2002.03.05 14:53 "Photoshop Alpha Channel", by Phil Hulme

2002.03.06 07:36 "Re: Photoshop Alpha Channel", by Peter Montgomery


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.

  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. However, the message nenver appeared on the list, so I assume that the attachment caused it to fail. Why it didn't bounce back, I don't know. Anyway, what it showed is that PhotoShop autopmatically checks the "Flatten Image" and then grays it out (making it un-clickable). Since PhotoShop prevents me from not falttening the image, I'm not quite sure how I am supposed to save it "with transparency". If I choose "Save" or "Save As", the only file option is PSD.

This is inexcusable.

This is user error.

See above.

I should be able to save a TIFF, PNG, or TGA and still have my alpha channel.

No, you should still have transparency.

PNG and TGA support transparency, not arbitrary alpha channels. TIFF supports transparency AND arbitrary alpha channels.

I have no need for an aribitrary alpha channel. I simply want the opacity of each indiviual pixel as I painted it to be preserved in the TIFF file. See above for further details.

Of course, if you open a TIFF file that has a proper associated alpha channel, PhotoShop will properly display the image. Too bad you then have to save it as a PSD in order to maintain the alpha.

You don't have to do that - as explained in your manual. In Photoshop 6, TIFF and PNG handle transparency just fine. TGA did not, that was a bug (it wrote the first alpha channel into the transparency slot and read it back as an alpha channel instead of transparency). That is fixed in PS 7.

I'm using PhotoShop 5.0, so information pertaining to PhotoShop 6 and above is not specifically useful to me. As I mentioned in my original post, Adobe would have gotten an upgrade or three out of me if they knew how to handle alpha properly in any file format other than the one they created.