Do transparent pixels cause a stencil buffer operation (increment, decrement, etc.) to be executed?

My understanding was that only opaque pixels cause a stencil buffer operation, transparent pixels have no effect on the buffer. However in my application transparent pixels do seem to cause the stencil buffer to be written.

Do I have to set up a special blendstate in order to achieve transparent pixels to have no effect?


DirectX9 was the last revision to support alpha-testing.

For DirectX10+:
In fact, transparent pixels do have an influence on the stencil buffer. In order to avoid altering the depth/stencil, you omit the pixel:

if (pixel.a < 0.1) discard;

clip(pixel.a < 0.1f ? -1 : 1); //omit if parameter evaluates to less than 0

clip(pixel.a - 0.1f);

This uses 0.1f to determine "close enough" similar to float.Epsilon.

Be aware: This disables early-z testing and may cause significant and/or unacceptable performance loss.

  • 2
    \$\begingroup\$ Sounds right. "Transparent" pixels by default update all ouput buffers, unless manually discard them or you have alpha testing on. \$\endgroup\$ – Sean Middleditch Feb 5 '13 at 22:13
  • 1
    \$\begingroup\$ As a minor note, testing if a == 0 might be a bad idea because of texture sampling interpolation. You should do if (a < 0.01) or so, or even define a higher cutoff value like 0.5. \$\endgroup\$ – maul Feb 5 '13 at 22:15
  • \$\begingroup\$ thanks for both of your comments. @Sean: If i understood correctly, alpha testing has been removed in DirectX 11, so using the shader is the only way? \$\endgroup\$ – thumbmunkeys Feb 5 '13 at 22:47
  • \$\begingroup\$ guess so. shaders are the future. \$\endgroup\$ – Sean Middleditch Feb 6 '13 at 1:50
  • \$\begingroup\$ @Sean: right, it is just that an if in a shader always makes me a bit nervous. But according to this, probably there is no reason to be: stackoverflow.com/questions/5340237/… \$\endgroup\$ – thumbmunkeys Feb 6 '13 at 10:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.