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); 

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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).