Here is a mini tutorial I posted in the Avsim painter's forum a couple years back. The particular thread has long since vanished for all but the most advanced users of the search function, and the images I originally posted in context with the tutorial have been reduced to hyperlinks. Since I put a moderate amount of effort into typing this up and editing the screenshots, and I still feel it may be of use to someone at some point, I'd like for it to once again see the light of day. Thanks
I notice a lot of discussion about importing and exporting alpha channels in DXTBmp. There's really no need to do this if you're using PhotoShop to edit your textures. The alpha channel can be easily edited from within PhotoShop at the same time you're working with the main RGB channels. In fact I believe it's much easier to do it this way because you have the RGB channels to use as a guide when placing an alpha fill. This allows you to place your filled areas precicely where they need to be without guesswork or fiddling.
I've done up a small tutorial here that shows my method of editing alpha channels. I'm using PhotoShop 7, but I'm sure the basic procedure is the same using any graphics editor capable of working with alpha channels.
This is just a simple example I've created specifically for the purpose of explanation. I'm adding an alpha fill to the aluminum underbelly of this repaint to simulate polished aluminum in the sim. These are bits and pieces of a repaint I did for the iFDG A319 using the official iFDG paintkit.
Here I'm working on the right fwd part of the fuselage. So far I've copied two layers from my full-length master .psd; the base paintjob and the polished aluminum underbelly as a seperate layer. I've pasted them onto the "fuse.bmp" blank template that came in the paintkit, flipped them horizontally, and slid them into position over the template with the move tool.
I've added the aluminum underbelly as a seperate layer rather than merging it with the base paintwork because I'm going to use it's selection to fill the alpha channel in the appropriate area. First I must create an alpha channel because this .bmp template doesn't have one (24 bit). This is done in the "channels pallette", if you don't see the channels pallette, you must enable it in the PS "Window" menu.
When you create the alpha channel it will automatically be pure black so you must fill it. You can fill it with white, but in this case I'm going to use a very light gray (#F4F4F4) which will give my entire fuselage a bit of overall reflectivity. I set the foreground color to my desired shade of gray and then use "fill/foreground color" to fill the alpha channel.
Next I want to darken up the alpha channel in the area underlying my aluminum underbelly, to add extra reflectivity and make it look like polished aluminum in the sim. I click on the RGB channel in the channels pallette, then return to the layers pallette and click on the "underbelly" layer to select it. Now I will load it's selection from the "Select" menu.
(Alternatively you can simply click the "underbelly" layer in the layers pallette while holding down the CTRL key, but do explore the "Select > Load Selection" menu because there are a lot of possibilities for loading selections that aren't available through shortcut keystrokes such as "invert" or "intersect with selection")
With the selection loaded I now switch back to the channels pallette and click on the alpha channel. Notice the underbelly area is still selected, all I need to do now is fill the area with a darker gray. This image shows the selected underbelly on the RGB, and the corresponding area on the alpha channel after filling it with gray:
Doing it this way precisely places the alpha fill directly beneath the underbelly area on the finished texture .bmp. Attempting this with an exported alpha channel would obviously require either some guesswork or pasting the exported alpha channel onto the main .psd as a new layer in order to get the fill area positioned properly. The latter is pointless in my opinion when you're already using a graphics editor capable of working with alpha channels - you have the tools, use them.
Now I'll repeat the process for the right-rear, left-front, and left-rear portions of the fuselage, and save this file as a .psd. This enables me to re-open it and modify the alpha, or add an alpha fill to another part of the texture later if desired.
Once I'm ready to try out the texture in the sim, I simply re-open the .psd, flatten the image, and do a "Save as", saving it as a 32 bit .bmp with the appropriate filename for the particular model I'm working with. The .psd remains completely intact for future editing if desired.
Finally I open the saved .bmp in DXTBmp for a final save to make it DX compatible. Notice that you can now see the newly created alpha channel in DXTBmp's alpha preview window. No need to export or import anything. I then save it as either "Extended 32 bit 888-8" or "DXT3" and fire up the sim to see how it looks. This final save for DX compatibility is really the only reason I have DXTBmp installed. Everything else gets done in PhotoShop.
I hope that clears up some of the myths associated with importing and exporting alpha channels, and sheds some light on alpha editing in PhotoShop.