第十八章 Post-Processing

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本文介绍如何在游戏渲染引擎中实现后处理效果，包括离屏渲染、全屏四边形组件的创建及使用，以及如何灵活地支持不同的后处理效果。

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第十八章 Post-Processing

Post-processing是指在场景渲染之后，使用一些图形技术对场景进行处理。比如，把整个场景转换为grayscale（灰度）样式或使场景中明亮的区域发光。本章将编写一些post-processing effects，并集成到C++渲染引擎框架中。

Render Targets

到目前为止，所有的示例程序都是直接把场景渲染到back buffer，这是一个2D texture用于在渲染完成之后把图像显示到屏幕上。但是在post-processing应用程序中，首先把场景渲染到一个中间层的texture buffer中，然后把post-processing effect应用到该texture。最后使用一个全屏的四边形（由两个三角形组成，包含整个屏幕区域）渲染最终要显示到屏幕上的图像。
下面的步骤概括了post-processing的处理过程：
1、将一个off-screen render target（离屏渲染目标）绑定到管线的output-merger阶段。
2、在off-scrent render target上绘制场景。
3、恢复back buffer，并作为绑定到output-merger阶段的render target。
4、使用off-scrent render target的texture作为一种post-processing effect的输入buffer，绘制一个全屏的四边形。
为了更容易的执行这些步骤，首先创建一个FullScreenRenderTarget类，该类的声明代码如列表18.1所示。

列表18.1 Declaration of the FullScreenRenderTarget Class

#pragma once

#include "Common.h"

namespace Library
{
    class Game;

    class FullScreenRenderTarget
    {
    public:
        FullScreenRenderTarget(Game& game);
        ~FullScreenRenderTarget();

        ID3D11ShaderResourceView* OutputTexture() const;
        ID3D11RenderTargetView* RenderTargetView() const;
        ID3D11DepthStencilView* DepthStencilView() const;

        void Begin();
        void End();

    private:
        FullScreenRenderTarget();
        FullScreenRenderTarget(const FullScreenRenderTarget& rhs);
        FullScreenRenderTarget& operator=(const FullScreenRenderTarget& rhs);

        Game* mGame;
        ID3D11RenderTargetView* mRenderTargetView;
        ID3D11DepthStencilView* mDepthStencilView;
        ID3D11ShaderResourceView* mOutputTexture;
    };
}

FullScreenRenderTarget类中的成员变量看起来非常熟悉，在Game类中已经包含了同样的数据类型ID3D11RenderTargetView和ID3D11DepthStencilView。这些类型的变量是用于把一个render target和depth-stencil buffer绑定到管线的output-merger阶段。与Game类不同的是，在FullScrentRenderTarget类中还包含有一个ID3D11ShaderResourceView类型的成员变量，表示render target底层的2D texture buffer。这种类型的输出texture可以作为post-processing shaders的输入数据。FullScreenRenderTarget::Begin()和FullScreenRenderTarget::End()函数分别用于把render target绑定以output-merger阶段和恢复back buffer。列表18.2列出了FullScrennRenderTarget类的实现代码。

列表18.2 Implementation of the FullScreenRenderTarget Class

#include "FullScreenRenderTarget.h"
#include "Game.h"
#include "GameException.h"

namespace Library
{
    FullScreenRenderTarget::FullScreenRenderTarget(Game& game)
        : mGame(&game), mRenderTargetView(nullptr), mDepthStencilView(nullptr), mOutputTexture(nullptr)
    {
        D3D11_TEXTURE2D_DESC fullScreenTextureDesc;
        ZeroMemory(&fullScreenTextureDesc, sizeof(fullScreenTextureDesc));
        fullScreenTextureDesc.Width = game.ScreenWidth();
        fullScreenTextureDesc.Height = game.ScreenHeight();
        fullScreenTextureDesc.MipLevels = 1;
        fullScreenTextureDesc.ArraySize = 1;
        fullScreenTextureDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
        fullScreenTextureDesc.SampleDesc.Count = 1;
        fullScreenTextureDesc.SampleDesc.Quality = 0;
        fullScreenTextureDesc.BindFlags = D3D11_BIND_RENDER_TARGET | D3D11_BIND_SHADER_RESOURCE;
        fullScreenTextureDesc.Usage = D3D11_USAGE_DEFAULT;

