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On 04.07.2010 15:06, Ive wrote:
> On 04.07.2010 12:24, clipka wrote:
>> Some other image file formats, such as OpenEXR and TIFF, officially use
>> associated - or premultiplied - alpha, which means that the data is
>> stored in the file already pre-composed against a black background, so
>> simply ignoring alpha would indeed give what you'd expect.
>
>
> To clarify and to prevent a new bug within POV-Ray:
>
> I do not know where you did get the information/impression that TIFF
> "officially use associated - or premultiplied - alpha". This is not true
> as TIFF allows *everything*.
> You might have misread the TIFF 6.0 specs section 18: Associated Alpha
> Handling. This section applies only *IF* the "ExtraSamples" tag is
> present and the value of the tag reads 1. In most cases (in all if the
> TIFF was written by any Adobe software) the value of the ExtraSample tag
> will be 0 - i.e. unassociated, non-pre-multiplied, straight.
>
> -Ive
>
Umm, and to correct myself an "ExtraSample" value of 0 actually means
undefined but was always used by Adobe to indicate unassociated alpha,
(but could actually mean something else e.g. no "alpha" data at all).
The correct value to identify unassociated, non-pre-multiplied alpha
would be 2.
-Ive
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