Cg: omit depth write

Question

I am outputting depth in Cg in a branch, like so:

ZWrite On
..
void frag(v2f IN, out color : COLOR, out depth : DEPTH) {
 if (statement) {
  color = float4(1);
 } else {
  color = float4(0);
  depth = 0;
 }
}

However as you see I omit writing the depth in the first condition. This results in undefined behaviour, but I believe this is common practice in GLSL (omitting writing to glFragDepth will result in the original depth).

What should I do to get the original depth in the first condition in Cg when having a depth value for output?

Answer 1

YMMV w/ this script. The code, as I recall, needed to be targeted to old implementations of OpenGL or else you'd get an error like shader registers cannot be masked related to this D3D issue.

But I believe you can pull the depth from the camera depth texture and rewrite it out. You do need to calculate a projected position first using ComputeScreenPos in the vertex shader. Documentation is non-existent, AFAIK, for the functions Linear01Depth and LinearEyeDepth so I can't tell you what the performance hit might be.

Shader "Depth Shader" { // defines the name of the shader 
SubShader { // Unity chooses the subshader that fits the GPU best
  Pass { // some shaders require multiple passes
     ZWrite On
     CGPROGRAM // here begins the part in Unity's Cg

     #pragma vertex vert 
     #pragma fragment frag
     #include "UnityCG.cginc"
     struct v2f
     {
        float4 position : POSITION;
        float4 projPos : TEXCOORD1;
     };

     v2f vert(float4 vertexPos : POSITION)
     {
        v2f OUT;
        OUT.position = mul(UNITY_MATRIX_MVP, vertexPos);
        OUT.projPos = ComputeScreenPos(OUT.position);
        return OUT;
     }

     //camera depth texture here
     uniform sampler2D _CameraDepthTexture; //Depth Texture
     void frag(v2f IN, out float4 color:COLOR, out float depth:DEPTH) // fragment shader
     {
        color = float4(0);
         // use eye depth for actual z...
        depth = LinearEyeDepth (tex2Dproj(_CameraDepthTexture, UNITY_PROJ_COORD(IN.projPos)).r); 

        //or this for depth in between [0,1]
        //depth = Linear01Depth (tex2Dproj(_CameraDepthTexture, UNITY_PROJ_COORD(IN.projPos)).r);
     }

     ENDCG // here ends the part in Cg 
  }
}
}