【URP】Unity[RendererFeatures]屏幕空间环境光遮蔽SSAO

【从UnityURP开始探索游戏渲染】专栏-直达

SSAO概述与作用

SSAO(Screen Space Ambient Occlusion)是一种基于屏幕空间的全局环境光遮蔽技术，它通过计算场景中物体间的遮蔽关系来增强场景的深度感和真实感。在Unity URP中，SSAO通过Renderer Feature实现，作为URP渲染管线的扩展模块插入到渲染流程中。

SSAO的主要作用包括：

• 增强场景深度感知，使物体间的接触区域产生自然阴影
• 提升场景细节表现，特别是角落和凹陷处的视觉效果
• 无需额外光照计算即可增强场景的空间感
• 相比传统AO技术性能开销更低

SSAO发展历史

SSAO技术起源于2007年，由Crytek公司在《孤岛危机》中首次实现并商业化应用。随后该技术经历了多个发展阶段：

• ‌早期SSAO‌(2007-2010)：基于深度缓冲的简单采样，存在明显的噪点和性能问题
• ‌HBAO‌(2010-2013)：NVIDIA提出的Horizon-Based AO，提高了精度但计算量较大
• ‌SSDO‌(2013-2015)：Screen Space Directional Occlusion，考虑了光线方向
• ‌现代SSAO‌(2015至今)：结合了降噪技术和自适应采样，如GTAO(Ground Truth AO)

Unity自2018版开始将SSAO集成到URP中，通过Renderer Feature方式提供灵活的配置选项。

SSAO实现原理

SSAO在URP中的实现主要分为以下步骤：

• ‌深度/法线信息采集‌：从摄像机深度纹理和法线纹理获取场景几何信息
• ‌采样点生成‌：在像素周围半球空间内生成随机采样点
• ‌遮蔽计算‌：比较采样点深度与场景深度，计算遮蔽值
• ‌模糊处理‌：通过双边滤波消除噪点
• ‌合成输出‌：将AO效果与场景颜色混合

SSAO核心原理

• ‌环境光遮蔽基础‌

  AO通过模拟物体表面因几何遮挡导致的环境光衰减，增强场景深度感。其数学本质是法线半球面上可见性函数的积分计算。SSAO在屏幕空间利用深度/法线缓冲近似这一过程，避免传统AO的复杂光线求交。

• ‌屏幕空间实现机制‌

  • ‌深度重建‌：通过深度缓冲和相机投影矩阵反推像素的世界坐标，公式为：

    float3 clipVec = float3(ndcPos.x, ndcPos.y, 1.0) * _ProjectionParams.z;
    float3 viewVec = mul(unity_CameraInvProjection, clipVec.xyzz).xyz;
    

  • ‌法向半球采样‌：在像素法线方向构建半球采样核，对比周围深度值计算遮蔽因子。深度更高的采样点计数越多，遮蔽效果越强。

URP实现流程

• ‌关键组件‌
  • ‌Renderer Feature‌：需创建独立Feature并配置ScriptableRenderPassInput.Normal以获取法线缓冲。
  • ‌Shader计算‌：结合_CameraNormalsTexture和深度图进行世界坐标重建与遮蔽计算。
• ‌示例代码

  • SSAORendererFeature.cs

    using UnityEngine;
    using UnityEngine.Rendering;
    using UnityEngine.Rendering.Universal;
    
    public class SSAORendererFeature : ScriptableRendererFeature {
        class SSAOPass : ScriptableRenderPass {
            public override void OnCameraSetup(CommandBuffer cmd, ref RenderingData renderingData) {
                ConfigureInput(ScriptableRenderPassInput.Normal);
            }
            // 实现Execute方法进行SSAO计算
        }
        public override void Create() {
            m_SSAOPass = new SSAOPass();
            m_SSAOPass.renderPassEvent = RenderPassEvent.AfterRenderingOpaques;
        }
    }
    

  • SSAO.shader

    Shader "Hidden/SSAO" {
        Properties {
            _Radius ("采样半径", Range(0.1, 5)) = 1
            _Intensity ("强度", Range(0, 10)) = 1
        }
        SubShader {
            Pass {
                // 深度重建与采样核计算代码
            }
        }
    }
    

参数解析

参数	作用	典型值
_Radius	控制采样范围	0.5-2.0
_Intensity	遮蔽强度	1.0-3.0
_SampleCount	采样点数量	16-32

