[Solved] Rendering to a specific Texture2DArray slice

Hi there!

I'm currently trying to build a post-effect shader, attached to a camera, based on a number N of previously rendered frames.
To store them, I'd like to use a RenderTexture with a single Texture2DArray color render buffer. This Tex2DArray would then be used as a circular buffer: at each rendered frame, the index i in the Tex2DArray is incremented (modulo N frames), the rendered frames is copied in the ith slice of the Tex2DArray, and finally the whole Tex2DArray is processed in a subsequent shader.

My problem is that in the final shader that uses the Tex2DArray, it seems that only the slice 0 is valid, and it is always the last rendered frame (so rendering in a specific slice does not seem to work in my case, I always write in the first one).

I've read the documentation (https://docs.unity3d.com/Manual/SL-TextureArrays.html) and related answer (https://docs.unity3d.com/Manual/SL-TextureArrays.html), but neither helped me solve my problem.

Here is a simplified version of the code I use (where I scrapped obvious inits, runtime re-allocs, etc.) ; if someone could point out what I'm doing wrong, it would be great ! :)
The shader at the end is supposed to render N vertical stripes on the screen, each stripe being extracted from one specific slice of the Tex2DArray. However, in my renders only the first stripe (slice 0) is not black, and is contains always the last rendered frame.

Also, I'm pretty new to Unity / C# / D3D, so if I wrote something silly / pointless / etc. also not directly related to my problem, feel free to correct me! :)

Thanks a lot in advance for your time!

C# script attached to the camera

 public class MyClass : MonoBehaviour
 {
     [...]

     private int _samples = ...            // number of frames in Tex2DArray
     private RenderTexture _frames = null; // will hold the Textute2DArray
     private Material _matProcess = null;  // material used to process the whole _frames

     void OnEnable()
     {
         // Testing for 2D array textures support, creation of Material with the final shader, etc.
         [...]
     }

     [...]

     void OnRenderImage( RenderTexture source, RenderTexture destination )
     {
         // Power of two dimensions needed for Tex2DArray
         int potw = Mathf.NextPowerOfTwo( source.width );
         int poth = Mathf.NextPowerOfTwo( source.height );

         if ( _frames == null )
         {
             // RenderTexture creation without a depth buffer
             _frames = new RenderTexture( potw, poth, 0, RenderTextureFormat.ARGB32 );

             _frames.dimension = UnityEngine.Rendering.TextureDimension.Tex2DArray;
             _frames.volumeDepth = _samples;

             _frames.useMipMap = false;
             _frames.autoGenerateMips = false;
             _frames.filterMode = FilterMode.Point;

             // Given https://docs.unity3d.com/ScriptReference/Graphics.SetRenderTarget.html
             // I tested to the sRGBWrite flag, without success
             // GL.sRGBWrite = false;

             _frames.Create();
         }

         // Record last frame
         if ( _lastFrameCount != Time.frameCount )
         {
             _lastFrameCount = Time.frameCount;

             _lastFrameIdx = ( _lastFrameCount % _samples );

             // Setting the render target to the proper Tex2DArray slice
             Graphics.SetRenderTarget( _frames, 0, CubemapFace.Unknown, _lastFrameIdx );

             // Blitting the source frame in the Tex2DArray
             // Here I also tried to blit with a specific shader that reads the input texture
             // with no more success (just in case)
             Graphics.Blit( source, _frames );

             // I also tried to use instead an explicit copy texture, but end up with an error:
             // "Graphics.CopyTexture can only copy between same texture format groups
             // (d3d11 base formats: src=9 dst=27)"
             // Graphics.CopyTexture(
             //     source, 0, 0, 0, 0, source.width, source.height,
             //     _frames, _lastFrameIdx, 0, 0, 0
             // );
         }

         // Finally, post-process all previous frames

         // Side question: does it need to be set at each rendered frame?
         _matProcess.SetInt( "_samples", _samples );
         _matProcess.SetTexture( "_frames", _frames );

         Graphics.Blit( source, destination, _matProcess ); // 'source' not used directly
     }
 }

A simple shader that uses the Tex2DArray, that actually display N vertical slices from the Tex2DArray.

 Shader "Hidden/Tex2DArrayProcess"
 {
     Properties
     {
         _frames( "frames", 2DArray ) = "white" {}
         _samples( "samples", Int ) = 8
     }
     SubShader
     {
         // No culling or depth
         Cull Off ZWrite Off ZTest Always
         Pass
         {
             CGPROGRAM

             #pragma vertex   vert
             #pragma fragment frag
             #pragma target   3.5
             #include "UnityCG.cginc"

             //------------------------------------------------------------------------------- //

             struct vertInput
             {
                 float4 vertex : POSITION;
                 float2 uv     : TEXCOORD0;
             };

             struct vertOutput
             {
                 float4 screenPos : SV_POSITION;
                 float4 uvs       : TEXCOORD0;
             };

             //------------------------------------------------------------------------------- //

             UNITY_DECLARE_TEX2DARRAY( _frames );

             int _samples;

             //------------------------------------------------------------------------------- //

             vertOutput vert ( vertInput v )
             {
                 vertOutput o;

                 o.screenPos = UnityObjectToClipPos( v.vertex );

                 o.uvs = float4( v.uv, 0.0, 0.0 );

                 return o;
             }

             //------------------------------------------------------------------------------- //

             fixed4 frag( vertOutput vInput ) : SV_Target
             {
                 float3 uvs = vInput.uvs.xyz;

                 // We select the slice based on horizontal tex coord, leading to vertical stripes
                 // from different slices
                 uvs.z = floor( uvs.x * _samples );

                 return UNITY_SAMPLE_TEX2DARRAY( _frames, uvs );
             }

             ENDCG
         }
     }
 }