Unity command buffer that modifies screenspace shadow mask

_readme.txt

Super small example of a command buffer based approach that modifies screenspace shadow mask.

1. computes (very crude & simple) SSAO from the depth buffer, after depth is rendered.
2. modifies screenspace shadow texture (from the main directional light), my multiplying in the SSAO term into regular shadow term.
3. now all objects that receive shadows also get illumination darkened by SSAO; it essentially becomes "part of the shadow"

ComputeOcclusionFromDepth.shader

Shader "Hidden/ComputeOcclusion"
{
	Properties
	{
		_MainTex ("", 2D) = "white" {}
	}
	SubShader
	{
		Pass
		{
			Cull Off ZWrite Off ZTest Always

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma target 3.0
			#include "UnityCG.cginc"

			struct appdata_t {
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f {
				float4 vertex : SV_POSITION;
				float2 uv : TEXCOORD0;
			};

			v2f vert (appdata_t v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = v.uv;
				return o;
			}

			sampler2D_float _CameraDepthTexture;

			// Fairly bad & noisy "SSAO" computation from the depth
			// texture alone.

			float3 normal_from_depth(float depth, float2 uv)
			{  
			  float2 offset1 = float2(0.0,_ScreenParams.w-1);
			  float2 offset2 = float2(_ScreenParams.z-1,0.0);
			  
			  float depth1 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, uv + offset1);
			  depth1 = LinearEyeDepth (depth1);
			  float depth2 = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, uv + offset2);
			  depth2 = LinearEyeDepth (depth2);
			  
			  float3 p1 = float3(offset1, depth1 - depth);
			  float3 p2 = float3(offset2, depth2 - depth);
			  
			  float3 normal = cross(p1, p2);
			  normal.z = -normal.z;
			  
			  return normalize(normal);
			}			

			float rand(float2 co){
    			return frac(sin(dot(co ,float2(12.9898,78.233))) * 43758.5453);
			}

			fixed4 frag (v2f i) : SV_Target
			{
				const float base = 0.2;

				const float falloff = 0.001;
				const float radius = 0.3;

				const int kSampleCount = 16;
				float3 kSamples[kSampleCount] = {
				float3( 0.5381, 0.1856,-0.4319), float3( 0.1379, 0.2486, 0.4430),
				float3( 0.3371, 0.5679,-0.0057), float3(-0.6999,-0.0451,-0.0019),
				float3( 0.0689,-0.1598,-0.8547), float3( 0.0560, 0.0069,-0.1843),
				float3(-0.0146, 0.1402, 0.0762), float3( 0.0100,-0.1924,-0.0344),
				float3(-0.3577,-0.5301,-0.4358), float3(-0.3169, 0.1063, 0.0158),
				float3( 0.0103,-0.5869, 0.0046), float3(-0.0897,-0.4940, 0.3287),
				float3( 0.7119,-0.0154,-0.0918), float3(-0.0533, 0.0596,-0.5411),
				float3( 0.0352,-0.0631, 0.5460), float3(-0.4776, 0.2847,-0.0271)
				};

				float depth = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, i.uv.xy);
				depth = LinearEyeDepth (depth);


				float3 position = float3(i.uv, depth);
				float3 normal = normal_from_depth(depth, i.uv);

				float3 random = normalize(float3(rand(i.uv), rand(i.uv.yx), 0));

				float r = radius/depth;
				float occ = 0.0;
				for(int i=0; i < kSampleCount; i++)
				{
					float3 ray = r * reflect(kSamples[i], random);
					float3 hemi_ray = position + sign(dot(ray,normal)) * ray;

					float sd = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, saturate(hemi_ray.xy));
					sd = LinearEyeDepth (sd);
					float diff = depth - sd;

					occ += step(falloff, diff);
				}

				float ao = 1.0 - occ / kSampleCount;
				ao += base;
				ao *= ao;
				return ao;
			}
			ENDCG
		}
	}
}

