Why my Blit( <RenderTexture> , <Material> ) doesn't work?

Can someone tell why this doesn't work:

Code (CSharp):

1. public void Blit ( RenderTexture rt , Material mat )
2. {
3.     Graphics.Blit( rt , mat );
4. }

...while this works just fine:

Code (CSharp):

1. public void Blit ( RenderTexture rt , Material mat )
2. {
3.     var copy = RenderTexture.GetTemporary( rt.descriptor );
4.     {
5.         Graphics.Blit( rt , copy );
6.         Graphics.Blit( copy , rt , mat );
7.     }
8.     RenderTexture.ReleaseTemporary( copy );
9. }

It's confusing because it throws no errors and documentation states no special requirements about this first overload.

 

What are you expecting Graphics.Blit(rt, mat); to do? If it's to replicate the code you have in the second option, that's not what it does. That option is equivalent to:

Graphics.Blit(rt, null, mat, -1);

It will do a blit to the current active texture target. You can't read from and render to the same render textures.* To apply a shader to a render texture you have to render the results to a new render texture, or copy the original and blit back. CopyTexture might be a faster option than a straight blit too.

* iOS devices can do it, but only using specially written shaders, and only from and to the same pixel. Compute shaders can do it using RWTextures, but there are a lot of gotchas, including the possibility of some reads being from already modified pixels meaning the shader won't act like you expect.

 

Oh, that makes so much sense now. Thank you for this clarification!