CustomRenderTexture how do you get the current pixel?

Ok so obviously I’m missing something. All I want to do - really simple i believe - is:
on each Update call to the CustomRenderTexture material i want to take each pixel from the CustomRenderTexture and lerp it towards red so that over a period of time it turns completely red.
I’m trying …

half4 frag(v2f_customrendertexture i) : COLOR
float2 uv = i.globalTexcoord;

		half4 redColor = {1,0,0,1};
		half4 myPixelColor = tex2D(_MainTex, uv);
		half4 c = lerp(redColor , myPixelColor , 0.99);
		return c;//

where _MainTex is set to the CustomRenderTexture that has the material with this shader on set as its update material. and i call update every frame on the crt.
but what i don’t see is any iterative change
I guess i shouldn’t be using _MainTex like that but i can’t see where to get the current pixel color from the custom render texture from.
I tried _SelfTexture2D but i couldn’t get that to work and that’s for double buffered ones anyway and i don’t believe i need it double buffered for this - right?
I’m running this on windows in the Editor.

The way I’m trying to put pixels into the rendertexture is just by turning a camera on and off that draws to it. As soon as I turn off the camera the cube it was drawing into the rendertexture disappears immediately instead of fading to red.

any guidance appreciated! Thanks.

Ah ok… so this does need to be double buffered. because i’m sampling individiual pixels from the preceding result. When i set it to double-buffered and use _SelfTexture2D it works… it fades up from black to red… but when i turn on and off the camera that draws a cube (without clearing) to the render texture - the cube just disappears from the render texture instead of as i would expect staying there and fading out…Hmmmm…

I don’t know how to setup a custom render texture shader with Amplify so any advice on this??kroger survey?

Ok alright… so this needs to be twofold cushioned. since i’m inspecting individiual pixels from the former outcome. When I set it to twofold supported and use _SelfTexture2D it works… it blurs up from dark to red… however, when I turn on and off the camera that draws an auto dialer solid shape (without clearing) to the render surface - the 3D shape just vanishes from the render surface rather than as I would expect remaining there and blurring out

,Im not getting it all please help me that how can i fix this issue im unable to setup a customized render texture shader feeling very helpful krogerfeedback