Re [PD] pix_freeframe

Mathieu Bouchard matju at
Fri Sep 8 03:04:52 CEST 2006

On Fri, 1 Sep 2006, chris clepper wrote:
> On 9/1/06, chris clepper <cgclepper at> wrote:
>> On 9/1/06, Mathieu Bouchard <matju at> wrote:
>> > if you're in 8 bits per value, then "444" is 24 bpp, "422" is 16 bpp, 
>> > and the others are 12 bpp, but those "bits per pixel" are assuming a 
>> > "pixel" is defined only in terms of the Y channel. If you do it only 
>> > in terms of UV channels, then everything is reversed, as "444" is 24 
>> > bpp, "422" is 32 bpp, and the others are 48 bpp.
>> It is important to state that all of the pixels are 24 bit in every YCbCr
>> system.  Each pixel can have a unique value even with the shared CbCr.

Well, I hope so!

If it didn't have a value of its own, it wouldn't be a pixel.

> each pixel has a discrete 24 bits of information in YCbCr system.  The 
> subsampling means that pixels share some of the chroma (Cb and Cr) 
> values and not that some pixels lack those color values.

Why is it important to make that distinction? There's no point in saying 
that pixels share chromas vs that pixels lack chromas. It depends on how 
you think of the pixel (e.g. is a pixel a point or is it a square?). To 
complicate matters, in practice, various types of interpolation are used, 
which leads the concepts of "sharing" and "lacking" to be a lot blurrier 
(pun intended).

BTW, congrats for the first mention of YCbCr in this thread. For anyone 
who doesn't know, Cb = blue chroma, Cr = red chroma, and those are 
synonymous with U and V, and also with B-Y and R-Y. Some people insist on 
distinguishing YCbCr, YUV and Y(B-Y)(R-Y) to mean slightly different 
things and may also use apostrophes and stuff, but in the current 
situation, one can't really expect a consistent notation from person to 

  _ _ __ ___ _____ ________ _____________ _____________________ ...
| Mathieu Bouchard - tél:+1.514.383.3801 -
| Freelance Digital Arts Engineer, Montréal QC Canada

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