I'm making a program that manipulates the color channels of a bitmap separately. So I can paint on more red without changing any green, blue or alpha for example.

Here's code for painting on more red using the filled circle primitive.

al_set_target_bitmap(my_bitmap);
al_set_blender(allegro5.color.ONE, allegro5.color.ONE, al_map_rgba_f(1, 1, 1, 1));

allegro5.primitives.draw_filled_circle(x, y, 10, al_map_rgba_f(0.2, 0, 0, 1));

al_set_current_display(my_display);
set_blender(allegro5.color.ALPHA, allegro5.color.INVERSE_ALPHA, map_rgba_f(1, 1, 1, 1));

My problem now is that I wanna be able to reduce the amount of red too. But it seems there's only funcionality in place to let me add color.
Is there a way to erase some red from the bitmap?

I've been thinking about it, and can't find a way.

What extensions are you talking about?
And does anyone have a different solution?

EDIT: I do know it's possible to blend in a darker color over something brighter. But the regular way to do that is by using the alpha channel for translucency and that means it will pull down r, g and b.

I just wish I could do that but only affect one color channel. :/

(Emphasis mine)

glColorMask() turns a particular RGBA element off entirely

I'll take a look on colormask, maybe it can solve the problem. At least enough to meet my needs.

As for the shader route, I looked into that and it turns out blending takes place after your shader. So the blending stage isn't programmable and I've seen conflicting opinions about the readability of the rendering target which I think is required if I want to blend incoming fragments with what's previously on the target.

The rendering target should always be readable in a shader, AFAIK... what good would they be if not? So instead of doing the blending stage, you do your blending in the shader. Luckily they're very fast.

so...he should make a blender function himself with it.

I guess I can assume it has the getr/getg/getb and rgb_to_hsv etc.

Don't guess, just read the manual.

Anyway, yes, you probably can do it that way. But it doesn't play nice with the hardware, as Trent said. It's the Allegro 4 way of doing things, but Allegro 5 is (will be) a different thing entirely.

Maybe you could glCopyPixels to/from a memory buffer and fiddle with the red component on your own.

If you're doing all that, aren't you using lighting? Just change the lights...

Uh, why can't you do blending in a shader? I thought that was most of the reason for shaders. To implement awesome things that the fixed function stuff couldn't even dream of.

No I'm working on a heightmap editor. The heightmap uses the color channels of one texture (splat texture) to blend four different textures. (This is different blending, I'm not blending with the target, only the textures which are used for the fragment that comes into the shader)

So for the editor, I al_set_target_bitmap(splat_texture) and paint the color channels, which makes it look like I'm painting textures onto my heightmap.

Anyway, I've tried glColorMask now and it works perfectly. At least rgb works just as I want, now I just need to figure out how to paint the alpha channel.

The heights aren't controlled by any bitmap, only the texturing of it.
Here's an explanation.
http://www.gamedev.net/reference/articles/article2238.asp

For the height points I'm just using arrays now, haven't even gotten around to saving it to any file format.