[BUG] Hair alpha dithering glitches in 4.17

Development Rendering
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1

After switching from 4.15 to 4.17 we noticed that hair, especially short hair, is rendering much worse.
Dither glitches appear in places where opacity is set to 0.
Below comparison of rendering hair on photorealistic human sample (lef 4.15, right 4.17):

204062-hairrenderingdifference.jpg
204062-hairrenderingdifference.jpg1403×815 239 KB

And how it looks like in our case:

204063-image-2017-08-10-14-30-41-307.png

2

Hello Gomo,

Thank you for letting us know of this issue, we have created a ticket for this issue which you can view here:

Cheers!

3

For some reason using dithering node resolved this issue in Photorealistic Character sample but not in our implementation, so in some cases it helps.
I’ll investigate it further.

4

To fix, disable dithering in material details and replace it with a dithering node.

5

In “Engine\Shaders\Private\MaterialTemplate.ush”
search for GetMaterialCoverageAndClipping
and comment out

float2 DepthGrad = {
    ddx( Parameters.SvPosition.z ),
    ddy( Parameters.SvPosition.z )
};
Dither = frac( Dither + dot( DepthGrad, float2( 4093, 3571 ) ) );

Seems to be doing some sort of filtering? Maybe someone can explain why this was added.
After in console type in the command "recompileshaders changed"

6

Thanks, I’ll check this out.

7

AFAIK we experienced this bug when Opacity Mask Clip Value was different than default.

8

Hey mate. Go into project settings, anti-aliasing method inside the engine rendering tab, it should be set to either Temporal Anti-Aliasing (TAA) or Temporal Super-resolution (TSR). Also make sure to activate Anti-Aliasing.