[URP]Why does URP draw A "fullscreen Quad" instead of A "fullsceen Triangle" in postprocessing?

I found that in URP(7.5) , they draw a fullscreen quad to do postprocessing, while in HDRP and PPSV2, A Triangle is used for postprocessing。AFAIK， use a Triangle is more performance friendly, I wonder why URP doesn't use that Triangle version?

 

https://forum.unity.com/threads/use-a-giant-triangle-instead-of-quad-for-postprocessing.664465/ peripheral discussion but yeah it seems faster on some hardware.

 

Yeah, that's why I don't understand why URP does not use this TRIANGLE method like it in PPSV2

 

7.5 is quite old, did you check a newer URP? 2021.x versions of Unity control what version of URP is installed. I wonder if it has been addressed? 12 is the current.

 

Either way the performance difference will be extremely negligible. You're talking a single vertex and triangle.

 

The fullscreen triangle isn't really about optimizing the number of vertices rendered. It's about optimizing the small 2x2 (sometimes bigger) tile the GPU executes fragment shaders in. With a typical fullscreen quad you get a diagonal where the two triangles meet which causes the GPU tile to run fragment shaders on fragments that have already been processed so it does more work. With the fullscreen triangle you don't have that issue which means fragment shaders would run an optimal amount of times. This can make a measurable difference on mid to low end hardware and VR also. In my experience, a fullscreen triangle is no worse than a fullscreen quad but can be a decent improvement over it depending on what shaders are running.

 

Indeed and it's never usually in isolation. A post effect stack can do 10-20 blits without breaking sweat, and that could add up quite a lot for a VR or switch title as you say.

 

oh wow, that's wild, i'm a bit surprised that modern GPU hardware can't optimize around that but it's probably a hard problem