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Date: Tue, 4 Feb 2003 09:23:35 -0800
From: Daniel McCoy <mccoy@pixar.com>
To: Dante Allegria <danteallegria@yahoo.ca>
Cc: Chris Cox <ccox@adobe.com>, tiff@olympiakos.com
Subject: Re: Pre-multiplied alpha
Message-ID: <20030204172335.GA5714@melete.pixar.com>
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From: Dante Allegria <danteallegria@yahoo.ca>
>  --- Chris Cox <ccox@adobe.com> wrote: > 
> > Sorry, but nothing changed in that regard.
> > 
> > TIFF specifies that images with transparency (at least one associated
> > alpha channel) must be premultiplied (black for RGB/Grayscale, white 
> > for CMYK), and Photoshop has always and will continue to treat it 
> > that way until the TIFF specification changes.
> 
> Hi Chris,
> 
> Perhaps I misstated what I had meant. I have a test image file that has
> its colour data pre-multiplied with its alpha already, while also
> supplying the alpha plane. In the ExtraSamples tag, it is set to a
> value of 1 (Associated alpha data with pre-multiplied color). For more
> info, see Section 18 of the TIFF6 standard. In Photoshop 6, it seems to
> think that this tag is set to 2 and not 1. Attached is a sample tiff
> demonstrating the problem.

The tags in that image look right for a premultiplied image,
but the pixels all have an alpha value of zero, even in the red pixels.
Images with a color value > alpha value are sort of ill-defined and 
something tells me that may not be what you intended.

I don't have Photoshop handy (I'm at a linux box),
Pixar's internal tools can show the image just fine, as can shake
but the gimp sees the zero alpha and just seems to ignore the color.
I do know that a few Photoshop revisions ago, it was common practice around here
to run tiff files through a filter that clamped all the color channels
to be <= alpha before sending them to Photoshop.

I'm not sure what Adobe assumes should be done with over-alpha colors,
but in general such images but in practice, software handling of this
kind of image is pretty varied depending on the assumptions of the authors.
If you have a package that understands high-dynamic range images
it'll likely handle them fine, but a package which expects a normalized
color range, (like I would assume Photoshop is), having color > alpha can
break a whole lot of software assumptions.

Daniel McCoy	Pixar

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