Unity SRP 管线【第四讲：URP 阴影】

URP 全文源码解析参照

引入

在UniversalRenderer.cs/ line 505行处
此处已经准备好了所有渲染数据（所有数据全部存储在了renderingData中）
我们只用renderingData中的数据初设置mainLightShadows

bool mainLightShadows = m_MainLightShadowCasterPass.Setup(ref renderingData);
bool additionalLightShadows = m_AdditionalLightsShadowCasterPass.Setup(ref renderingData);
bool transparentsNeedSettingsPass = m_TransparentSettingsPass.Setup(ref renderingData);

进入函数 m_MainLightShadowCasterPass.Setup(ref renderingData);

MainLightShadowCasterPass.cs

一、MainLightShadowCasterPass 创建

public MainLightShadowCasterPass(RenderPassEvent evt)

在UniversalRenderer管线创建的时候，我们已经做了创建

我们设置MainLightShadow在RenderingShadows（Event = 50）之前

// UniversalRenderer.cs
public UniversalRenderer(UniversalRendererData data) : base(data)
{
	...
	m_MainLightShadowCasterPass = new MainLightShadowCasterPass(RenderPassEvent.BeforeRenderingShadows);
	m_AdditionalLightsShadowCasterPass = new AdditionalLightsShadowCasterPass(RenderPassEvent.BeforeRenderingShadows);
	...
}

创建MainLightShadowCasterPass

参数：

基类参数

public RenderPassEvent renderPassEvent { get; set; }
int renderTargetWidth { get; set; }
int renderTargetHeight { get; set; }

自定义参数

const int k_MaxCascades = 4;
const int k_ShadowmapBufferBits = 16;
float m_CascadeBorder;                  // 边缘模糊度（0~1）
float m_MaxShadowDistanceSq;			// 最大阴影距离的平方
int m_ShadowCasterCascadesCount;		// 当前设置的Cascades数量

RenderTargetHandle m_MainLightShadowmap;           //临时阴影纹理Shader中的索引ID
internal RenderTexture m_MainLightShadowmapTexture;//实际临时阴影纹理，以及设置(深度为16位)

Matrix4x4[] m_MainLightShadowMatrices;// 阴影矩阵
ShadowSliceData[] m_CascadeSlices;    // 级联阴影切片数据,包含(
						//splitData、       级联阴影数据（由UNITY内置函数提供）
						//offsetX/Y、       级联阴影偏移
						//resolution        级联阴影分辨率
						//shadowTransform   级联阴影矩阵
Vector4[] m_CascadeSplitDistances;    // 包围球数据（xyz：位置，w：半径）

GPU ID参数

private static class MainLightShadowConstantBuffer
{
    public static int _WorldToShadow;
    public static int _ShadowParams;
    public static int _CascadeShadowSplitSpheres0;
    public static int _CascadeShadowSplitSpheres1;
    public static int _CascadeShadowSplitSpheres2;
    public static int _CascadeShadowSplitSpheres3;
    public static int _CascadeShadowSplitSphereRadii;
    public static int _ShadowOffset0;
    public static int _ShadowOffset1;
    public static int _ShadowOffset2;
    public static int _ShadowOffset3;
    public static int _ShadowmapSize;
}

MainLightShadowConstantBuffer._WorldToShadow = Shader.PropertyToID("_MainLightWorldToShadow");
MainLightShadowConstantBuffer._ShadowParams = Shader.PropertyToID("_MainLightShadowParams");
MainLightShadowConstantBuffer._CascadeShadowSplitSpheres0 = Shader.PropertyToID("_CascadeShadowSplitSpheres0");
MainLightShadowConstantBuffer._CascadeShadowSplitSpheres1 = Shader.PropertyToID("_CascadeShadowSplitSpheres1");
MainLightShadowConstantBuffer._CascadeShadowSplitSpheres2 = Shader.PropertyToID("_CascadeShadowSplitSpheres2");
MainLightShadowConstantBuffer._CascadeShadowSplitSpheres3 = Shader.PropertyToID("_CascadeShadowSplitSpheres3");
MainLightShadowConstantBuffer._CascadeShadowSplitSphereRadii = Shader.PropertyToID("_CascadeShadowSplitSphereRadii");
MainLightShadowConstantBuffer._ShadowOffset0 = Shader.PropertyToID("_MainLightShadowOffset0");
MainLightShadowConstantBuffer._ShadowOffset1 = Shader.PropertyToID("_MainLightShadowOffset1");
MainLightShadowConstantBuffer._ShadowOffset2 = Shader.PropertyToID("_MainLightShadowOffset2");
MainLightShadowConstantBuffer._ShadowOffset3 = Shader.PropertyToID("_MainLightShadowOffset3");
MainLightShadowConstantBuffer._ShadowmapSize = Shader.PropertyToID("_MainLightShadowmapSize");
m_MainLightShadowmap.Init("_MainLightShadowmapTexture");

二、MainLightShadowCasterPass 初始化

public bool Setup(ref RenderingData renderingData){}

1. 初始化数据（矩阵、cascade分割、切片）

Clear();

