alpha_test.md

In the fixed-function pipeline, we use alpha testing to discard pixels.

GlStateManager.alphaFunc(alphaFunc, alphaRef);

Here are alphaFuncs.

Constant	Meaning
GL_NEVER	Never passes
GL_LESS	Passes if alpha < alphaRef
GL_EQUAL	Passes if alpha == alphaRef
GL_LEQUAL	Passes if alpha <= alphaRef
GL_GREATER	Passes if alpha > alphaRef
GL_NOTEQUAL	Passes if alpha != alphaRef
GL_GEQUAL	Passes if alpha >= alphaRef
GL_ALWAYS	Always passes

And of course alphaRef is between 0.0 and 1.0.

When alpha testing is enabled, for each fragment (pixel):

• The pixel’s alpha value is compared with alphaRef using alphaFunc
• If it fails the test, the pixel is discarded. That is, not writing it to color buffer, no depth test, and no blend.

So, when you don't want to blend things and want a hard cutoff, you could do

GlStateManager.enableAlpha();
GlStateManager.alphaFunc(GL11.GL_GREATER, 0.0f);

Don't forget to restore states if needed, but be aware of the trade-offs.

boolean alphaTest = GL11.glIsEnabled(GL11.GL_ALPHA_TEST);
GL11.glGetInteger(GL11.GL_ALPHA_TEST_FUNC, intBuffer);
int alphaFunc = intBuffer.get(0);
GL11.glGetFloat(GL11.GL_ALPHA_TEST_REF, floatBuffer);
float alphaRef = floatBuffer.get(0);

// render

if (alphaTest)
    GlStateManager.enableAlpha();
else
    GlStateManager.disableAlpha();
GlStateManager.alphaFunc(alphaFunc, alphaRef);