Targa alpha differences between Photoshop 7 and CS3

Question

We're using Targa images for games work and need precise control over alpha levels.
In Photoshop 7, it seems that grey levels in the alpha channel are transferred directly to that Targa. For example, a grey level of 50% in the alpha channel produces an alpha value of 128 in the Targa file, as you would expect.
In Photoshop CS3, I'm getting 105 for 50% grey. And in order to get 128 I have to set my Photoshop colour to #959595.
This is a serious incompatibility for us. We can no longer generate correct alpha using Photoshop gradients. Also, some of us are using 7 and some CS3, and we need automation to work! There seems to be some sort of curve being applied to the alpha.
Firstly, does anyone else see this? I'm beginning to doubt my sanity.
Is there a way to fix it? When I say 128 I want 128!
(I've searched the forums and knowledgebase for "targa alpha" and Google for "photoshop targa alpha" and turned up nothing relevant.)

Answer 1

progress, I know that the values are often the same when you look at them in Photoshop, but they are not stored in the TGA as those values. Also, because the range 0-255 for alpha is mapped through a colour management curve, the values are NOT always preserved by save/open, but rounded.
Proof: using colour settings "North America General purpose 2" make a small (e.g. 32x32) image with white background. Make an alpha channel. Set your foreground colour to 62, 62, 62. Fill the alpha channel. Save as TGA, open, check the alpha channel. It's now 61, 61, 61.
If you save a TGA with alpha channel 61 grey, then try to change it to 62, then save it, you get an identical file.
But really the point is this: alpha is not colour. It is transparency. It should not be subject to colour management. You say "the visual is maintained" but alpha is transparency, not colour, and the visual is _not_ maintained if it is transformed through colour management. This is because we are trying to manipulate images where "128 alpha" means exactly 50% transparency, as you would expect. But with Photoshop this is not what you get _in_the_file_.
Ramón, if Chris means that Photoshop writes "128" in the file for "grey 128" in the alpha channel then he's wrong, as shown by my hex dumps earlier in this thread. This is why I wonder what he means.