Converting a GM8->GM7 file right now scrambles the transparency considerably. Understandable, but I have a couple of suggestions to make converted sprites less unrecognizable.
1. When removing the transparency channel, convert all fully transparent pixels to the same color. Probably that default GM7 green, as GM users will understand this.
2. If no fully transparent pixels exist, save the sprite as non-transparent in the gm6 file. This will fully preserve any GM7>8>7 conversion.
3. Provide an option to replace the bottom left pixel with the transparent color. This will minimally damage the sprite while still saving transparency.
4. Alternatively, have an option to add a transparent row to the sprites. Adjust the bounding box accordingly.
I think number 1 and 2 are the most important. Number 3 and 4 help also, too, but I feel they should be optional.
useradd
more /root/Bugs/GM8 Transparency Suggestions.thread
Another option would be to split the colour and alpha channels and generate a script that makes use of sprite_set_alpha_from_sprite. For fully automatic conversion, you could insert a call to the script at the start of the room creation code for the first room.