ScriptableRenderContext.BeginRenderPass

参数

描述

调度新渲染通道的开头。在任何时候只能有一个渲染通道处于活动状态。

渲染通道是一种在可编程渲染管线环境中切换渲染目标的全新方式。与 SetRenderTargets 函数不同，渲染通道指定了明确的渲染开头和结尾以及渲染表面上的显式加载/存储操作。

渲染通道还允许在同一渲染通道中运行多个子通道，其中，像素着色器能够读取渲染通道中的当前像素值。这样可以高效实现基于瓦片的 GPU 上的各种渲染方法，例如延迟渲染。

渲染通道在 Metal (iOS) 和 Vulkan 上原生实现，但 API 能够通过仿真在所有渲染后端上完全正常运行（使用旧版 SetRenderTargets 调用并通过纹素抓取读取当前像素值）。

渲染通道机制具有以下限制：
- 所有附件必须拥有相同的分辨率和 MSAA 样本数
- 先前子通道的渲染结果仅通过着色器中的 UNITY_READ_FRAMEBUFFER_INPUT(x) 宏
在相同的屏幕空间像素坐标中可用； 附件不能绑定为纹理或直到渲染通道结束才能访问
- iOS Metal 不允许从 Z 缓冲区读取，因此需要其他渲染目标来执行这项操作
- 每个渲染通道允许的最大附件数量目前是 8 + 深度，但请注意，不同的 GPU 可能 有更严格的限制。


另请参阅：BeginSubPass、EndRenderPass、BeginScopedRenderPass、BeginScopedSubPass。

一个有关如何在可编程渲染管线中使用渲染通道 API 实现延迟渲染的简单示例：

using UnityEngine;
using UnityEngine.Rendering;
using Unity.Collections;

public static class DeferredRenderer
{
    public static void ExecuteRenderLoop(Camera camera, CullingResults cullResults, ScriptableRenderContext context)
    {
        // Create the attachment descriptors. If these attachments are not specifically bound to any RenderTexture using the ConfigureTarget calls,
        // these are treated as temporary surfaces that are discarded at the end of the renderpass
        var albedo = new AttachmentDescriptor(RenderTextureFormat.ARGB32);
        var specRough = new AttachmentDescriptor(RenderTextureFormat.ARGB32);
        var normal = new AttachmentDescriptor(RenderTextureFormat.ARGB2101010);
        var emission = new AttachmentDescriptor(RenderTextureFormat.ARGBHalf);
        var depth = new AttachmentDescriptor(RenderTextureFormat.Depth);

        // At the beginning of the render pass, clear the emission buffer to all black, and the depth buffer to 1.0f
        emission.ConfigureClear(new Color(0.0f, 0.0f, 0.0f, 0.0f), 1.0f, 0);
        depth.ConfigureClear(new Color(), 1.0f, 0);

        // Bind the albedo surface to the current camera target, so the final pass will render the Scene to the screen backbuffer
        // The second argument specifies whether the existing contents of the surface need to be loaded as the initial values;
        // in our case we do not need that because we'll be clearing the attachment anyway. This saves a lot of memory
        // bandwidth on tiled GPUs.
        // The third argument specifies whether the rendering results need to be written out to memory at the end of
        // the renderpass. We need this as we'll be generating the final image there.
        // We could do this in the constructor already, but the camera target may change on the fly, esp. in the editor
        albedo.ConfigureTarget(BuiltinRenderTextureType.CameraTarget, false, true);

        // All other attachments are transient surfaces that are not stored anywhere. If the renderer allows,
        // those surfaces do not even have a memory allocated for the pixel values, saving RAM usage.

        // Start the renderpass using the given scriptable rendercontext, resolution, samplecount, array of attachments that will be used within the renderpass and the depth surface
        var attachments = new NativeArray<AttachmentDescriptor>(5, Allocator.Temp);
        const int depthIndex = 0, albedoIndex = 1, specRoughIndex = 2, normalIndex = 3, emissionIndex = 4;
        attachments[depthIndex] = depth;
        attachments[albedoIndex] = albedo;
        attachments[specRoughIndex] = specRough;
        attachments[normalIndex] = normal;
        attachments[emissionIndex] = emission;
        context.BeginRenderPass(camera.pixelWidth, camera.pixelHeight, 1, 1, attachments, depthIndex);
        attachments.Dispose();

        // Start the first subpass, GBuffer creation: render to albedo, specRough, normal and emission, no need to read any input attachments
        var gbufferColors = new NativeArray<int>(4, Allocator.Temp);
        gbufferColors[0] = albedoIndex;
        gbufferColors[1] = specRoughIndex;
        gbufferColors[2] = normalIndex;
        gbufferColors[3] = emissionIndex;
        context.BeginSubPass(gbufferColors);
        gbufferColors.Dispose();

        // Render the deferred G-Buffer
        // RenderGBuffer(cullResults, camera, context);

        context.EndSubPass();

        // Second subpass, lighting: Render to the emission buffer, read from albedo, specRough, normal and depth.
        // The last parameter indicates whether the depth buffer can be bound as read-only.
        // Note that some renderers (notably iOS Metal) won't allow reading from the depth buffer while it's bound as Z-buffer,
        // so those renderers should write the Z into an additional FP32 render target manually in the pixel shader and read from it instead
        var lightingColors = new NativeArray<int>(1, Allocator.Temp);
        lightingColors[0] = emissionIndex;
        var lightingInputs = new NativeArray<int>(4, Allocator.Temp);
        lightingInputs[0] = albedoIndex;
        lightingInputs[1] = specRoughIndex;
        lightingInputs[2] = normalIndex;
        lightingInputs[3] = depthIndex;
        context.BeginSubPass(lightingColors, lightingInputs, true);
        lightingColors.Dispose();
        lightingInputs.Dispose();

        // PushGlobalShadowParams(context);
        // RenderLighting(camera, cullResults, context);

        context.EndSubPass();

        // Third subpass, tonemapping: Render to albedo (which is bound to the camera target), read from emission.
        var tonemappingColors = new NativeArray<int>(1, Allocator.Temp);
        tonemappingColors[0] = albedoIndex;
        var tonemappingInputs = new NativeArray<int>(1, Allocator.Temp);
        tonemappingInputs[0] = emissionIndex;
        context.BeginSubPass(tonemappingColors, tonemappingInputs, true);
        tonemappingColors.Dispose();
        tonemappingInputs.Dispose();

        // present frame buffer.
        // FinalPass(context);

        context.EndSubPass();

        context.EndRenderPass();
    }
}