性能优化建议

• 降低采样数（如16个）并配合噪声纹理
• 使用双边滤波消除噪点
• 仅在高端设备启用（移动端需谨慎）

完整Unity URP实现示例

以下是完整的SSAO Renderer Feature实现流程：

• SSAORendererFeature.cs

  using UnityEngine;
  using UnityEngine.Rendering;
  using UnityEngine.Rendering.Universal;
  
  public class SSAORendererFeature : ScriptableRendererFeature
  {
      [System.Serializable]
      public class SSAOSettings
      {
          public RenderPassEvent renderPassEvent = RenderPassEvent.AfterRenderingOpaques;
          public Material blitMaterial = null;
          public float radius = 0.5f;
          public float intensity = 1.0f;
          public float power = 2.0f;
          public int sampleCount = 16;
          public float bias = 0.025f;
          public float downsampling = 1;
          public bool blur = true;
          public float blurRadius = 1.0f;
      }
  
      public SSAOSettings settings = new SSAOSettings();
      private SSAORenderPass ssaoPass;
  
      public override void Create()
      {
          ssaoPass = new SSAORenderPass(settings);
      }
  
      public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData)
      {
          if (settings.blitMaterial == null)
          {
              Debug.LogWarning("Missing SSAO material");
              return;
          }
          renderer.EnqueuePass(ssaoPass);
      }
  }
  
  public class SSAORenderPass : ScriptableRenderPass
  {
      private Material ssaoMaterial;
      private SSAORendererFeature.SSAOSettings settings;
      private RenderTargetIdentifier source;
      private RenderTargetHandle tempTexture;
      private RenderTargetHandle tempTexture2;
  
      public SSAORenderPass(SSAORendererFeature.SSAOSettings settings)
      {
          this.settings = settings;
          this.renderPassEvent = settings.renderPassEvent;
          tempTexture.Init("_TempSSAOTexture");
          tempTexture2.Init("_TempSSAOTexture2");
      }
  
      public void Setup(RenderTargetIdentifier source)
      {
          this.source = source;
      }
  
      public override void Configure(CommandBuffer cmd, RenderTextureDescriptor cameraTextureDescriptor)
      {
          if (settings.downsampling > 1)
          {
              cameraTextureDescriptor.width = (int)(cameraTextureDescriptor.width / settings.downsampling);
              cameraTextureDescriptor.height = (int)(cameraTextureDescriptor.height / settings.downsampling);
          }
          cmd.GetTemporaryRT(tempTexture.id, cameraTextureDescriptor, FilterMode.Bilinear);
          cmd.GetTemporaryRT(tempTexture2.id, cameraTextureDescriptor, FilterMode.Bilinear);
      }
  
      public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData)
      {
          CommandBuffer cmd = CommandBufferPool.Get("SSAO");
  
          // Set SSAO material properties
          ssaoMaterial = settings.blitMaterial;
          ssaoMaterial.SetFloat("_Radius", settings.radius);
          ssaoMaterial.SetFloat("_Intensity", settings.intensity);
          ssaoMaterial.SetFloat("_Power", settings.power);
          ssaoMaterial.SetInt("_SampleCount", settings.sampleCount);
          ssaoMaterial.SetFloat("_Bias", settings.bias);
  
          // First pass - generate AO
          Blit(cmd, source, tempTexture.Identifier(), ssaoMaterial, 0);
  
          if (settings.blur)
          {
              // Second pass - horizontal blur
              ssaoMaterial.SetVector("_Direction", new Vector2(settings.blurRadius, 0));
              Blit(cmd, tempTexture.Identifier(), tempTexture2.Identifier(), ssaoMaterial, 1);
  
              // Third pass - vertical blur
              ssaoMaterial.SetVector("_Direction", new Vector2(0, settings.blurRadius));
              Blit(cmd, tempTexture2.Identifier(), tempTexture.Identifier(), ssaoMaterial, 1);
          }
  
          // Final pass - composite
          Blit(cmd, tempTexture.Identifier(), source, ssaoMaterial, 2);
  
          context.ExecuteCommandBuffer(cmd);
          CommandBufferPool.Release(cmd);
      }
  
      public override void FrameCleanup(CommandBuffer cmd)
      {
          cmd.ReleaseTemporaryRT(tempTexture.id);
          cmd.ReleaseTemporaryRT(tempTexture2.id);
      }
  }
  

• SSAO.shader

  Shader "Hidden/SSAO"
  {
      Properties
      {
          _MainTex ("Texture", 2D) = "white" {}
      }
  
      SubShader
      {
          Cull Off ZWrite Off ZTest Always
  
          Pass // 0: Generate AO
          {
              CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
              #include "UnityCG.cginc"
  
              struct appdata
              {
                  float4 vertex : POSITION;
                  float2 uv : TEXCOORD0;
              };
  
              struct v2f
              {
                  float2 uv : TEXCOORD0;
                  float4 vertex : SV_POSITION;
              };
  
              v2f vert(appdata v)
              {
                  v2f o;
                  o.vertex = UnityObjectToClipPos(v.vertex);
                  o.uv = v.uv;
                  return o;
              }
  
              sampler2D _MainTex;
              sampler2D _CameraDepthNormalsTexture;
              float _Radius;
              float _Intensity;
              float _Power;
              int _SampleCount;
              float _Bias;
  
              float3 GetPosition(float2 uv)
              {
                  float depth;
                  float3 normal;
                  DecodeDepthNormal(tex2D(_CameraDepthNormalsTexture, uv), depth, normal);
                  float4 pos = float4(uv * 2 - 1, depth * 2 - 1, 1);
                  pos = mul(unity_CameraInvProjection, pos);
                  return pos.xyz / pos.w;
              }
  
