Getting the alpha value of opaque geometry in a render texture.

Unity Engine
1

I am interested in storing some values from opaque objects into the alpha channel of a render texture. I got the idea from listening to a Unite talk where they used the trick to crate a bloom post effect without using hdr. Here is the link

.

I tried using CommandBuffers to get the texture right after Opaque geometry is rendered:

myCam = this.gameObject.GetComponent<Camera>();

        if (XRSettings.enabled)
            originalTextureDescriptor = XRSettings.eyeTextureDesc;
        else
            originalTextureDescriptor = new RenderTextureDescriptor(Screen.width, Screen.height); // Not XR

        originalTextureDescriptor.colorFormat = RenderTextureFormat.ARGB32;

        CommandBuffer buf = new CommandBuffer();
        buf.name = "grab alpha";

        int screenCopyID = Shader.PropertyToID("_AlphaMask");
        buf.GetTemporaryRT(screenCopyID, originalTextureDescriptor);
        buf.Blit(BuiltinRenderTextureType.CurrentActive, screenCopyID);
        buf.SetGlobalTexture("_HDRmask", screenCopyID);

        myCam.AddCommandBuffer(CameraEvent.AfterForwardOpaque, buf);

And use this test surface shader for opaque objects:

 SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 200

        Blend SrcAlpha OneMinusSrcAlpha, One One
        ColorMask RGBA
        ZWrite On

        CGPROGRAM
        #pragma surface surf Lambert keepalpha
        #pragma target 3.0


        sampler2D _MainTex;

        struct Input
        {
            float2 uv_MainTex;
        };

        half _Glossiness;
        half _Metallic;
        fixed4 _Color;

        UNITY_INSTANCING_BUFFER_START(Props)
            // put more per-instance properties here
        UNITY_INSTANCING_BUFFER_END(Props)

        void surf (Input IN, inout SurfaceOutputStandard o)
        {
            // Albedo comes from a texture tinted by color
            fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
            o.Albedo = c.rgb;
            o.Alpha = c.a;
        }

Even though i use ColorMask RGBA and keepalpha tag all the opaque geometry has alpha 1 in the render texture. My cameras clear flag is set to solid color (0,0,0,0). Any advice would be appreciated.

2

In the surface shader the declaration should be: void surf (Input IN, inout SurfaceOutput o).
So i figured that the result from the surface shader is passed through the lighting code declared in a cginc that for some types changes the alpha back to 1. Writing your own lighting code solves the problem, and there is no specific need to use command buffers for this.