2002.03.06 08:15 "Re: Photoshop Alpha Channel", by Chris Cox

First, alpha is not the same as transparency. Transparency is a subset of possible alpha channels.

Fine. Semantically speaking, I want an RGBA image where the alpha channel defines the coverage of each pixel over the background, thereby preserving each individual pixel's relative opacity. Please got to Alvy Ray Smith's site at http://www.alvyray.com/ and download his papers "Image Compositing Fundamentals" and "Alpha and the History of Digital Compositing" if there is any confusion as to the specific type of pre-multiplied alpha that should be present in a TIFF file's associated alpha channel. As Alvy pretty much invented the alpha channel as we know it today, I use his approach as the standard.

His standard has been added to over the years, and abstracted far beyond his original usage.

Look in 3D applications which ask you to specify _which_ alpha channel you want to control any one of a dozen shading parameters..... They treat the channels about like Photoshop: color data, transparency, and "other". Transparency and "other" are all alpha channels.

1. Open a blank image
2. Draw anything on it. Make it small enough that you can see the background checker pattern.
3. Save it as a PSD.
4. Save it as a TIFF.
5. Open both the PSD and TIFF and compare them. The PSD will maintain the alpha channel while the TIFF file will have been merged over white with no sign of transparency.

Second, you didn't save the TIFF with transparency. If you save the TIFF with transparency, it won't be flattened.

I sent this message earlier with a small JPEG attached which shows the "Save A Copy As" dialog box in PhotoShop 5.0.

Ok, 4 years ago Photoshop didn't support transparency in TIFF. There have been 2 full versions since then, and we're about to release the third.

Chris