在 Unity 的 URP 中实现屏幕空间描边效果

public class SelectionRendering : ScriptableRendererFeature
{
	SelectionPass selectionPass;
	public Shader SelectionBlitShader;
	public Color OutlineColor;
	public Color SelectionColor;
	public float SampleDistance;
	public float SampleDistanceReferenceWidth;

	public uint RenderingLayerMask = uint.MaxValue;

	public override void Create()
	{
		if (SelectionBlitShader == null)
		{
			SelectionBlitShader = Shader.Find("SelectionRendering/SelectionBlit");
		}
		// 建立对应的 ScriptableRenderPass
		selectionPass = new SelectionPass(this);
		// 将 ScriptableRenderPass 的渲染时机指定为所有其他渲染操作完成之后
		selectionPass.renderPassEvent = RenderPassEvent.AfterRendering;
	}

	public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData)
	{
		// 在编辑器中不渲染描边
		if (renderingData.cameraData.cameraType == CameraType.Game)
		{
			// 把创建的 ScriptableRenderPass 放入渲染队列
			renderer.EnqueuePass(selectionPass);
		}
	}
	public override void SetupRenderPasses(ScriptableRenderer renderer, in RenderingData renderingData)
	{
		if (renderingData.cameraData.cameraType == CameraType.Game)
		{
			// 把当前相机的 RenderTarget 传给创建的 ScriptableRenderPass
			selectionPass.SetTarget(renderer.cameraColorTargetHandle);
		}
	}
}

class SelectionPass : ScriptableRenderPass
{
	public static readonly int SampleDistanceShaderId = Shader.PropertyToID("_SampleDistance");
	public static readonly int BlitTextureSizeShaderId = Shader.PropertyToID("_BlitTextureSize");
	public static readonly int SelectionColorShaderId = Shader.PropertyToID("_SelectionColor");
	public static readonly int OutlineShaderId = Shader.PropertyToID("_OutlineColor");
	public SelectionRendering feature;
	private RTHandle selection;
	private RTHandle selectionDepth;
	private RTHandle cameraColor;
	private List<ShaderTagId> shaderTagIdList = new List<ShaderTagId> {
		new ShaderTagId("UniversalForward"),
		new ShaderTagId("UniversalForwardOnly"),
		new ShaderTagId("LightweightForward"),
		new ShaderTagId("SRPDefaultUnlit")
	};

	public Material selectionBlitMaterial;

	private static readonly ShaderTagId SelectionShaderTagId = new ShaderTagId("Selection");

	public SelectionPass(SelectionRendering feature)
	{
		this.feature = feature;
		// 在构造函数中创建描边时使用的材质
		this.selectionBlitMaterial = CoreUtils.CreateEngineMaterial(feature.SelectionBlitShader);
		profilingSampler = new ProfilingSampler("SelectionPass");
	}

	public void SetTarget(RTHandle cameraColor)
	{
		this.cameraColor = cameraColor;
	}

	public override void OnCameraSetup(CommandBuffer cmd, ref RenderingData renderingData)
	{
		RenderTextureDescriptor descriptor = renderingData.cameraData.cameraTargetDescriptor;
		descriptor.colorFormat = RenderTextureFormat.ARGB32;
		// 分配深度 RenderTexture
		RenderingUtils.ReAllocateIfNeeded(ref selectionDepth, descriptor, wrapMode: TextureWrapMode.Clamp, name: "_selection");
		// 分配颜色 RenderTexture
		descriptor.depthBufferBits = 0;
		RenderingUtils.ReAllocateIfNeeded(ref selection, descriptor, wrapMode: TextureWrapMode.Clamp, name: "_selection");

		// 设置描边材质的信息，这里我希望描边宽度与分辨率成正比所以做了一些计算
		selectionBlitMaterial.SetFloat(SampleDistanceShaderId, feature.SampleDistance * ((float)descriptor.width) / feature.SampleDistanceReferenceWidth);
		selectionBlitMaterial.SetVector(BlitTextureSizeShaderId, new Vector4(descriptor.width, descriptor.height));
		selectionBlitMaterial.SetColor(SelectionColorShaderId, feature.SelectionColor);
		selectionBlitMaterial.SetColor(OutlineShaderId, feature.OutlineColor);

		// 指定 Render Target 和渲染前 RenderTexture 的清空方式
		// 这里需要注意的是，不能只使用颜色 RenderTexture 而不使用深度 RenderTexture，否则会报错
		ConfigureTarget(selection, selectionDepth);
		ConfigureClear(ClearFlag.All, new Color(0, 0, 0, 0));
	}

	public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData)
	{
		// 在编辑器中不渲染描边
		if (renderingData.cameraData.cameraType != CameraType.Game)
		{
			return;
		}
		// 分配渲染用的 CommandBuffer
		CommandBuffer cmd = CommandBufferPool.Get(name: "Selection");
		// ProfilingScope 仅仅是为了方便在 Profiler 中显示
		using (new ProfilingScope(profilingSampler))
		{
			// 指定 DrawingSettings，这里使用了 URP 默认的 Shader Pass
			DrawingSettings drawingSettings = CreateDrawingSettings(shaderTagIdList, ref renderingData, renderingData.cameraData.defaultOpaqueSortFlags);
			RendererListParams rendererListParams = new RendererListParams(
				renderingData.cullResults,
				drawingSettings,
				// 指定 FilteringSettings，这里我们只关心渲染层
				new FilteringSettings(RenderQueueRange.all, -1, feature.RenderingLayerMask)
			);

			// 构建 RendererList
			RendererList rendererList = context.CreateRendererList(ref rendererListParams);

			// 渲染 RendererList
			cmd.DrawRendererList(rendererList);
			context.ExecuteCommandBuffer(cmd);
			cmd.Clear();

			// 准备绘制描边
			Blitter.BlitCameraTexture(cmd, selection, cameraColor, selectionBlitMaterial, 0);

			// 执行
			context.ExecuteCommandBuffer(cmd);
			cmd.Clear();
		}
		// 回收 CommandBuffer
		CommandBufferPool.Release(cmd);
	}
}