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2010.06.14 06:40 "Re: Class Y (YCbCr) tiffs with 420 subsampling", by Thomas Richter
Joris Van Damme (AWare Systems) schrieb:
> Thomas,
>
>> I checked into the tiff directory reader (tif_dirread.c), which again
>> calls TIFFScanlineSize(), which again only divides by the subsampling in
>> X direction, but rather ignores the subsampling in Y direction - hence
>> getting an invalid result.
>
> The problem is that the functionality of TIFFScanlineSize has not been
> definined for subsampled images, not in code nor in documentation. At
> some time, some parts of the library depended on some type of return
> in this case, others depended on another type of return, and yet other
> parts had their own local modified copy of the code. Attempts have
> been made to sort it out, but no attempt has been consistent over time.
Ok, I see. Thus, I guess we agree that at least the usage of
TIFFScanlineSize in the directory reader is incorrect as the semantics
of TIFFScanlineSize isn't well defined
enough to give it any specific meaning in the case of subsampling being
use. One can then compute the proper/expected plane size by a second
function that is still
to be implemented.
What about TIFFScanlineSize()? I suppose to simply fail over and return
with an error in case subsampling is enabled is then a fair option, then
see what it breaks
and fix that on a case by case basis? Either by giving
TIFFScanlineSize() a defined sematics, or by splitting it into several
functions?
So long,
Thomas