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Subpasses, alpha blending, FPK/Early ZS

Ivan Kozlovskikh

Hi! We're still in the process of deciding how to better apply subpasses to our application. From other vendors we heard that render passes with transparency may disable hidden surface removal optimization, and Mali docs state that

Blending has a significant impact on performance since blending disables many of the important optimizations that remove fragment overdraw, such as early ZS testing and FPK. 
The negative impact is especially noticeable for user interfaces and 2D games that use multiple layers of sprites.

Can you please elaborate on that, specifically:

  • What is the trigger for optimizations disable? Does driver scan render pass for render pipelines with blending enabled and disables FPK/EZS if it detects any?
  • If subpass fusion is taking effect, all the drawcalls inside different subpasses become a part of a single render pass; does that mean that even if we draw blended objects late in the pass, all of the drawcalls will execute without FPK/EZS?
  • If FPK/EZS are disabled, what other metrics in Streamline, other than FPK/Early ZS killed quads should we look for?
  • If subpass fusion is disabled by the phone vendor, does that still affect FPK/EZS for all of the render pass, or only the subpasses that contain transparent geometry?