              float3 GetNormal(float2 uv)
              {
                  float depth;
                  float3 normal;
                  DecodeDepthNormal(tex2D(_CameraDepthNormalsTexture, uv), depth, normal);
                  return normal;
              }
  
              float random(float2 uv)
              {
                  return frac(sin(dot(uv, float2(12.9898, 78.233))) * 43758.5453);
              }
  
              float3 getSampleKernel(int i, float2 uv)
              {
                  float r = random(uv * (i+1));
                  float theta = random(uv * (i+2)) * 2 * 3.1415926;
                  float phi = random(uv * (i+3)) * 3.1415926 * 0.5;
  
                  float x = r * sin(phi) * cos(theta);
                  float y = r * sin(phi) * sin(theta);
                  float z = r * cos(phi);
  
                  return normalize(float3(x, y, z));
              }
  
              float frag(v2f i) : SV_Target
              {
                  float3 pos = GetPosition(i.uv);
                  float3 normal = GetNormal(i.uv);
  
                  float occlusion = 0.0;
                  for(int j = 0; j < _SampleCount; j++)
                  {
                      float3 sampleKernel = getSampleKernel(j, i.uv);
                      sampleKernel = reflect(sampleKernel, normal);
  
                      float3 samplePos = pos + sampleKernel * _Radius;
                      float4 sampleClipPos = mul(unity_CameraProjection, float4(samplePos, 1.0));
                      sampleClipPos.xy /= sampleClipPos.w;
                      sampleClipPos.xy = sampleClipPos.xy * 0.5 + 0.5;
  
                      float sampleDepth = GetPosition(sampleClipPos.xy).z;
                      float rangeCheck = smoothstep(0.0, 1.0, _Radius / abs(pos.z - sampleDepth));
                      occlusion += (sampleDepth >= samplePos.z + _Bias ? 1.0 : 0.0) * rangeCheck;
                  }
  
                  occlusion = 1.0 - (occlusion / _SampleCount);
                  return pow(occlusion, _Power) * _Intensity;
              }
              ENDCG
          }
  
          Pass // 1: Blur
          {
              CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
              #include "UnityCG.cginc"
  
              struct appdata
              {
                  float4 vertex : POSITION;
                  float2 uv : TEXCOORD0;
              };
  
              struct v2f
              {
                  float2 uv : TEXCOORD0;
                  float4 vertex : SV_POSITION;
              };
  
              v2f vert(appdata v)
              {
                  v2f o;
                  o.vertex = UnityObjectToClipPos(v.vertex);
                  o.uv = v.uv;
                  return o;
              }
  
              sampler2D _MainTex;
              float4 _MainTex_TexelSize;
              float2 _Direction;
  
              float frag(v2f i) : SV_Target
              {
                  float2 texelSize = _MainTex_TexelSize.xy;
                  float result = 0.0;
                  float weightSum = 0.0;
  
                  for(int x = -2; x <= 2; x++)
                  {
                      float weight = exp(-(x*x) / (2.0 * 2.0));
                      float2 offset = _Direction * x * texelSize;
                      result += tex2D(_MainTex, i.uv + offset).r * weight;
                      weightSum += weight;
                  }
  
                  return result / weightSum;
              }
              ENDCG
          }
  
          Pass // 2: Composite
          {
              CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
              #include "UnityCG.cginc"
  
              struct appdata
              {
                  float4 vertex : POSITION;
                  float2 uv : TEXCOORD0;
              };
  
              struct v2f
              {
                  float2 uv : TEXCOORD0;
                  float4 vertex : SV_POSITION;
              };
  
              v2f vert(appdata v)
              {
                  v2f o;
                  o.vertex = UnityObjectToClipPos(v.vertex);
                  o.uv = v.uv;
                  return o;
              }
  
              sampler2D _MainTex;
              sampler2D _SSAOTex;
  
              float4 frag(v2f i) : SV_Target
              {
                  float4 color = tex2D(_MainTex, i.uv);
                  float ao = tex2D(_SSAOTex, i.uv).r;
                  return color * ao;
              }
              ENDCG
          }
      }
  }
  

SSAO参数详解与使用指南

参数含义与调整建议

• ‌Radius 半径‌
  • 含义：控制采样点的搜索半径
  • 范围：0.1-2.0
  • 用例：小半径适合细节丰富的场景，大半径适合开阔场景
• ‌Intensity 强度‌
  • 含义：控制AO效果的强度
  • 范围：0.5-4.0
  • 用例：值越大，遮蔽效果越明显
• ‌Power 幂次‌
  • 含义：控制AO效果的对比度
  • 范围：1.0-4.0
  • 用例：值越大，暗部越暗，亮部越亮
• ‌Sample Count 采样数‌
  • 含义：每个像素的采样点数
  • 范围：8-32
  • 用例：值越高效果越平滑但性能消耗越大
• ‌Bias 偏移‌
  • 含义：防止自遮蔽的偏移量
  • 范围：0.01-0.1
  • 用例：值过小会产生噪点，值过

---

【从UnityURP开始探索游戏渲染】专栏-直达

（欢迎点赞留言探讨，更多人加入进来能更加完善这个探索的过程，）