Graphic artefacts when using GrabPass with custom camera Rect (Aspect Ratio enforced)

Hi,

I am currently observing graphical artefacts when using a shader with GrabPass in conjonction with an aspect ratio enforcer for my main camera...

First of all the AspectRatioEnforcer script, attached to the main camera:

public class AspectRatioEnforcer : MonoBehaviour

 {
         /// <summary>
         /// Additional "background" camera which allows setting a color for the 
         /// otherwise undefined "Letterbox" & "Pillarbox" zones
         /// </summary>
         private Camera backgroundCam;
 
         public float targetAR;
         private int lastScreenWidth = 0;
         private int lastScreenHeight = 0;
 
         void Update()
         {
             // Check the screen height and witdh & enforce aspect Ratio if they have
             // changed
             //-----------------------------------------------------------------------
             if ((Screen.height != lastScreenHeight) || (Screen.width != lastScreenWidth)) 
             {
                 lastScreenWidth        = Screen.width;
                 lastScreenHeight     = Screen.height;
                 EnforceAR();
             }
         }
 
         /// <summary>
         /// Forces the camera to use specified aspect ratio
         /// </summary>
         private void EnforceAR()
         {
             // Retrieve current window's AR & compare to target one
             //-----------------------------------------------------------------------
             float windowAR         = (float)Screen.width / (float)Screen.height;
             float scaleheight    = windowAR / this.targetAR;
             
             // If scaled height is less than current height, add letterbox
             //-----------------------------------------------------------------------
             if (scaleheight < 1.0f)
             {  
                 Rect rect    = this.GetComponent<Camera>().rect; 
                 rect.width    = 1.0f;
                 rect.height = scaleheight;
                 rect.x         = 0;
                 rect.y         = (1.0f - scaleheight) / 2.0f;
 
                 this.GetComponent<Camera>().rect = rect;
             }
 
             // If scaled width is less than current width, add pillarbox
             //-----------------------------------------------------------------------
             else
             {
                 float scalewidth = 1.0f / scaleheight;
 
                 Rect rect    = this.GetComponent<Camera>().rect; 
                 rect.height = 1.0f;
                 rect.width     = scalewidth;
                 rect.x         = (1.0f - scalewidth) / 2.0f;
                 rect.y         = 0;
 
                 this.GetComponent<Camera>().rect = rect;
             }
                             
             // Create background cam if it doesn't exist yet
             //-----------------------------------------------------------------------            
             if (!backgroundCam)
             {
                 backgroundCam                     = new GameObject("BackgroundCam", typeof(Camera)).GetComponent<Camera>();
                 backgroundCam.depth             = -2;
                 backgroundCam.clearFlags         = CameraClearFlags.SolidColor;
                 backgroundCam.backgroundColor     = Color.green;
                 backgroundCam.cullingMask         = 0;
             }
         }

And here a very simple shader that uses a grabPass and simply displays the color of the grabbed texture:

 Shader "Custom/SimpleGrab"
  {
     Properties {}
 
     SubShader 
     {    
 
         Tags { "RenderType"    ="Opaque"                         
                "Queue"        ="Transparent"}                    
 
         LOD 200
         Lighting Off    
         
         GrabPass { }
         
         Pass 
         {
             Tags{"LightMode" = "Always"}    
             
              CGPROGRAM
              #include "UnityCG.cginc"
              #pragma vertex         vert            
              #pragma fragment     frag            
 
             sampler2D     _GrabTexture;            
             float4         _GrabTexture_TexelSize;
 
             struct vertexInput
             {
                 float4 position    :POSITION;        
                 float2 uv       :TEXCOORD0;        
             };
             
              struct vertexOutput
              {
                  float4 pos        :SV_POSITION;    
                  float2 uv        :TEXCOORD0;
                  float4 grabUV    :TEXCOORD4;
              };
 
              vertexOutput vert(vertexInput input)
              {
                 vertexOutput     output;
                 output.pos         = mul(UNITY_MATRIX_MVP, input.position);
                 output.uv         = input.uv;
                 output.grabUV    = ComputeGrabScreenPos(output.pos);
                  return             output;
              }
              
             float4 frag(vertexOutput input):COLOR
             {
                 return tex2D( _GrabTexture, float2(input.grabUV.x, input.grabUV.y));
             }
             
              // End CG programing
             //-----------------------------------------------------------------------
              ENDCG
         }

And now the actual issue: - I am using an orthogonal camera looking at a white Rect (Quad with non-uniform scaling) with an Ellipse drawn on top of it - I put the exact same quad in front of it and apply the aforementioned shader => 90% of the time it is fine, but with some specific aspect ratios I get the following artefact: (Note: the pillar / Letterbox zones have been made green for debugging purposes, normaly they are white)

=> as you can see, there is a black, inconsistent line at the top of my main camera's rect... (sometimes it's on top, and sometimes on the side, it depends on current window's AR) => I think it has to do with the _GrabTexture not being resized properly when I resize the camera rect. I strongly suspect this because I repeatedly get the following error in the console: rect.width <= rt->GetGLWidth() && rect.height <= rt->GetGLHeight()

I can't find any way to configure the Texture targeted in the GrabPass, so I don't know if this is a missing feature, a known bug to be solved, or if I'm doing something wrong... I'll appreciate any help, thanks in advance!

To reproduce:

• Main camera, orthogonal, @ (0, 0, -10), scale (1,1,1), rotation (0,0,0), size 5, depth -1

• White background quad @ (0, 0, 0), scale (12.04, 10, 1)

• Quad with shader @ (0, 0, -1), scale (12.04, 10, 1)

• Target aspect ratio of the Aspect ratio enforcer: 1.204

• Test Window size: 733 * 1056