Screenspace coordinates

screenspace.shader

CGPROGRAM
	// Pass a clean copy of the VP matrix.
	// Shader.SetGlobalMatrix("_MyMatrixVP", Camera.current.projectionMatrix*Camera.current.worldToCameraMatrix);
	// Unity applies poorly documented magic to UNITY_MATRIX_VP that you don't want.
	float4x4 _MyMatrixVP;
	
	struct VertexOutput {
		// Pass the screen space coordinate as a new varying.
		// Don't re-use SV_POSITION in your frag shader, it only works on non-DirectX platforms IIRC.
		float4 screenSpace : TEXCOORD1;
	};
	
	VertexOutput VShader(VertexInput v){
		VertexOutput o;
		
		// Multiply by MVP matrix, mostly as normal.
		float4 screenSpace = mul(_MyMatrixVP, mul(unity_ObjectToWorld, float4(v.position, 1.0)));
		
		// Add this line if you want your screen space coords to go from (0, 1) instead of (-1, 1) for texturing.
		o.screenSpace = screenSpace + screenSpace.w;
	}

	fixed4 FShader(VertexOutput v) : SV_Target {
		// In your Fragment shader either use projective texturing:
		float3 color = tex2Dproj(_SFLightMap, v.screenSpace);
		
		// Or manually divide by the perspective coordinate:
		float3 color = tex2D(_SFLightMap, v.screenSpace.xy/v.screenSpace.w);
	}
ENDCG