        HRESULT hr;
        ID3D11Texture2D* fullScreenTexture = nullptr;
        if (FAILED(hr = game.Direct3DDevice()->CreateTexture2D(&fullScreenTextureDesc, nullptr, &fullScreenTexture)))
        {
            throw GameException("IDXGIDevice::CreateTexture2D() failed.", hr);
        }

        if (FAILED(hr = game.Direct3DDevice()->CreateShaderResourceView(fullScreenTexture, nullptr, &mOutputTexture)))
        {
            throw GameException("IDXGIDevice::CreateShaderResourceView() failed.", hr);
        }

        if (FAILED(hr = game.Direct3DDevice()->CreateRenderTargetView(fullScreenTexture, nullptr, &mRenderTargetView)))
        {
            ReleaseObject(fullScreenTexture);
            throw GameException("IDXGIDevice::CreateRenderTargetView() failed.", hr);
        }

        ReleaseObject(fullScreenTexture);

        D3D11_TEXTURE2D_DESC depthStencilDesc;
        ZeroMemory(&depthStencilDesc, sizeof(depthStencilDesc));
        depthStencilDesc.Width = game.ScreenWidth();
        depthStencilDesc.Height = game.ScreenHeight();
        depthStencilDesc.MipLevels = 1;
        depthStencilDesc.ArraySize = 1;
        depthStencilDesc.Format = DXGI_FORMAT_D24_UNORM_S8_UINT;
        depthStencilDesc.SampleDesc.Count = 1;
        depthStencilDesc.SampleDesc.Quality = 0;
        depthStencilDesc.BindFlags = D3D11_BIND_DEPTH_STENCIL;
        depthStencilDesc.Usage = D3D11_USAGE_DEFAULT;            

        ID3D11Texture2D* depthStencilBuffer = nullptr;
        if (FAILED(hr = game.Direct3DDevice()->CreateTexture2D(&depthStencilDesc, nullptr, &depthStencilBuffer)))
        {
            throw GameException("IDXGIDevice::CreateTexture2D() failed.", hr);
        }

        if (FAILED(hr = game.Direct3DDevice()->CreateDepthStencilView(depthStencilBuffer, nullptr, &mDepthStencilView)))
        {
            ReleaseObject(depthStencilBuffer);
            throw GameException("IDXGIDevice::CreateDepthStencilView() failed.", hr);
        }

        ReleaseObject(depthStencilBuffer);
    }

    FullScreenRenderTarget::~FullScreenRenderTarget()
    {
        ReleaseObject(mOutputTexture);
        ReleaseObject(mDepthStencilView);
        ReleaseObject(mRenderTargetView);
    }

    ID3D11ShaderResourceView* FullScreenRenderTarget::OutputTexture() const
    {
        return mOutputTexture;
    }

    ID3D11RenderTargetView* FullScreenRenderTarget::RenderTargetView() const
    {
        return mRenderTargetView;
    }

    ID3D11DepthStencilView* FullScreenRenderTarget::DepthStencilView() const
    {
        return mDepthStencilView;
    }

    void FullScreenRenderTarget::Begin()
    {	
        mGame->Direct3DDeviceContext()->OMSetRenderTargets(1, &mRenderTargetView, mDepthStencilView);
    }

    void FullScreenRenderTarget::End()
    {
        mGame->ResetRenderTargets();
    }
}

