Cg: omit depth write - unity3d

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
}
}
}