OpenGL - How to read Alpha from a texture

Question

I want the alpha value to be read for each pixel from the texture, so that some pixels completely disappear. The texture file(targa format) does contain the proper alpha channel.

Screenshot: http://i43.tinypic.com/2i79s1x.png

Here are the options I am using:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_BGR,  //changing GL_BGR to anything else doesn't do a thing :? also tried GL_BGRA.
GL_UNSIGNED_BYTE, targaImage);

I have also tried most of the combinations of parameters for the glBlendFunc but none achieves the effect, alhtough I might have skipped it. This is the one that gets the regular blending done right(based on the alpha from glColor):

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Rectangle drawing:

    glColor4f(1,1,1,0.5);
    mRect->Render();

If I set alpha to 1 it is fully opaque, but there is still white in the bottom right, meaning that the alpha is read from the texture but the white polygon beneath it is visible. So I need to make the polygon disappear somehow, but the texture to remain visible.

So that's how I achieve this in the picture. I have also experimented with this:

glAlphaFunc(GL_GREATER, 0.49);
glEnable(GL_ALPHA_TEST);

It only proves that the alpha of each ,,fragment'' of my rectangle is 0.5.

This texture file has a gradient that has full red around the blue circle in the middle, but the alpha goess from 0 in the top-left to full in the bottom-right(it's not the red color fading to white).

I would supply the whole code but it has more than 2k lines and I have split everything into classes, so I am just pulling out the parts I think are important.

Do I need my own shader to do this? I have only made my first contact with OpenGL and C++ a couple of weeks ago and I'm not into them yet, so if that's the solution I would appreciate a link to a tutorial that deals with alpha and GLSL.

Thank you :)

Answer 1

It looks like you're using the old, fixed function pipeline. With that you must properly configure the texture environment. Specifically you want the texture to modulate or replace the base color. Either is fine, but I presume replace mode is better suited for you.

After binding the texture set the mode using glTexEnvi, in your case specifically

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

Also I want to remind you, that you actually must enable blending (glEnable(GL_BLEND);)

If you look at the reference page for glTexEnv I think you easily grasp how ridiculously complex the state space for the fixed function texture environment pipeline became. I strongly suggest you don't bother with it and go directly for using the programmable pipeline, i.e. fragment shaders. Yes, their learning curve is significantly steeper, but with shaders you can actually write something legible like

#version 330
uniform sampler3D sampler_albedo;
uniform sampler2D sampler_diffuse;
out vec4 outcolor;

main()
{
    float albedo = texture(sampler_albedo, texcood);
    vec4 diffuse = texture(sampler_diffuse, texcoord);
    outcolor = mix(basecolor, diffuse.rgb, diffuse.a) * albedo;
}

instead of spending over 15 lines setting up the texture environment and register combiners.