在FullScreenRenderTarget类的构造函数中，包含了该类的大部分实现代码。在该构造函数中，首先构建一个D3D11_TEXTURE2D_DESC结构体类型的实例，并用于创建render target的底层texture buffer。在Game类的初始化过程中这些步骤并不是显式定义的，因为在构建swap chain时就会隐含的创建back buffer。其中在D3D11_TEXTURE2D_DESC结构体实例化时指定了两个bind flags：D3D11_BIND_RENDER_TARGET和D3D_BIND_SHADER_RESOURCE。这两个flags值表示该texture可以同时作为一个render target和一个shader的输入数据。
创建完ID3D11Texture2D类型的texture之后，就可以使用该texture创建shader resource view和render target view。并对外提供了访问这两个views变量的公有函数接口；并且在这两个变量初始化之后就可以释放对texture的引用了。对于depth-stencil view变量的初始化使用类似的操作步骤。
在FullScrentRenderTarget::Begin()函数中，直接调用ID3D11DeviceContext::OMSetRenderTargets()函数把render target view和depth-stencil view绑定到管线的output-merger阶段。而FullScreenRenderTarget::End()函数中则是调用Game::ResetRenderTargets()函数，该函数同样是调用OMSetRenderTargets()函数重置绑定为Game类的中render target view和detph-stencil view变量。
对于FullScreenRenderTarget类的调用方式如下：
mRenderTarget->Begin();
// 1. Clear mRenderTarget->RenderTargetView()
// 2. Clear mRenderTarget->DepthStencilView()
// 3. Draw Objects

mRenderTarget->End();

这种方法首先把objects渲染到FullScreenRenderTarget实例下的2D texture，并且可以通过调用FullScreenRenderTarget::OutputTexture()函数访问该texture变量。最后调用FullScreenRenderTarget::End()函数，就可以把所有的渲染数据写到back buffer中。

A Full-Screen Quad Component

现在我们可以把场景渲染到一个off-screen texture中，接下来需要把一种effect应用到该texture并在屏幕上显示应用的输出结果。在这样一个系统结构中需要封装渲染部分的代码，否则在程序之间将会出现重复多余的代码，但是又要使渲染代码足够灵活，以及支持任意的post-processing effects。列表18.3列出了FullScreenQuad类的声明代码。

列表18.3 Declaration of the FullScreenQuad Class

#pragma once

#include <functional>
#include "DrawableGameComponent.h"

namespace Library
{
	class Effect;
	class Material;
	class Pass;

	class FullScreenQuad : public DrawableGameComponent
	{
		RTTI_DECLARATIONS(FullScreenQuad, DrawableGameComponent)

	public:
		FullScreenQuad(Game& game);
		FullScreenQuad(Game& game, Material& material);
		~FullScreenQuad();

		Material* GetMaterial();
		void SetMaterial(Material& material, const std::string& techniqueName, const std::string& passName);
		void SetActiveTechnique(const std::string& techniqueName, const std::string& passName);
		void SetCustomUpdateMaterial(std::function<void()> callback);

		virtual void Initialize() override;
		virtual void Draw(const GameTime& gameTime) override;

	private:
		FullScreenQuad();
		FullScreenQuad(const FullScreenQuad& rhs);
		FullScreenQuad& operator=(const FullScreenQuad& rhs);

		Material* mMaterial;
		Pass* mPass;
		ID3D11InputLayout* mInputLayout;

		ID3D11Buffer* mVertexBuffer;
		ID3D11Buffer* mIndexBuffer;
		UINT mIndexCount;
		std::function<void()> mCustomUpdateMaterial;
	};
}

