glPoint Alpha Blending issue

Question

I am having issues alpha blending glPoints. The alpha blending works correctly when the glPoint is over the background but when a glPoint overlaps another glPoint the background color is visible rather then the underlying glPoint.

enter image description here

// create texture

    GLuint tex;
    glGenTextures(1, &tex);
    glBindTexture(GL_TEXTURE_2D, tex);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, spriteData);

    // Draw

    glEnable(GL_DEPTH_TEST);
    glEnable(GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glBindTexture(GL_TEXTURE_2D, tex);
    ....
    glDrawArrays(GL_POINTS, 0, numberOfPoints);

    // Frag Shader

    uniform sampler2D Texture; 

    void main(void) 
    {
        gl_FragColor = texture2D(Texture, gl_PointCoord); 
    }

What am i doing wrong?

Looks like the alpha value of your sprite texture isn't 0 for the parts you want to be transparent from here. — radical7, Sep 11 '13 at 03:26
The alpha is definitely 0. I added if(color.a < 0.5) color.a = 0; to make sure — user346443, Sep 11 '13 at 03:54

score 2 · Answer 1 · answered Sep 11 '13 at 04:37

It looks like the depth test is causing your issues. What happens is, as you describe, a point in front is drawn first and the depth value is replaced. Then the point behind is rasterized but all the fragments fail the depth test so nothing more is rendered/blended.

As Andon M. Coleman pointed out, you really need to sort the fragments in order of depth for correct alpha blending (exact order independent transparency is currently impractical for particles although you could try some of the approximate techniques. averaging all colour using alpha values as a weight can give decent results too).
Especially for the particle density you have and the lack of variation among particles there probably won't be much difference between sorting and not sorting. In this case, make sure you draw all the opaque stuff first, with depth testing enabled and then draw the particles. You want to keep depth testing enabled so your particles aren't drawn when they're behind opaque things however you don't want them to obscure each other - you want to use the depth buffer for testing but not write depth values. For this, use glDepthMask

@user346443 Sometimes for particles like these, you can even get away with an additive blend mode, depending on the desired effect. If this is supposed to be snow, then that would be a bad idea, but if they're little balls of light then `glBlendFunc (GL_SRC_ALPHA, GL_ONE)` would avoid having to depth sort. — Andon M. Coleman, Sep 11 '13 at 13:56

glPoint Alpha Blending issue

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