LightCommandBufferSSAO.cs

using UnityEngine;
using UnityEngine.Rendering;
using System.Collections.Generic;

public class LightCommandBufferSSAO : MonoBehaviour
{
	public Mesh m_Quad;

	public Shader m_OcclusionShader;
	private Material m_OcclusionMaterial;

	public Shader m_ShadowMaskModulateShader;
	private Material m_ShadowMaskModulateMaterial;

	private CommandBuffer m_Buffer;
	private CommandBuffer m_OcclusionBuffer;

	private Light m_Light;
	private Camera m_Camera;

	private void Cleanup()
	{
		if (m_Buffer != null)
			m_Light.RemoveCommandBuffer (LightEvent.AfterScreenspaceMask, m_Buffer);
		m_Buffer = null;
		if (m_OcclusionBuffer != null && m_Camera != null)
			m_Camera.RemoveCommandBuffer (CameraEvent.AfterDepthTexture, m_OcclusionBuffer);
		m_OcclusionBuffer = null;

		Object.DestroyImmediate (m_OcclusionMaterial);
		Object.DestroyImmediate (m_ShadowMaskModulateMaterial);
	}


	void OnDisable()
	{
		Cleanup();
	}
	
	void OnEnable ()
	{
		Cleanup();
		m_Light = GetComponent<Light>();
		m_Camera = Camera.main;
		if (!m_ShadowMaskModulateMaterial)
		{
			m_ShadowMaskModulateMaterial = new Material(m_ShadowMaskModulateShader);
			m_ShadowMaskModulateMaterial.hideFlags = HideFlags.HideAndDontSave;
		}
		if (!m_OcclusionMaterial)
		{
			m_OcclusionMaterial = new Material(m_OcclusionShader);
			m_OcclusionMaterial.hideFlags = HideFlags.HideAndDontSave;
		}

		if (m_OcclusionBuffer == null)
		{
			// compute occlusion after camera's depth texture is done
			m_OcclusionBuffer = new CommandBuffer();
			m_OcclusionBuffer.name = "Compute ambient occlusion";
			int texID = Shader.PropertyToID("_GlobalOcclusionTexture");
			m_OcclusionBuffer.GetTemporaryRT (texID, -1, -1, 0, FilterMode.Bilinear);
			m_OcclusionBuffer.Blit (BuiltinRenderTextureType.CurrentActive, texID, m_OcclusionMaterial);

			m_Camera.AddCommandBuffer (CameraEvent.AfterDepthTexture, m_OcclusionBuffer);
		}

		if (m_Buffer == null)
		{
			// after light's screenspace shadow mask is computed, modulate it by occlusion
			// texture
			m_Buffer = new CommandBuffer();
			m_Buffer.name = "Modify screenspace mask";
			m_Buffer.DrawMesh(m_Quad, Matrix4x4.identity, m_ShadowMaskModulateMaterial);

			int screenCopyID = Shader.PropertyToID("_ShadowMapCopy");
			m_Buffer.GetTemporaryRT (screenCopyID, -1, -1, 0, FilterMode.Bilinear);
			m_Buffer.Blit (BuiltinRenderTextureType.CurrentActive, screenCopyID);

			m_Light.AddCommandBuffer (LightEvent.AfterScreenspaceMask, m_Buffer);
		}
	}
}

MultiplyShadowMaskWithOcclusion.shader

Shader "Hidden/MultiplyShadowMaskWithOcclusion"
{
	SubShader
	{
		Pass
		{
			Cull Off ZWrite Off ZTest Always
			Blend DstColor Zero

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			struct appdata_t {
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f {
				float4 vertex : SV_POSITION;
				float2 uv : TEXCOORD0;
			};

			v2f vert (appdata_t v)
			{
				v2f o;
				v.vertex.xy += 0.5;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = v.uv;
				return o;
			}

			sampler2D _GlobalOcclusionTexture;

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = tex2D(_GlobalOcclusionTexture, i.uv);
				return col;
			}
			ENDCG
		}
	}
}

Does not work in single pass or single pass instanced