Alpha masks with OpenGL

Question

I want to use an alpha mask in OpenGL so that white(1)=visible and black(0)=hidden.

So what I do is I write something in the alpha component of the framebuffer using glColorMask(False, False, False, True) (I'm using python, you see) and then draw some geometry above it using blending.

But it isn't working: I tried filling the alpha buffer completly with 0 and then drawing some geometry that should thus not be visible. But it always shows up, the alpha buffer is completly ignored.

# Clear alpha buffer to 0, and clear color buffer.
# After this, the alpha buffer should probaby be filled with 0.
glClearColor(0, 0, 0, 0)
glClear(GL_COLOR_BUFFER_BIT)

# Disable blending.
glDisable(GL_BLEND)

# Disable color writing.
glColorMask(False, False, False, True)

# Set color to a white with alpha 0.
glColor4f(1, 1, 1, 0)

# Now draw a fullscreen quad.
# After this, the alpha buffer should really be filled with 0.
# Shouldn't it?
glBegin(GL_QUADS)
glVertex2f(0, 0)
glVertex2f(320, 0)
glVertex2f(320, 480)
glVertex2f(0, 480)
glEnd()

# Enable color writing.
glColorMask(True, True, True, True)

# Enable blending so that incoming fragments are multiplied
# by alpha values already in the buffer.
glEnable(GL_BLEND)
glBlendFunc(GL_DST_ALPHA, GL_ONE)

# Set color to a white with alpha 1.
glColor4f(1, 1, 1, 1)    

# Now draw a triangle.
# It should not be visible because alpha in framebuffer is 0
# and 0 * 1 = 0.
glBegin(GL_TRIANGLES)
glVertex2f(20, 50)
glVertex2f(300, 50)
glVertex2f(160, 210)
glEnd()

(Yes, the projection matrices are right so my screen ranges from 0/0 to 320/240.)

The triangle shouldn't be visible, what did I do wrong?

Answer 1

Use glAlphaFunc( GL_GREATER, 0.5 );

Answer 2

Try asking for an alpha buffer when you create your GL context, if you aren't already.