【URP】Unity[视差遮挡贴图]原理剖析实践

技术分享 5个月前 (10-25) 0 999+

【从UnityURP开始探索游戏渲染】专栏-直达

视差遮挡贴图（Parallax Occlusion Mapping, POM）介绍

视差遮挡贴图是视差贴图技术的高阶实现，通过‌光线步进（Raymarching）算法‌精确计算视线与高度图的交点，模拟复杂表面（如砖墙、岩石）的几何遮挡效果。相比标准视差贴图，POM能更真实地表现深度变化和自阴影，适用于高精度材质表现。

核心原理

‌分层深度检测‌将视线方向在切线空间分解为多层（通常8-15层），逐层采样高度图，通过二分法逼*视线与表面的交点。
‌动态采样优化‌根据视角与法线的夹角动态调整采样层数（**行视角增加层数，垂直视角减少层数），*衡精度与性能。
‌遮挡关系重建‌通过交点处的UV偏移修正纹理采样位置，模拟凹凸表面的光线遮挡效果。

Unity URP 实现示例与原理详解

代码关键点解析

‌动态采样层数‌
- 通过lerp(_MaxSamples, _MinSamples, saturate(dot(float3(0,0,1), viewDirTS)))实现视角自适应分层，优化性能。
‌光线步进循环‌
- 循环比较当前层高度与采样深度，找到首个交点区间，避免全精度遍历。
‌二分法优化‌
- 在初步交点区间内使用lerp插值计算精确UV，减少采样次数。

ParallaxOcclusionMapping.shader

Shader "Universal Render Pipeline/POM" {     Properties     {         _MainTex("Albedo", 2D) = "white" {}         _NormalMap("Normal Map", 2D) = "bump" {}         _HeightMap("Height Map", 2D) = "white" {}         _ParallaxScale("Height Scale", Range(0, 0.1)) = 0.05         _MinSamples("Min Samples", Int) = 8         _MaxSamples("Max Samples", Int) = 15     }      SubShader     {         Tags { "RenderPipeline"="UniversalPipeline" }          HLSLINCLUDE         #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"          TEXTURE2D(_MainTex);    SAMPLER(sampler_MainTex);         TEXTURE2D(_NormalMap);  SAMPLER(sampler_NormalMap);         TEXTURE2D(_HeightMap);  SAMPLER(sampler_HeightMap);         float _ParallaxScale;         int _MinSamples, _MaxSamples;          float2 ParallaxOcclusionMapping(float3 viewDirTS, float2 uv)         {             // 动态计算采样层数             int numSamples = (int)lerp(_MaxSamples, _MinSamples, saturate(dot(float3(0,0,1), viewDirTS)));             float layerHeight = 1.0 / numSamples;             float2 deltaUV = _ParallaxScale * viewDirTS.xy / viewDirTS.z / numSamples;              // 光线步进初始化             float currentLayerHeight = 0;             float2 currentUV = uv;             float currentDepth = 1 - SAMPLE_TEXTURE2D(_HeightMap, sampler_HeightMap, currentUV).r;              // 分层检测             [loop]             for (int i = 0; i < 15; ++i) {                 if (currentLayerHeight >= currentDepth) break;                 currentUV -= deltaUV;                 currentDepth = 1 - SAMPLE_TEXTURE2D(_HeightMap, sampler_HeightMap, currentUV).r;                 currentLayerHeight += layerHeight;             }              // 二分法精确交点             float2 prevUV = currentUV + deltaUV;             float prevDepth = currentDepth - layerHeight;             float weight = (currentLayerHeight - currentDepth) / (prevDepth - currentDepth);             return lerp(currentUV, prevUV, weight);         }         ENDHLSL          Pass         {             HLSLPROGRAM             #pragma vertex vert             #pragma fragment frag              struct Attributes             {                 float4 positionOS : POSITION;                 float2 uv : TEXCOORD0;                 float3 normalOS : NORMAL;                 float4 tangentOS : TANGENT;             };              struct Varyings             {                 float4 positionCS : SV_POSITION;                 float2 uv : TEXCOORD0;                 float3 viewDirTS : TEXCOORD1;             };              Varyings vert(Attributes IN)             {                 Varyings OUT;                 VertexPositionInputs posInput = GetVertexPositionInputs(IN.positionOS.xyz);                 OUT.positionCS = posInput.positionCS;                  // 转换视角方向到切线空间                 VertexNormalInputs normInput = GetVertexNormalInputs(IN.normalOS, IN.tangentOS);                 float3 viewDirWS = GetWorldSpaceViewDir(posInput.positionWS);                 OUT.viewDirTS = TransformWorldToTangent(viewDirWS,                      normInput.tangentWS, normInput.bitangentWS, normInput.normalWS);                  OUT.uv = IN.uv;                 return OUT;             }              half4 frag(Varyings IN) : SV_Target             {                 // 计算POM偏移后的UV                 float2 pomUV = ParallaxOcclusionMapping(normalize(IN.viewDirTS), IN.uv);                  // 采样最终纹理                 half4 albedo = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, pomUV);                 half3 normalTS = UnpackNormal(SAMPLE_TEXTURE2D(_NormalMap, sampler_NormalMap, pomUV));                  return half4(albedo.rgb, 1);             }             ENDHLSL         }     } }

技术对比与性能建议

特性	标准视差贴图	陡峭视差贴图	POM
‌采样次数‌	单次	5-15次	8-15次+二分法
‌陡峭表面精度‌	低	中	高
‌*适用台**‌	移动端	PC/主机	高端PC
‌推荐参数‌	`Scale=0.02`	`Scale=0.05`	`Scale=0.03-0.07`