DDX/DDY in post process when preview ES3.1

Hi,all
I am trying to use DDX/DDY to calculate normal in post process material,but it has like shadow acne happen when i using preview ES3.1, and it all good on SM5. any explain about this?
ES3.1
ES31
SM5
SM5

Seems like relate to precision of depth.
Mobile use Texture2DSample(MobileSceneTextures.SceneColorTexture, MobileSceneTextures.SceneColorTextureSampler, ScreenUV).a

the format is RGBA16

in deferred , it use Texture2DSample(MobileSceneTextures.SceneDepthTexture, MobileSceneTextures.SceneDepthTextureSampler, ScreenUV).r

the format is D32S8

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