How do I choose which alpha chanel is used in 32bit textures?

I finally upgraded to Photoshop CS2, and everything is fine with one exception; If I have an image with multiple alpha channels, and I export to a 32 TGA file, no transparency information is included. If I remove all but one of the alpha channel sit works fine.

How can I choose which alpha channel will be used to control transparency in the output? I'd really prefer not to have to delete all my extra channels in every image.

Don't know if it's (easily) doable and can't find such option at the moment, but i'd simply save the original file in Photoshop format to keep all alpha channels intact, then whenever needed 'export' separate version of that file for SL, by combining/deleting the extra alpha channels from the original and saving the result as Targa image...

There's no way to choose. TGA is only meant to support one alpha channel. It's not intended to be an archival format in any way. It's an output format, period.

For a loose analogy, think of TGA for image output the same way you might think of a MOV file or even a DVD for movie output. Let's say for a minute that you're a film maker instead of a texture artist. While you're in the process of making your movie, you're gonna be working with raw footage, special effects, editing tracks, etc. You wouldn't shoot straight to DVD and then go "how come I can't edit this?" You'd use the appropriate source format/material every step of the way in the process of creating the movie. Then, once you're done, and you want to show your movie to your friends, you wouldn't dream of hauling your whole editing suite over to their house; you'd just show them the finished movie, the output format, meaning you might bring a DVD to their house or you might E-mail them a MOV file. They don't need to see every step in how the movie was made; they just need to see the finished product

Well, as a texture artist, your PSD file is your raw footage, your special effects, etc. Photoshop is your editing suite. While you're working on your image, you've got all the individual elements, layers, channels, masks, etc, that all add up together to form the whole image, but when it comes time to show that image (meaning upload it to SL), you don't want or need all that source stuff any more. What you want now is a nice, clean, simple output format with as little constructionary data in it as possible. That's what TGA is. SL doesn't need to see every layer and every channel any more than your movie audience would need to see every edit and every overlay. All SL needs to know is what color is each pixel, and how transparent is it. So that's the only data the TGA contains. Try to cram in anything more, and it won't work, as you've discovered.

The good news is since TGA is one of the oldest and THE most commonly used file format for texturing, common workflow technique for it is well established. If you use more than one alpha channel in your creation process, and you want/need to preserve them, the common thing to do is to copy them onto hidden layers, and then delete the extra channels as you go.

For example, let's say you've got an alpha you're using as a lighting map for LayerX. Well, after you apply the lighting effect, copy the channel to a new layer called "LayerX_Lightmap" or something, and then delete the channel itself. Just keep LayerX_Lightmap turned off, and it won't affect the image in any way. However, should you ever need to re-use the lightmap, all the information is there, preserved in that hidden layer, and you can copy it back to a channel at any time.

This of course is all assuming you're using alphas for purposes other than transparency. If you're not doing that, then there's no point whatsoever in EVER using more than one alpha channel anyway.

I wonder if I could ask Nepenthe's question in a slightly different way.

The question relates to Photoshop.

I often want to create two tga's, one of which differs from the other only because the alpha channel is different.

I maintain a psd file with a number of RGB layers, which I assemble in different ways. For example, I select layer 1,2,3 and save it as a tga file, then I select layer 1,2,4 and save it as a tga file, etc.

I know I can have a couple of different alpha channels in a psd file (but not a tga file), but whenever I have tried to use the same trick with alpha channels (e.g., select alpha 1 and save the image as a tga, then select alpha 2 and save image as another tga) it doesn't work. Instead, I have to keep ONE alpha channel, and change it by copying something to it.

Is there a way to choose one of a number of alpha channels before saving it as a tga, in the same way as I can chose several rgb channels?

Lee's Question is the same as mine, sorry if that wasn't clear.

I have a .PSD with multiple alpha channels. I want to save this to a .TGA file with a single alpha channel, chossen from the list of alpha channels in the original .PSD. The reason I have multiple alpha channels is the .PSD file contains both skins and clothes, and there is an obvious difference in the alpha channels needed for these.

My current workaround involves creating a seperate action for each alpha channel which duplicates the image, removess all but one alpha channel, and then saves the .TGA. Annoying, but usable for now.

I think Chosen answered that.

The answer is that a targa can't have more than one alpha channel in it, there's no way to directly do what you're hoping to. Sorry.

The workaround that you've got will work just fine.

The method Chosen was suggesting was rather similar. You make copies of all the channels you want and save them as layers. When it comes to selecting the final alpha channel you select amongst the archived layers, copy and paste the right one into the alpha channel, hide your other archive layers, just as you hide the template layers, save. Boom, only one alpha channel (to go with one red, one green, one blue) and photoshop and SL are both happy. It's a wee bit more work for you, or rather different work and without trying them I'm not sure which is the fastest way to go, but they seem much of a muchness to me - except Chosen's method means if you press save at the wrong time you've not lost your stuff.

Remember: There Can Be Only One.

The trick that I use, to keep several "Alpha" choices in my files is to create the Alpha in the normal way, then create a New Layer, go to Select > Load Selection, and Fill the selection (Option/alt+Delete) with some color or other. (I often color-code them, for easy selecting later.)

That puts a perfect copy of the Alpha on the layer. I name it for whatever alpha it is (in case I get brain dead, and forget the color coding.) Now I have it, forever. (Would this be the elusive Alpha Layer? (Please don't hit me, Chosen! ))

To use it, all I have to do is Command/ctrl click on the thumbnail image for that layer, and go to Select > Load Selection. I Replace whatever Alpha is there, and I'm all set.

I leave all these layers Hidden, all the time, since you don't need to Show a layer to use the Command/ctrl-click-the-thumbnail trick. That way, none of them are ever accidentally left showing.

Makes it simple to switch which Alpha I need, for whatever I'm doing. For things like my Hawaiian shirts, which have 2 different Alphas for the top, and 4 for the bottom, having all of them on the template saves all sorts of grief.

Hope this helps!

Edited to include remark about leaving the "Alpha" layers hidden at all times.