2. 一些条件判断，未通过的话将ShadowMap置为默认值

… 这里不作为重点

3. 设置数据

SetUp函数主要是将数据整理保存到类内参数中供类使用
其中设置的数据有：

（1）m_ShadowCasterCascadesCount

光源级联阴影数量

（2）RenderTexture 大小

renderTargetWidth;
renderTargetHeight;

（3）提取级联阴影的光照矩阵以及包围球

1. Vector4[] m_CascadeSplitDistances[cascadeIndex] 包围球数据（xyz：位置，w：半径）
2. ShadowSliceData[] m_CascadeSlices; 级联阴影切片数据

public struct ShadowSliceData
{
    public Matrix4x4 viewMatrix;
    public Matrix4x4 projectionMatrix;
    public Matrix4x4 shadowTransform;//projectionMatrix * viewMatrix
    public int offsetX;			     //ShadowAtlasMap行偏移
    public int offsetY;				 //ShadowAtlasMap列偏移
    public int resolution;			 //分辨率
    public ShadowSplitData splitData;//splitData包含筛选信息
}

提取函数

for (int cascadeIndex = 0; cascadeIndex < m_ShadowCasterCascadesCount; ++cascadeIndex)
{
    bool success = ShadowUtils.ExtractDirectionalLightMatrix(
    	ref renderingData.cullResults, 
    	ref renderingData.shadowData,
        shadowLightIndex, 
        cascadeIndex, 
        renderTargetWidth, 
        renderTargetHeight, 
        shadowResolution, 
        light.shadowNearPlane,
        out m_CascadeSplitDistances[cascadeIndex], 
        out m_CascadeSlices[cascadeIndex]);

    if (!success)
        return SetupForEmptyRendering(ref renderingData);
}

（4）获取新的临时阴影纹理

m_MainLightShadowmapTexture = ShadowUtils.GetTemporaryShadowTexture(renderTargetWidth, renderTargetHeight, k_ShadowmapBufferBits);

（5）存储renderingData中的阴影数据

m_MaxShadowDistanceSq = renderingData.cameraData.maxShadowDistance * renderingData.cameraData.maxShadowDistance;
m_CascadeBorder = renderingData.shadowData.mainLightShadowCascadeBorder;

4. 绑定渲染目标和清除状态

public override void Configure(CommandBuffer cmd, RenderTextureDescriptor cameraTextureDescriptor)
{
    ConfigureTarget(new RenderTargetIdentifier(m_MainLightShadowmapTexture), m_MainLightShadowmapTexture.depthStencilFormat, renderTargetWidth, renderTargetHeight, 1, true);
    ConfigureClear(ClearFlag.All, Color.black);
}

5. 执行 Execute

public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData)
{
    if (m_CreateEmptyShadowmap)
    {
        SetEmptyMainLightCascadeShadowmap(ref context);
        return;
    }

    RenderMainLightCascadeShadowmap(ref context, ref renderingData.cullResults, ref renderingData.lightData, ref renderingData.shadowData);
}

三、Execute 阴影参数设置

RenderMainLightCascadeShadowmap(
			ref context, 
			ref renderingData.cullResults, 
			ref renderingData.lightData, 
			ref renderingData.shadowData);

参数设置多引用ShadowsUtils单例中的函数设置
进入函数RenderMainLightCascadeShadowmap

// settings 中为阴影所需要的数据
var settings = new ShadowDrawingSettings(cullResults, shadowLightIndex);
settings.useRenderingLayerMaskTest = UniversalRenderPipeline.asset.supportsLightLayers;

for (int cascadeIndex = 0; cascadeIndex < m_ShadowCasterCascadesCount; ++cascadeIndex)
{
    settings.splitData = m_CascadeSlices[cascadeIndex].splitData;

    // 偏移量
    Vector4 shadowBias = ShadowUtils.GetShadowBias(ref shadowLight, shadowLightIndex, ref shadowData, m_CascadeSlices[cascadeIndex].projectionMatrix, m_CascadeSlices[cascadeIndex].resolution);
    ShadowUtils.SetupShadowCasterConstantBuffer(cmd, ref shadowLight, shadowBias);
    
    CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.CastingPunctualLightShadow, false);
    ShadowUtils.RenderShadowSlice(cmd, ref context, ref m_CascadeSlices[cascadeIndex],
        ref settings, m_CascadeSlices[cascadeIndex].projectionMatrix, m_CascadeSlices[cascadeIndex].viewMatrix);
}

shadowData.isKeywordSoftShadowsEnabled = shadowLight.light.shadows == LightShadows.Soft && shadowData.supportsSoftShadows;
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.MainLightShadows, shadowData.mainLightShadowCascadesCount == 1);
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.MainLightShadowCascades, shadowData.mainLightShadowCascadesCount > 1);
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.SoftShadows, shadowData.isKeywordSoftShadowsEnabled);

SetupMainLightShadowReceiverConstants(cmd, shadowLight, shadowData.supportsSoftShadows);

1. SetupShadowCasterConstantBuffer

// 阴影偏移
cmd.SetGlobalVector("_ShadowBias", shadowBias);

