Transparent parts of texture are opaque black instead

Question

I render a sprite twice, one on top of the other. The sprites have transparent parts, so I should be able to see the bottom sprite under the top sprite. The transparent parts are black (the clear colour) and opaque instead though and the topmost sprite blocks the bottom sprite.

My fragment shader is trivial:

uniform sampler2D texture;
varying vec2 f_texcoord;
void main()
{
    gl_FragColor = texture2D(texture, f_texcoord);
}

I have glEnable(GL_BLEND) and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) in my initialization code. My texture comes from a PNG file that I load with libpng. I'm sure to use GL_RGBA when initializing the texture with glTexImage2D (otherwise the sprites look like noise).

Edit: Here's a screenshot.

Answer 1

What do you get if you try and swap the order you draw them in?

Transparent pixels still create depth buffer entries. If you're drawing the back one after the first it's not processing the pixels behind the front image, making it look like there's no alpha.

For reference, take a look at the Painter's alogrithm.

Answer 2

I don't think your textures have the desired alpha inside them, so calling gl_FragColor = texture2D(texture, f_texcoord) won't generate any transparent fragments. You should pass the alpha through as a varying from the vertex program to the fragment program if you want transparency.

EDIT: Teodron suggestion writing the following code to verify that alpha information is getting passed to the shaders (this is in the fragment shader and color is a varying which is assigned in the vertex program). vec4 color = texture2D(texture, f_texcoord); if (color.w < 1) gl_FragColor = vec4(1,0,0,1); else gl_FragColor = vec4(0,1,0,1);

Answer 3

If you know that those pixels have zero alpha and you want to discard them, a trivial solution is:

if (color.a <= 0.0) {
    discard;
}

There are better options probably, but this does work. There is no claim that this is good for performance on my behalf however.