Prevent Render Texture clearing

Questions & Answers
1

In a nutshell,
How can I prevent RenderTextures from being cleared after every frame?

I have a script that writes high intensity values to a render texture and I want to do an image effect with this over multiple frames while constantly drawing more to it; kinda like a camera with no clear flags. The problem with this is that Unity is damn dedicated to clear the RenderTexture on every frame with the camera content. Is it possible to prevent this somehow or make a workaround?

Here’s my simple script for reference.

Unity 5.3.1p1

using UnityEngine;

[RequireComponent(typeof(Camera))]
public class TemporalTest : MonoBehaviour {

    public Shader temporalTestShader;
    public RenderTexture rt;
    public Material temporalMaterial;

    void Start () {
        rt = new RenderTexture(Screen.width, Screen.height, 0);
        temporalMaterial = new Material(temporalTestShader);
    }

    void OnRenderImage(RenderTexture source, RenderTexture destination) {

        // rt gets filled correctly here
        // but it's cleared every frame for no reason causing nuclear headaches

        temporalMaterial.SetTexture("_TemporalMap", rt);
        Graphics.Blit(source, rt, temporalMaterial, 0);
    }
}
2

Ok, after hours of debugging I found out that it was the Blit function that is clearing the rendertexture and not unity in between OnRenderImage calls.

The problem was that you can’t do

mat.SetTexture("something", sometex);
Blit(A, B, mat); // Add A to "something" in shader and save to some result B

and expect it to write the result nicely to B. Instead you have to use temporary rendertextures and do

mat.SetTexture("something", sometex);
RenderTexture temp = RenderTexture.GetTemporary(...);
Blit(A, temp, mat); // Add A to "something", save to temp
Blit(temp, B); // Copy temp to some result texture B
RenderTexture.Release(temp);

I don’t know why it has to be done this way or why can’t I use B straight as a result and so on but at least it works.