How to blit-erase one texture from another?

Unity Engine
1

I’m trying to erase values from R8_UNORM texture via Graphics.Blit;
Got one texture that stores values that needs to be subtracted or “erased” from the second texture.

The issue I’m having is that instead of removing values based on the texture, it removes them from the texture based on its fillrate???

Before painting to the blit texture:
6126836--667871--before.png

After painting:
6126836--667877--after.png

Instead of removing only values from the second texture, it dims whole texture;

Textures / material initialization:

           _drawRT = new RenderTexture(_textureRes, _textureRes, 0, GraphicsFormat.R8_UNorm, 0);
           _blitRT = new RenderTexture(_textureRes, _textureRes, 0, GraphicsFormat.R8_UNorm, 0);
      
           _blitMaterial = new Material(_gs.ClearBlitShader);
           _blitMaterial.SetTexture(ShaderConst.BlitClearTexture, _blitRT);

Here’s what I’ve tried for blitting:

// Subtract blitRT from drawRT
           RenderTexture rt = RenderTexture.GetTemporary(_textureRes, _textureRes, 0, GraphicsFormat.R8_UNorm);
           Graphics.Blit(_drawRT, rt);
           Graphics.Blit(rt, _drawRT, _blitMaterial);
      
           rt.Release();

BlitClear shader (used on _blitMaterial):

Shader "Unlit/BlitClear"
{
    Properties
    {
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100

        Pass
        {
            ZTest Always Cull Off ZWrite Off
       
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            sampler2D _ClearTexture;
       
            float4 _MainTex_ST;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
           
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                float2 uvs = i.uv;
           
                fixed bit = tex2D(_ClearTexture, uvs);
                fixed4 col = tex2D(_MainTex, uvs);
           
                if (bit > 0) return fixed4(0,0,0,0);
           
                return col;
            }
            ENDCG
        }
    }
}

Any ideas what can be the cause?

2

Soo, the first issue with the whole texture being painted is caused by the UV’s (no need to use TRANSFORM_TEX on those).
The second one (dimming) is related to the sRGB? Probably need to figure that one out.

3

Okay, I’ve tried using everything, like GL.sRGBWrite, setting rt’s srgb readwrite via constructor to sRGB / linear;
No results whatsoever, so I’ve done conversion via shader and done return col * 2.2; instead.

Did the trick for me. Would like to know why the heck conversion not working correctly though.
If anyone figures out, please let me know.

4

Aaaand the blit doesn’t work correctly on the android device. FML
Texture looks like a bunch of dots instead of being solid.