glEnable(GL_ALPHA_TEST) gives invalid enum (seems to be depreciated - code works though - but why?)

Question

Quick question - title says it all:

In my OpenGL-code (3.3), I'm using the line

glEnable(GL_ALPHA_TEST);

I've been using my code for weeks now and never checked for errors (via glGetError()) because it works perfectly. Now that I did (because something else isn't working), this line gives me an invalid enum error. Google revealed that glEnable(GL_ALPHA_TEST) seems to be depreciated since OpenGL 3 (core profile?) or so and I guess, that is the reason for the error.

But that part of the code still does exactly what I want. Some more code:

glDisable(GL_CULL_FACE);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_ALPHA_TEST);
// buffer-stuff
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);

glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 9, NumParticles);

So, did I put something redundant in there? I'm drawing particles (instanced) on screen using 2 triangles each (to give a quad) and in the alpha-chanel of the particle-color, I'm basically setting a circle (so 1.0f if in the circle, otherwise 0.0f). Depth-testing of course for not drawing particles from the back infront of particles further in front and glBlendFunc() (and as I understood glEnabled(GL_ALPHA_TEST)) for removing the bits not in the circle. I'm still learning OpenGL and am trying to understand, why that code actually works (for once) and why I apparently don't need glEnable(GL_ALPHA_TEST)...

Answer 1

Yes, I'm using discard in the fragment shader. Otherwise, I just used to code above, so I guess, only one depth value (standard?).

discard is the replacement for glEnable(GL_ALPHA_TEST);.

So, did I put something redundant in there?

Yes discard and glEnable(GL_ALPHA_TEST); would be redundant if you use a profile for which glEnable(GL_ALPHA_TEST); still exists and if you use discard for every fragment with an alpha for which the glAlphaFunc would discard that fragment.

Since you are in a profile for which the glEnable(GL_ALPHA_TEST); does not exist anymore, the glEnable(GL_ALPHA_TEST); has no effect in your code and can be removed.

Answer 2

Alpha test is a (since ages deprecated) method to only draw fragments when they match some alpha function. Nowadays this can easily be done inside a shader by just discarding the fragments. Alpha testing in itself is also very limited, because it can only decide to draw a fragment or not.

In general, enabling GL_ALPHA_TEST without setting a proper glAlphaFunc will do nothing since the default comparison function is GL_ALWAYS which means that all fragments will pass the test.

Your code doesn't seem to rely on alpha testing, but on blending (I assume that since you are setting the glBlendFunc). Somewhere in your code there's probably also a glEnable(GL_BLEND).