Do Not Use Discard

Avoid forcing depth-test processing in the texture stage as this will decrease performance in the early depth rejection architectures.

Applications should avoid the use of the discard operation in the fragment shader as using it will not improve performance. Most mobile graphics cores use a form of tile based deferred rending (TBDR) and using discard negates some of the benefits of this type of architecture. If possible, an application should prefer alpha blending over discarding.

Applications should also avoid alpha testing. When an alpha-tested primitive is submitted, early depth testing, such as PowerVR’s Hidden Surface Removal (HSR), can discard fragments that are occluded by other fragments closer to the camera. Unlike opaque primitives which would also perform depth writes at this pipeline stage, alpha-tested primitives cannot write data to the depth buffer until the fragment shader has executed and fragment visibility is known. These deferred depth writes can impact performance, as subsequent primitives cannot be processed until the depth buffers are updated with the alpha tested primitive’s values.

For optimal performance, consider alpha blending instead of alpha test to avoid costly deferred depth write operations. To ensure HSR removes as much overdraw as possible, submit draws in the following order:

1. Opaque

2. Alpha-tested

3. Blended.