit)
• Cb horizontal factor (relative to first digit), except when zero. Zero indicates that Cb horizontal factor is equal to second digit, and, in addition, both Cr and Cb are subsampled 2:1 vertically. Zero is chosen for the bandwidth calculation formula (see below) to remain correct.
• Alpha horizontal factor (relative to first digit). May be omitted if alpha component is not present.

To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present).

From Microsoft Help

• 4:4:4 means no downsampling of the chroma channels.
• 4:2:2 means 2:1 horizontal downsampling, with no vertical downsampling. Every scan line contains four Y samples for every two U or V samples.
• 4:2:0 means 2:1 horizontal downsampling, with 2:1 vertical downsampling.
• 4:1:1 means 4:1 horizontal downsampling, with no vertical downsampling. Every scan line contains four Y samples for every U or V sample. 4:1:1 sampling is less common than other formats, and is not discussed in detail in this article.

esempio di visualizzazione: http://msdn2.microsoft.com/en-us/library/ms867704.aspx

Video subtype (i codici four cc più famosi per il video non compresso):
AYUV 4:4:4
YUY2 4:2:2
UYVY 4:2:2
YV12 4:2:0

Video di esempio
http://trace.eas.asu.edu/yuv/index.html

RGB formats

rgb24 un byte per colore (8bit*3 per ogni pixel)
rgb555 5 bit per ogni colore (5bit*3 per ogni pixel messi su due byte) 32K colori
rgb565 6 bit solo per il verde (5bit*2 + 6, messi su due byte) 65K colori