Help needed on CustomPass pixel picking to RenderTexture

Hello! I could really use some help, I've been stuck on this for a few days now.

(marginally important context, I'm porting this project over to HDRP: https://andrea-intg.itch.io/clayxels ).

My aim is to render some flat colors on a texture and read them back at specific coords, this is to do pixel-picking on my proceduraly drawn primitives.
I had this working on the legacy pipeline using OnRenderImage, but on HDRP I'm really struggling to understand what I should be doing.

EDIT:
I first tried with a custompass, did not work but then someone suggested I should be using commandbuffers for this.
All I'd need to know is if there's something obviously wrong about this piece of code.

EDIT again:
I thought it worked, but I was just reading some other randomly active rendertexture.
After properly setting RenderTexture.active = pickingRenderTexture, I get only black pixels.

Code (CSharp):

1.  
2. Texture2D pickingTextureResult = new Texture2D(1, 1, TextureFormat.ARGB32, false);
3. Rect rectReadPicture = new Rect(this.mousePosX, this.mousePosY, 1, 1);
4.  
5. RenderTexture pickingRenderTexture = RenderTexture.GetTemporary(Screen.width, Screen.height, 0, RenderTextureFormat.ARGB32);
6.  
7. RenderTargetIdentifier rtID = new RenderTargetIdentifier(this.pickingRenderTexture);
8.  
9. CommandBuffer commandBuffer = new CommandBuffer();
10. commandBuffer.SetRenderTarget(rtID);
11. commandBuffer.ClearRenderTarget(true, true, Color.clear, 1f);
12.  
13. commandBuffer.DrawProceduralIndirect(new Matrix4x4(), myMat, 0, MeshTopology.Triangles, myIndirectDrawArgsBuffer);
14.  
15. Graphics.ExecuteCommandBuffer(commandBuffer);
16.  
17. RenderTexture.active = this.pickingTextureResult;
18. this.pickingTextureResult.ReadPixels(this.rectReadPicture, 0, 0);
19. this.pickingTextureResult.Apply();
20.  
21. Color32 pickCol = this.pickingTextureResult.GetPixel(0, 0);
22. Debug.Log(pickCol);
23.  
24. RenderTexture.ReleaseTemporary(this.pickingRenderTexture);
25.  

 

I'm personally very much looking forward to the HDRP compatible release of Clayxels so I'll second this request!

@Remy_Unity @SebLagarde or anyone?

 

The current status is as follows:
I get meaningful values from the rendertarget if I use CommandBuffer.DrawRenderer on some geometry, but not with CommandBuffer.DrawProcedural.
I've stripped down my shader to just draw a triangle, there's no indirect args involved, no structuredBuffer, to make sure everything checks out I'm also drawing the same shader from the same material with Graphics.DrawProcedural.

 

The final solution to my pixel-picking for HDRP had nothing to do with HDRP at all.
I ended up implementing the commandbuffer using this solution:
https://forum.unity.com/threads/help-getting-comandbuffer-drawprocedural-on-a-rendertexture.840412/