Discussion How to efficiently extract a render texture from the Built-In render pipeline?

Hi all,
this is more a question about whether I am doing this right.
It's working but I do worry about efficiency.

So, I have implemented an OnPreRender() and an OnPrenderImage(src, dest) method to get access to the image before any UI is rendered. What I want is to copy the "src" render texture to another (much smaller) render texture which I then apply some effect on and use elsewhere (I show it in the UI to emulate some translucency effects).

It works fine, yet I am not 100% sure this is the best way to do it.

Here is the gist of the code:

Code (CSharp):

1. Material MaterialWithSomeEffect;
2. RenderTexture MyRenderTextureThatIsUsedElsewhere;
3. RenderTexture _rt;
4.  
5. void OnPreRender()
6. {
7.     // This is done due to an ancient (2016) Unity forum thread which mentions that
8.     // if no render texture is assigned Unity will copy the texture pixel by pixel (which is slow).
9.     // See: https://forum.unity.com/threads/post-process-mobile-performance-alternatives-
10.     //      to-graphics-blit-onrenderimage.414399/#post-2759255
11.     // Q: Is this still necessary?
12.     _rt = RenderTexture.GetTemporary(Screen.width, Screen.height, 16);
13.     Camera.main.targetTexture = _rt;
14. }
15.  
16. void OnRenderImage(RenderTexture src, RenderTexture dest)
17. {
18.     // See on PreRender
19.     Camera.main.targetTexture = null;
20.     RenderTexture.ReleaseTemporary(_rt);
21.  
22.     // Bascially all I need is a copy of src.
23.     Graphics.Blit(src, MyRenderTextureThatIsUsedElsewhere, MaterialWithSomeEffect);
24.  
25.     // Since we did render into a render texture (see OnPreRender) we now
26.     // have to blit the contents to the frame buffer.
27.     // Q: Is that correct?
28.     Graphics.Blit(src, null as RenderTexture);
29.  
30.     // Otherwise we will get a "OnRenderImage() possibly didn't write anything
31.     // to the destination texture!" warning.
32.     Graphics.Blit(src, dest);
33. }

It looks okay in the frame debugger, but I wonder if maybe the final "Graphics.Blit(src, dest)" can be axed or somehow merged with the "Graphics.Blit(src, null as RenderTexture)".

It seems inefficient to do the same blit twice. I tried Graphics.CopyTexture() instead of the final blit, yet then I get a "Graphics.CopyTexture can only copy between same texture format groups (d3d11 base formats: src=9 dst=27)" error.

Anyways, thanks for reading it all
Here is the link that's mentioned in the comments.