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What makes full screen alpha blending so expensive on mobile devices?

Discussion in 'Shaders' started by bfogerty, Apr 23, 2014.

  1. bfogerty

    bfogerty

    Joined:
    Nov 12, 2010
    Posts:
    58
    I have heard that doing full screen transparency effects on mobile devices is not reccomended due to performance issues. Why is this? Is it because you are creating 2 render textures and blending each pixel? Is there any other task going on behind the scenes that makes this costly? Also, would it generally be ok to run a full screen single pass blur post processing effect without dropping frame rate too much on mobile device? Thanks!
     
  2. Gibbonator

    Gibbonator

    Joined:
    Jul 27, 2012
    Posts:
    204
    I don't think alpha blending has any additional costs other than overdraw and ROP blending. Alpha test performance is poor on mobile GPUs that use tile based rendering (eg. PowerVR). I'm not sure what the exact details are, I think it's something to do with the way depth data is stored. For a fullscreen blur on mobile I found it much quicker to render the scene to a smaller render texture, blur that, then blit it to the main render texture.