Re [PD] pix_freeframe
matju at artengine.ca
Fri Sep 8 03:04:52 CEST 2006
On Fri, 1 Sep 2006, chris clepper wrote:
> On 9/1/06, chris clepper <cgclepper at gmail.com> wrote:
>> On 9/1/06, Mathieu Bouchard <matju at artengine.ca> 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
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 - http://artengine.ca/matju
| Freelance Digital Arts Engineer, Montréal QC Canada
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