// 世界空间光照方向，用于支持阴影法线偏移(normal bias).
Vector3 lightDirection = -shadowLight.localToWorldMatrix.GetColumn(2);
cmd.SetGlobalVector("_LightDirection", new Vector4(lightDirection.x, lightDirection.y, lightDirection.z, 0.0f));

// 世界空间光源位置
Vector3 lightPosition = shadowLight.localToWorldMatrix.GetColumn(3);
cmd.SetGlobalVector("_LightPosition", new Vector4(lightPosition.x, lightPosition.y, lightPosition.z, 1.0f));

2. 关闭精确光源阴影 PunctualLightShadow（点光源、方向光、聚光灯）

/*  "_CASTING_PUNCTUAL_LIGHT_SHADOW"  */
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.CastingPunctualLightShadow, false);

3. 渲染cascade阴影
   设置默认全局偏移（这应该就是为什么，在unity中，将light的bias都设为0，但还是没有明显的摩尔纹）

cmd.SetGlobalDepthBias(1.0f, 2.5f); // these values match HDRP defaults 

设置Viewport、矩阵信息

cmd.SetViewport(new Rect(shadowSliceData.offsetX, shadowSliceData.offsetY, shadowSliceData.resolution, shadowSliceData.resolution));
cmd.SetViewProjectionMatrices(view, proj);

渲染阴影，禁用硬件剪刀矩阵（不知道是什么）

context.DrawShadows(ref settings);
cmd.DisableScissorRect();

4. 是否开启主光源阴影、主光源级联阴影、主光源软阴影

/*"_MAIN_LIGHT_SHADOWS"*/
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.MainLightShadows, shadowData.mainLightShadowCascadesCount == 1);
/*"_MAIN_LIGHT_SHADOWS_CASCADE"*/
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.MainLightShadowCascades, shadowData.mainLightShadowCascadesCount > 1);
/*"_SHADOWS_SOFT"*/
CoreUtils.SetKeyword(cmd, ShaderKeywordStrings.SoftShadows, shadowData.isKeywordSoftShadowsEnabled);

5. 设置阴影接收常量

   1. 阴影贴图 “_MainLightShadowmapTexture”
   2. 光源矩阵 “_MainLightWorldToShadow”（级联矩阵为多个矩阵 Matrix4x4[]）
   3. 阴影参数 “_MainLightShadowParams”
      • x ：阴影强度
      • y ：是否软阴影（1为软，0为硬）
      • z ：阴影衰减部分的衰减斜率
      • w：横坐标0处阴影开始衰减部分的纵坐标
   4. 级联阴影数据：
      四个级联的包围球数据（4 * Vector4） _CascadeShadowSplitSpheres0、1、2、3
      四个级联的半径的平方（1 * Vector4） _CascadeShadowSplitSphereRadii
   5. 软阴影：
      阴影像素偏移（左下、右下、左上、右上） _MainLightShadowOffset0/1/2/3
      阴影偏移量（偏移x，偏移y，shadowMap宽，shadowMap长 _MainLightShadowmapSize

6. 在渲染结束后释放临时Texture

public override void OnCameraCleanup(CommandBuffer cmd)
{
    if (cmd == null)
        throw new ArgumentNullException("cmd");

    if (m_MainLightShadowmapTexture)
    {
        RenderTexture.ReleaseTemporary(m_MainLightShadowmapTexture);
        m_MainLightShadowmapTexture = null;
    }
}

GPU中的阴影数据

Shadows.hlsl

SCREENSPACE_TEXTURE(_ScreenSpaceShadowmapTexture);
SAMPLER(sampler_ScreenSpaceShadowmapTexture);

TEXTURE2D_SHADOW(_MainLightShadowmapTexture);
SAMPLER_CMP(sampler_MainLightShadowmapTexture);

TEXTURE2D_SHADOW(_AdditionalLightsShadowmapTexture);
SAMPLER_CMP(sampler_AdditionalLightsShadowmapTexture);

CBUFFER_START(MainLightShadows)
	// Last cascade is initialized with a no-op matrix. It always transforms
	// shadow coord to half3(0, 0, NEAR_PLANE). We use this trick to avoid
	// branching since ComputeCascadeIndex can return cascade index = MAX_SHADOW_CASCADES
	float4x4    _MainLightWorldToShadow[MAX_SHADOW_CASCADES + 1];
	float4      _CascadeShadowSplitSpheres0;
	float4      _CascadeShadowSplitSpheres1;
	float4      _CascadeShadowSplitSpheres2;
	float4      _CascadeShadowSplitSpheres3;
	float4      _CascadeShadowSplitSphereRadii;
	half4       _MainLightShadowOffset0;
	half4       _MainLightShadowOffset1;
	half4       _MainLightShadowOffset2;
	half4       _MainLightShadowOffset3;
	half4       _MainLightShadowParams;   // (x: shadowStrength, y: 1.0 if soft shadows, 0.0 otherwise, z: main light fade scale, w: main light fade bias)
	float4      _MainLightShadowmapSize;  // (xy: 1/width and 1/height, zw: width and height)
CBUFFER_END