Converting a RenderTexture to a Texture2D for use in a shader

I’m getting a Render Texture from a second camera and using it in a shader to find some alpha values in the current camera view. In OnRenderImage, I convert the RenderTexture to a simple Texture2D and set the material’s texture to this newly created Texture2D. This works perfectly and I get the effect I want.

However, its really expensive to convert a RenderTexture to a Texture2D every single frame so I would like to just set the material’s texture to the RenderTexture itself. This however, does not give the effect I want and I have no idea why.

What is so different between the RenderTexture’s pixels and the Texture2D’s pixels? Is there an better way to convert the RenderTexture into a Texture2D?

EDIT: Here’s how I’m converting it now:

void OnRenderImage(RenderTexture source, RenderTexture dest)
    if (mat.HasProperty("_RenderPaintTexture"))
    { = source;
        Texture2D texture = new Texture2D(source.width, source.height, TextureFormat.ARGB32, false);
        texture.ReadPixels(new Rect(0, 0, source.width, source.height), 0, 0);
        mat.SetTexture("_RenderPaintTexture", texture);

I’m not sure of the different between RenderTexture and Texture2D but Texture2D.Apply suggests that if you only need the texture on the GPU and not on the CPU then Graphics.CopyTexture would be faster.

Just try setting the camera to Orthographic to set everything in 2D