Photoshop and image alpha channels.

I cannot find much on the web about it, I mostly see tutorials on how to use an alpha channel. However, none of them address this issue.

I use photoshop cs3 extended.
I am using png and tga image formats

When using an alpha channel, you can set the alpha to absolutely transparent. However, when you do and save the image it removes the other channel information on the rgb channels. This is problematic because I have to use a 1 transparency value to retain my rgb channels. This causes issues as I need the full alpha but also the texture information.

This is for texture work on a game and the engine uses the alpha channel for a team color overlay on top of the rgb image. So I need all 4 channels rgb for texture, and the alpha is a team color overlay.

Is there a setting in photoshop to fix this issue? Is it a setting I am missing when saving?

Check to make sure those formats will allow saving layers. Was a similar post recently. Do a search. Seems like I remember png did not work, have to use psd, tiff or ?.

"Steve_ (smoth)" wrote in message

I cannot find much on the web about it, I mostly see tutorials on how to use an alpha channel. However, none of them address this issue.
I use photoshop cs3 extended.
I am using png and tga image formats

When using an alpha channel, you can set the alpha to absolutely transparent. However, when you do and save the image it removes the other channel information on the rgb channels. This is problematic because I have to use a 1 transparency value to retain my rgb channels. This causes issues as I need the full alpha but also the texture information.
This is for texture work on a game and the engine uses the alpha channel for a team color overlay on top of the rgb image. So I need all 4 channels rgb for texture, and the alpha is a team color overlay.

Is there a setting in photoshop to fix this issue? Is it a setting I am missing when saving?

Steve,

From an earlier thread;
"..Targa files when saved have a "bad" habit. If the file is saved and there is only one layer (and it's transparent) or it has a layer under it not active, Targa files default and put a white background under it. Bitmap (.bmp) files are similar but only a few programs will use the alpha channel when it opens. Jpegs (.jpg) files do not save with alpha channels.

You should have no problem with transparency saving as a PNG file.

TWK

I was unable to find such a post earlier. I also have to use either png or tga.

yeah I did a forum search and mostly it was people arguing that PNG doesn't support an 8 bit alpha channel. which is idiotic because it does.

Does anyone have any idea? I do not understand why alpha channel full transparency in photoshop destroys the rgb channels on those pixels, anyone?

On Tue, 29 Apr 2008 11:06:27 -0700, Steve_ wrote:

....

Does anyone have any idea? I do not understand why alpha channel full transparency in photoshop destroys the rgb channels on those pixels, anyone?

Photoshop uses pre-multiplied alpha for external image formats that support alpha, so RGB color channel values will be zero for transparent areas.

Targa format will save an explicit alpha channel along with the complete RGB data.

--
Mike Russell - http://www.curvemeister.com

I'll try again with TGA, is there any way to make png work? In the end I will be looking at over 400 textures on the game models alone, so I am trying to use something that will be smaller in size(tga gets pretty huge)

To get a smaller size file it has to throw away something.

Ok, so I tried a TGA this weekend and with the way my texture is done cs3 screws up the alpha unless I flatten before saving. PS6 did not do that, something is wrong with cs3's tga saving.

Adobe changed the way Targa files worked a couple of versions ago, claiming to fix them, but not realizing how they were actually used in the real world. There is a retro patch to restore the old behaviour. I don't remember exactly what it's called, but it's available on Adobe's main web site. Search for Targa.

thanks peter. Is adobe going to fix this on a more permanent way without some extra patch?

Steve,

Their claim was that they did fix it - the way targa files dealt with alpha channels was wrong all along - only they forgot to consult the real world to see how people had adapted to this "wrong" way of working. My guess, and it's only a guess, is that in their minds it's fixed and you'll have to use the patch to unfix it to your liking.

We frequently use targa's placed in Illustrator files and we need to be able to silhouette them with an alpha channel. We recently moved to CS3 for both Photoshop and Illustrator and of course the alpha's do not function the way they used to.

What I found in experimenting for a workaround is that Illustrator CS does in fact see the alpha's in .tga files, and we can even open the Illustrator CS files in CS3, and the transparency is preserved.

So for our purposes, it appears the problem lies with Illustrator and not Photoshop, though I'm not discounting the problems others are having with these issues.

I've also found that Illustrator either version) does NOT see the alpha's in .png's, .psd's or .tif's. But I don't think that's changed from previous versions afaik.