OpenGl alpha test - How to replace AlphaFunc deprecated?

Question

I'm trying to draw sphere with alpha, but I have a problem with my Z-buffer. Some pixels are transparent but write in the Zbuffer so a pixel opaque just behind is hidden.

Here are my settings:

gl Enable: gl DEPTH_TEST.
gl DepthFunc: gl LEQUAL.
gl Disable: gl CULL_FACE.
gl Enable: gl BLEND. 
gl BlendFunc: gl SRC_ALPHA with: gl ONE_MINUS_SRC_ALPHA.

I know the function glAlphaFunc(GREATER, aFloat) and enable(ALPHA_TEST) can do that but I read that it's a deprecated function since 3.1 version of OpenGL. How can I do a correct render without using ALPHAFunc? Does someone know a trick, or a way to do this via shaders?

In modern GL you're supposed to do that in fragment shader, e.g. if(result.a > threshold) discard;. — keltar
Your question is very similar to this: stackoverflow.com/questions/23280692/…. I'm not going to suggest it as a duplicate for now since it's a slightly different use case. But my answer in that question should still apply to your case. — Reto Koradi

Colonel Thirty Two Colonel Thirty Two · Accepted Answer · 2014-06-19T16:59:35

This is very easy to implement in a fragment shader:

uniform float alpha_threshold;

void main() {
    vec4 color = ...;

    if(color.a <= alpha_threshold) // Or whichever comparison here
        discard;

    gl_FragColor = color;
}

OpenGl alpha test - How to replace AlphaFunc deprecated?

1 Answers