Is discard bad for program performance in OpenGL?

Question

I was reading this article, and the author writes:

Here's how to write high-performance applications on every platform in two easy steps:
[...]
Follow best practices. In the case of Android and OpenGL, this includes things like "batch draw calls", "don't use discard in fragment shaders", and so on.

I have never before heard that discard would have a bad impact on performance or such, and have been using it to avoid blending when a detailed alpha hasn't been necessary.

Could someone please explain why and when using discard might be considered a bad practise, and how discard + depthtest compares with alpha + blend?

Edit: After having received an answer on this question I did some testing by rendering a background gradient with a textured quad on top of that.

• Using GL_DEPTH_TEST and a fragment-shader ending with the line "if( gl_FragColor.a < 0.5 ){ discard; }" gave about 32 fps.
• Removing the if/discard statement from the fragment-shader increased the rendering speed to about 44 fps.
• Using GL_BLEND with the blend function "(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)" instead of GL_DEPTH_TEST also resulted in around 44 fps.

Answer 1

It's hardware-dependent. For PowerVR hardware, and other GPUs that use tile-based rendering, using discard means that the TBR can no longer assume that every fragment drawn will become a pixel. This assumption is important because it allows the TBR to evaluate all the depths first, then only evaluate the fragment shaders for the top-most fragments. A sort of deferred rendering approach, except in hardware.

Note that you would get the same issue from turning on alpha test.