I need to use GIMP instead of Photoshop, but I have a problem doing some things with transparency:
I explain: I manipulate some tga files that have 4 channels, rgb and alpha.
Using Photoshop I'm able to draw into the 4 channels independently, and the alpha is used like the other channel: here is a shot:
If I try to open the same tga image (32 bits compressed) with gimp:
you can see that the 3 channels rgb are masked with the alpha channel, and the image is uneditable...