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2010.06.14 06:36 "Re: Class Y images with alpha channels", by Thomas Richter
Joris Van Damme (AWare Systems) schrieb:
> Thomas,
>
>> is there a possibility to add an alpha channel to a class Y (YCbCr) tiff
>> image? I don't see this in the rev.6 specs.
>
> Nor does the spec mention 12 bit per channel YCbCr, nor does the spec
> mention RGB with alpha and two unspecified channels, nor does it
> mention 20 bit per channel grayscale, etc. We ought to understand the
> 'class' concept in the spec as a (misleading) enumeration of 'typical
> usage', whilst the tag system makes perfect sense as to allow just any
> variation.
Well, but then how it's done if it is not specified?
Here are two concrete questions I cannot answer from the specs:
*) Given by bits/component is not divisible by 8, how do I pack the
samples? Given my understanding of the "RGB" class, I suppose I need to
bit-pack the samples
as tightly as possible, that is, if I have a 9 bit YCbCr (let's assume
this for a moment) with 420 subsampling, then I write 9 bits Y, 9 bits
Y, 9 bits Y, 9 bits Y, 9 bits Cb,
9 bits Cr, making a total of 6*9 = 54 bits. Now, the specs furthermore
say that I need to flush to the end of the byte at the end of a row.
What is the end of the row
here exactly? Actually, I'm writing not a single row, but two rows at
once. I suppose I need to flush at the end of the "row pair" in 420?
*) Given I have an alpha channel, how is it interleaved with the
remaining channels? Is the alpha channel subsampled (I suppose not, but
I don't know). Is it
YYYYCbCrAAAA or is it YYYYAAAACbCr, or YYYYCbCrA?
Again, no information or examples from the specs.
So long,
Thomas