Unfortunately there's no blend mode for division, so you will have to do this manually (pixel by pixel). You might want to create a DLL that can do this for you if the image is very large.
- is there a way to "un-premultiplied" (de-premultiplied?) the png saved by surface_save(), with GM ? Because the png saved is premultiplied, and I would like to use it on a non-premul layer (surface).
If the surface was created correctly, drawing it with draw_set_blend_mode_ext(bm_one, bm_inv_src_alpha) should work correctly. However, if you're using draw_surface_ext (or similar) and you're using the alpha argument, you have to premultiply the color argument too. Otherwise it won't work properly.
- When I use draw_set_blend_mode_ext(bm_one, bm_inv_src_alpha), the alpha of the surface isn't working as a normal alpha, which is logical because the surface isn't drawn in a "normal" blend mode. How can I have an alpha working correctly ?