2003.02.05 00:49 "Re: Pre-multiplied alpha", by Chris Cox

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.

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.

Photoshop 6 and 7 are reading it correctly (completely transparent because the associated alpha is all zero). Photoshop 5.5 is not reading it correctly because it fails to load the associated alpha as transparency. Photoshop 5.0 and older can't read it at all because it uses ZIP/FLATE compression.

Chris