Do Not Use Alpha Blend Unnecessarily

Be sure alpha blending is used only when required to make the most of deferred architectures and to save bandwidth.

Disable alpha blending wherever possible. If transparent objects are required, keep the number of transparent objects to a minimum. The reasoning behind this is that deferred renderers, such as PowerVR graphics cores, calculate the visibility of fragments before the corresponding fragment shader is invoked to process it. This prevents invisible fragments in the output image being processed unnecessarily.

If alpha blending is enabled, then the hardware used to determine a fragment’s visibility cannot be used. This is because the occluded (alpha-blended) fragment may impact the final rendered image. Due to this behaviour, enabling alpha blending eliminates the benefits of deferred rendering graphics architectures. This means the hardware is no longer able to make decisions about a fragment’s visibility and drop it from the pipeline. This will likely result in overdraw which is where fragments are being processed that are not actually visible in the final image. Overdraw can negatively impact the application’s performance, particularly if the application is already limited by rendering.