Unity - custom shadow from sun

Question

due to some features in materials I need to create my own directional shadow map.

Everything seems to work ok, until the moment where I compare depths.

First I add camera component to directional light with RenderTexture:

then in code, I set this texture and camera projection matrix as global variables for shaders:

Camera shadowCaptureCamera = GetComponent<Camera>();
Shader.SetGlobalTexture("_CustomShadowsTex", shadowCaptureCamera.targetTexture, UnityEngine.Rendering.RenderTextureSubElement.Depth);
           
Matrix4x4 worldToClipMatrix = shadowCaptureCamera.projectionMatrix * shadowCaptureCamera.worldToCameraMatrix;
Shader.SetGlobalMatrix("_CustomShadowsMatrix", worldToClipMatrix);

Then in shaders I do this:

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

    float4 worldPos = mul( unity_ObjectToWorld, float4( v.vertex.xyz, 1.0f) );
    float4 shadowPosition = mul( _CustomShadowsMatrix, worldPos );
    shadowPosition.xyz /= shadowPosition.w;
    shadowPosition.xy = shadowPosition.xy * 0.5f + 0.5f;
    o.shadowMapUV = shadowPosition;
    return o;
}

fixed4 frag(v2f i) : SV_Target
{

    ......

    float shadowDepth = tex2D( _CustomShadowsTex, i.shadowMapUV.xy ).r;             
    float atten = shadowDepth > abs(i.shadowMapUV.z) ? 1.0f : 0.0f;
    return atten;
}

Everything has value 1.0f.

Any idea what's wrong? It samples shadow at correct place, but something is wrong with values. I'm also not sure why i.shadowMapUV.z is <0.f. Looking at matrix's values it shouldn't be.

Answer 1

Ok, I got it. i.shadowMapUV.z is < 0.f because it's mapped to -1;1 between near;far. Forward is also at -Z, so correct way to calculate shadowPosition is:

float4 shadowPosition= mul( _CustomShadowsMatrix, float4(worldPos, 1.f) );
shadowPosition.xyz /= shadowPosition.w;
shadowPosition.xy = shadowPosition.xy * 0.5f + 0.5f;
shadowPosition.z = -shadowPosition.z * 0.5f + 0.5f;