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试了一下在unity中使用raytrace方式实现体积光,

运行效果如下

raytrace开销较大,采样加到200几乎卡得跑不动了

首先在光源处拍摄场景(unity对mainlight做了这个处理,并且是级联可设置)

基本原理是在全屏路径下,根据场景深度,还原出世界坐标

根据世界坐标判断是哪个裁切球

(如果不是级联阴影,比如spotlight就不需要这个操作)

然后将世界坐标变换到光源观察坐标light_view_pos

再根据投影矩阵对应到光源纹理的深度纹理

比较当前点在光源摄像机的深度,判断是不是处于遮挡(即阴影)

原理和纹理阴影的处理方式类似

附shader代码如下

Shader "lsc/RaytraceShader"
{Properties{_MainTex ("Texture", 2D) = "white" {}_raytrace_step_count("rayrace step count", Int) = 5_scale("scale", float) = 1.0}SubShader{// No culling or depthCull Off ZWrite Off ZTest AlwaysPass{HLSLPROGRAM#pragma vertex vert#pragma fragment frag#pragma multi_compile _ _MAIN_LIGHT_SHADOWS#pragma multi_compile _ _MAIN_LIGHT_SHADOWS_CASCADE#pragma multi_compile_fragment _ _ADDITIONAL_LIGHT_SHADOWS#pragma multi_compile_fragment _ _SHADOWS_SOFT#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Lighting.hlsl"struct appdata{float4 vertex : POSITION;float2 uv : TEXCOORD0;};struct v2f{float2 uv : TEXCOORD0;float4 vertex : SV_POSITION;float4 screen_pos : TEXCOORD1;};float4x4 _mtx_view_inv;float4x4 _mtx_proj_inv;TEXTURE2D_X_FLOAT(_CameraDepthTexture);SAMPLER(sampler_CameraDepthTexture);v2f vert (appdata v){v2f o;VertexPositionInputs vertexInput = GetVertexPositionInputs(v.vertex.xyz);o.vertex = vertexInput.positionCS;o.screen_pos = ComputeScreenPos(o.vertex);o.uv = v.uv;return o;}sampler2D _MainTex;int _raytrace_step_count;float _scale;float4 cal_world_pos_by_dep(float ndc_dep, float2 screen_space, out float4 view_pos){// 取出非线性深度与视深度float linearDepthZ = LinearEyeDepth(ndc_dep, _ZBufferParams);// 屏幕转ndcfloat4 ndc_pos;ndc_pos.xy = screen_space * 2.0 - 1.0;ndc_pos.zw = float2(ndc_dep, 1);// 添加齐次因子ndc_pos = ndc_pos * linearDepthZ;// 转成观察与世界坐标view_pos = mul(_mtx_proj_inv, ndc_pos);float4 world_pos = mul(_mtx_view_inv, float4(view_pos.xyz, 1));return world_pos;}float4 frag (v2f i) : SV_Target{float4 col = tex2D(_MainTex, i.uv);// 插值后的屏幕坐标去除齐次因子float2 screen_space = i.screen_pos.xy / i.screen_pos.w;// 取出非线性深度float org_depth = SAMPLE_TEXTURE2D_X(_CameraDepthTexture, sampler_CameraDepthTexture, screen_space).x;// 计算世界坐标float4 view_pos;float4 world_pos = cal_world_pos_by_dep(org_depth, screen_space, view_pos);float3 cam_wpos = GetCameraPositionWS();float3 v_step = (world_pos - cam_wpos) / _raytrace_step_count;float3 rt_start = cam_wpos;float shadow_atten = 0;UNITY_LOOPfor (int i = 0; i < _raytrace_step_count; i++)//循环,超级低效{float4 shadow_coord = TransformWorldToShadowCoord(rt_start);rt_start += v_step;Light mainLight = GetMainLight(shadow_coord);//这样产生了级联阴影采样shadow_atten += mainLight.shadowAttenuation;}shadow_atten = (shadow_atten / _raytrace_step_count) * _scale;col.rgb = col.rgb * shadow_atten;return col;}ENDHLSL}}
}

对应的urp管线cs代码

using UnityEngine;
using UnityEngine.Rendering;
using UnityEngine.Rendering.Universal;
using System;public class RayTraceFogRenderPassFeature : ScriptableRendererFeature
{class CustomRenderPass : ScriptableRenderPass{public Material raytrace_material_;public RenderTargetIdentifier render_target_color_;public RenderTargetHandle temp_render_target_;public int raytrace_count_ = 5;public float scale_ = 1.0f;// This method is called before executing the render pass.// It can be used to configure render targets and their clear state. Also to create temporary render target textures.// When empty this render pass will render to the active camera render target.// You should never call CommandBuffer.SetRenderTarget. Instead call <c>ConfigureTarget</c> and <c>ConfigureClear</c>.// The render pipeline will ensure target setup and clearing happens in a performant manner.public override void OnCameraSetup(CommandBuffer cmd, ref RenderingData renderingData){}// Here you can implement the rendering logic.// Use <c>ScriptableRenderContext</c> to issue drawing commands or execute command buffers// https://docs.unity3d.com/ScriptReference/Rendering.ScriptableRenderContext.html// You don't have to call ScriptableRenderContext.submit, the render pipeline will call it at specific points in the pipeline.public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData){if (!raytrace_material_)return;raytrace_material_.SetInt("_raytrace_step_count", raytrace_count_);raytrace_material_.SetFloat("_scale", scale_);{Camera cam = renderingData.cameraData.camera;var mtx_view_inv = cam.worldToCameraMatrix.inverse;var mtx_proj_inv = cam.projectionMatrix.inverse;raytrace_material_.SetMatrix("_mtx_view_inv", mtx_view_inv);raytrace_material_.SetMatrix("_mtx_proj_inv", mtx_proj_inv);}const string CommandBufferTag = "raytrace fog Pass";var cmd = CommandBufferPool.Get(CommandBufferTag);RenderTextureDescriptor opaqueDesc = renderingData.cameraData.cameraTargetDescriptor;opaqueDesc.depthBufferBits = 0;cmd.GetTemporaryRT(temp_render_target_.id, opaqueDesc);// 通过材质，将计算结果存入临时缓冲区cmd.Blit(render_target_color_, temp_render_target_.Identifier(), raytrace_material_);// 再从临时缓冲区存入主纹理cmd.Blit(temp_render_target_.Identifier(), render_target_color_);// 执行命令缓冲区context.ExecuteCommandBuffer(cmd);// 释放命令缓存CommandBufferPool.Release(cmd);// 释放临时RTcmd.ReleaseTemporaryRT(temp_render_target_.id);}// Cleanup any allocated resources that were created during the execution of this render pass.public override void OnCameraCleanup(CommandBuffer cmd){}}CustomRenderPass m_ScriptablePass;public Material raytrace_material_;public int raytrace_count_ = 5;public float scale_ = 1.0f;/// <inheritdoc/>public override void Create(){m_ScriptablePass = new CustomRenderPass();// Configures where the render pass should be injected.m_ScriptablePass.renderPassEvent = RenderPassEvent.AfterRenderingOpaques;}// Here you can inject one or multiple render passes in the renderer.// This method is called when setting up the renderer once per-camera.public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData){m_ScriptablePass.render_target_color_ = renderer.cameraColorTarget;m_ScriptablePass.raytrace_material_ = raytrace_material_;m_ScriptablePass.raytrace_count_ = raytrace_count_;m_ScriptablePass.scale_ = scale_;renderer.EnqueuePass(m_ScriptablePass);}
}