OpenGL alpha value makes texture whiter?

Question

I'm trying to load a texture with RGBA values but the alpha values just seem to make the texture more white, not adjust the transparency. I've heard about this problem with 3D scenes, but I'm just using OpenGL for 2D. Is there anyway I can fix this?

I'm initializing OpenGL with

glViewport(0, 0, winWidth, winHeight);
glDisable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDisable(GL_DEPTH_TEST);
glClearColor(0, 0, 0, 0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, winWidth, 0, winHeight); // set origin to bottom left corner
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor3f(1, 1, 1);

---

Screenshot: That washed out dotty image should be semitransparent. The black bits are supposed to be completely transparent. As you can see, there's an image behind it that isn't showing through.

The code to generate that texture is rather lengthy, so I'll describe what I did. It's a 40*30*4 array of type unsigned char. Every 4th char is set to 128 (should be 50% transparent, right?).

I then pass it into this function, loads the data into a texture:

void Texture::Load(unsigned char* data, GLenum format) {
    glEnable(GL_TEXTURE_2D);
        glBindTexture(GL_TEXTURE_2D, _texID);
        glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, _w, _h, format, GL_UNSIGNED_BYTE, data);
    glDisable(GL_TEXTURE_2D);
}

And...I think I just found the problem. Was initializing the full-sized texture with this code:

glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, _texID);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, tw, th, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, NULL);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glDisable(GL_TEXTURE_2D);

But I guess glTexImage2D needs to be GL_RGBA too? I can't use two different internal formats? Or at least not ones of different sizes (3 bytes vs 4 bytes)? GL_BGR works fine even when its initialized like this...

Answer 1

In the interest of others, I post my solution here.

The problem was that although my Load function was correct,

glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, _w, _h, GL_RGBA, GL_UNSIGNED_BYTE, data);

I was passing GL_RGB to this function

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, tw, th, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, NULL);

Which also needs to specify the correct number of bytes (four). From my understanding you can't use a different number of bytes for a SubImage, although I think you can use a different format if it does have the same number of bytes (i.e. mixing GL_RGB and GL_BGA is okay, but not GL_RGB and GL_RGBA).

Answer 2

Are there any overlapping primitives in your scene?

You are aware that you're calling the 3-parameter version of glColor, which sets the alpha to 1.0, right?

It could be helpful if you could post a screenshot, or otherwise describe what happens, say, when you draw two primitives with identical colors and differing alphas. In fact, any code demostrating the problem could help.

Edit:

I'd imagine that using TexImage with GL_RGB (for internalformat, the 3rd parameter) creates a 3-component texture with no alpha or alpha values implicitly initialized to 1, no matter what kind of pixel data you supply.

GL_BGR is not a valid value for this parameter, perhaps it is tricking your implementation into using a full 4-byte internal format? (Or a 2-byte one, as per GL_LUMINANCE_ALPHA) Or do you mean passing GL_BGR to your Texture::Load() function, which should not really be different from passing GL_RGB?

Answer 3

I think this should work, but it assumes the image has an alpha channel. If you try and load an image without an alpha channel you will get an exception or your application might crash. For non-alpha channel images use GL_RGB instead of GL_RGBA on the second parameter, right before setting the GL_UNSIGNED_BYTE.

void Texture::Load(unsigned char* data) {
  glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, _texID);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, tw, th, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
  glDisable(GL_TEXTURE_2D);
}