Alpha Test
In the fixed-function pipeline, we use alpha testing to discard pixels.
java
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
alphaRefusingalphaFunc - 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
java
GlStateManager.enableAlpha();
GlStateManager.alphaFunc(GL11.GL_GREATER, 0.0f);Don't forget to restore states if needed, but be aware of the trade-offs.
java
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);
Nullpinter