Ruby, openGL : change texture luminosity - ruby

Question

I have some problems with OpenGL and luminosity. Let me explain you my problem :
I drew this "sprite" (it's only a plane here) with a code like that :
sprite.set_active
left, right, top, bottom = 0.0, 1.0, 1.0, 0.0
glPushMatrix
glTranslate(#position.x - 16, #position.y, #position.z)
glRotate(-90 -#window.camera.horizontal_angle, 0, 1, 0)
glScale(chara.width, chara.height, 32.0)
begin
glEnable(GL_BLEND)
glBegin(GL_QUADS)
glColor4f(1.0, 1.0, 1.0, 1.0)
glTexCoord2d(left, top); glVertex3f(0, 1, 0.5)
glTexCoord2d(right, top); glVertex3f(1, 1, 0.5)
glTexCoord2d(right, bottom); glVertex3f(1, 0, 0.5)
glTexCoord2d(left, bottom); glVertex3f(0, 0, 0.5)
glEnd
glDisable(GL_BLEND)
rescue
end
glPopMatrix
My problem is with that line :
glColor4f(1.0, 1.0, 1.0, 1.0)
Well, I can put a number lesser than 1.0 to have a darker sprite, but I can't do the contrary. How can I do that ? How can I make the sprite be totally white, for example ?

Answer 1

To get full control over your fragment processing, the best approach is using the programmable pipeline, where you can implement exactly what you want with GLSL code.
But there are some options that could work for this case in the fixed pipeline. The simplest one is using a different GL_TEXTURE_ENV_MODE. The default value is GL_MODULATE, which means that the color you specified with glColor4f() is multiplied with the color from the texture. As you found, that allows you to make the texture darker, but not brighter.
You could try using GL_ADD instead. As the name suggests, this will produce the final output as the sum of the texture color and the color from glColor4f(). For example:
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_ADD);
glColor4f(0.2f, 0.2f, 0.2f, 0.0f);
would add 0.2 to the color components read from the texture.
There is more complex functionality in the fixed pipeline that gives you more control over how texture values are used to generate colors. You can find it by looking for "texture combiners". But in my personal opinion, you're much better off moving to the programmable pipeline if you need something complex enough to require texture combiners.