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March 2002
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2002.03.25 19:20 "Re: Photoshop TIFF+Alpha mysteries", by Chris Cox
First - you are confusing transparency with alpha channels. Alpha channels are a superset: they can be many things, only ONE of which is transparency. A transparency channel is a subset of possible alpha channels (one which is associated with the image data). At 11:37 AM -0700 3/25/02, Chris 'Xenon' Hanson wrote: > Maybe this is an FAQ. > > We use LIBTIFF in our software, and I am forever grateful for the brain > aneurysms it has prevented for me. > > In several places in our software we support the use of an Alpha channel > to indicate transparency amounts when compositing various images together. > > For example, I use Photoshop 5.5 to open a vector-based logo file I have > lying around. I set RGB Color, and anti-aliased. It converts it to a raster > image with transparency (but no Alpha channel). Fine. Just save as TIFF, there is no need to add an alpha channel or flatten - because you already have transparency. > Now, I do Select/Load Selection, choose Layer 1 Transparency and New > Selection. Now, Select/Save Selection, New Channel, named "Alpha". Now > I flatten the image. > > Ok. I appear to have an Alpha channel. Here I can do one of two things. > > I can save it as a 32-bit Targa file or I can save it as a TIFF file. In Photoshop 5.x, Targa used alpha channels instead of transparency. Thus it's using your blank alpha channel for the Targa file. > If I save it as Targa, it works fine in my app. The NULL areas where there > was no logo are interpreted as transparent. > > If I save it as TIFF, it doesn't exactly work. The areas that are > transparent in the TGA are opaque. Because you flattened the image and added an alpha channel. Lord only knows what your software is actually reading for the transparency. > A few pixels along the border between > the logo and the background area appear to become transparent. > > Loading the working TGA file back into Photoshop and resaving it as a TIFF > appears to make a similarly non-working TIFF. Resaving the non-working TIFF > as a 32-bit TGA makes a working TGA file. > > So, the actual Alpha 'channel' data in Photoshop is presumably identical > in all of the images. Perhaps Photoshop interprets that Alpha channel data > as it loads/saves to TIFF? Perhaps LIBTIFF is interpreting that Alpha channel > data differently as I load it? I'm not sure. All I know is that from the API > side of things, I have all indications that there is an alpha channel in the > TIFF image I load, yet when I go to use it in the way I am accustomed to, it > doesn't work the way i expect. I expect I'm missing some small detail. Yep - you're confused. > I've been thinking my next step is to add a a bit of debugging code to my > LIBTIFF loader to write the alpha channel out to a raw file so I may examine > it and see what exactly I'm getting out of LIBTIFF. However, I suspect this > is already a commonly-encountered problem and someone can just point me at > an answer I have overlooked. > > I considered that it might be an difference of premultipled versus non > premultipled alpha, but since the image data itself is the same, I can't see > how this could be. There is no transparency in your TIFF image - because you flattened the image. If there was transparency, then the TIFF would have an associated alpha channel and the color data would be premultiplied with white (white matted). Chris