在FullScreenQuad类中，成员变量mMaterial指向一个用于绘制四边形的material对象，并可以通过调用SetMaterial函数对该变量进行赋值。这样就可以在同一个FullScreenQuad实例对象的生存期内使用不同的materials。成员变量mPass和mInputLayout中存储了Draw()函数中使用的相关数据。另外两个非常熟悉的变量vertex和index buffers存储了该四边形对象的vertices和indices。在FullScreenQuad类中使用了一种新的数据类型std::function<T>，如果你之前没有见过的话，这是一种通用的函数封装语法，用于存储并调用函数，bind表达式，lambda表达式（回调以及关闭）。在这里是为了支持FullScreenQuad的调用者更新material shader中的变量。之所以这样做是因为在FullScreenQuad类中并不知道用于渲染四边形的是哪种material（因此也就无法知道material使用的shader输入变量）。FullScreenQuad类要完成的全部操作就是渲染一个四边形；而更新material变量的操作由该类对象的调用者完成。
列表18.4中列出了FullScreenQuad类的实现代码。

列表18.4 Implementation of the FullScreenQuad Class

#include "FullScreenQuad.h"
#include "Game.h"
#include "GameException.h"
#include "Material.h"
#include "VertexDeclarations.h"

namespace Library
{
	RTTI_DEFINITIONS(FullScreenQuad)

		FullScreenQuad::FullScreenQuad(Game& game)
		: DrawableGameComponent(game),
		mMaterial(nullptr), mPass(nullptr), mInputLayout(nullptr),
		mVertexBuffer(nullptr), mIndexBuffer(nullptr), mIndexCount(0), mCustomUpdateMaterial(nullptr)
	{
	}

	FullScreenQuad::FullScreenQuad(Game& game, Material& material)
		: DrawableGameComponent(game),
		mMaterial(&material), mPass(nullptr), mInputLayout(nullptr),
		mVertexBuffer(nullptr), mIndexBuffer(nullptr), mIndexCount(0), mCustomUpdateMaterial(nullptr)
	{
	}

	FullScreenQuad::~FullScreenQuad()
	{
		ReleaseObject(mIndexBuffer);
		ReleaseObject(mVertexBuffer);
	}

	Material* FullScreenQuad::GetMaterial()
	{
		return mMaterial;
	}

	void FullScreenQuad::SetMaterial(Material& material, const std::string& techniqueName, const std::string& passName)
	{
		mMaterial = &material;
		SetActiveTechnique(techniqueName, passName);
	}

	void FullScreenQuad::SetActiveTechnique(const std::string& techniqueName, const std::string& passName)
	{
		Technique* technique = mMaterial->GetEffect()->TechniquesByName().at(techniqueName);
		assert(technique != nullptr);

		mPass = technique->PassesByName().at(passName);
		assert(mPass != nullptr);
		mInputLayout = mMaterial->InputLayouts().at(mPass);
	}

	void FullScreenQuad::SetCustomUpdateMaterial(std::function<void()> callback)
	{
		mCustomUpdateMaterial = callback;
	}

	void FullScreenQuad::Initialize()
	{
		VertexPositionTexture vertices[] =
		{
			VertexPositionTexture(XMFLOAT4(-1.0f, -1.0f, 0.0f, 1.0f), XMFLOAT2(0.0f, 1.0f)),
			VertexPositionTexture(XMFLOAT4(-1.0f, 1.0f, 0.0f, 1.0f), XMFLOAT2(0.0f, 0.0f)),
			VertexPositionTexture(XMFLOAT4(1.0f, 1.0f, 0.0f, 1.0f), XMFLOAT2(1.0f, 0.0f)),
			VertexPositionTexture(XMFLOAT4(1.0f, -1.0f, 0.0f, 1.0f), XMFLOAT2(1.0f, 1.0f)),
		};

		D3D11_BUFFER_DESC vertexBufferDesc;
		ZeroMemory(&vertexBufferDesc, sizeof(vertexBufferDesc));
		vertexBufferDesc.ByteWidth = sizeof(VertexPositionTexture)* ARRAYSIZE(vertices);
		vertexBufferDesc.Usage = D3D11_USAGE_IMMUTABLE;
		vertexBufferDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER;

		D3D11_SUBRESOURCE_DATA vertexSubResourceData;
		ZeroMemory(&vertexSubResourceData, sizeof(vertexSubResourceData));
		vertexSubResourceData.pSysMem = vertices;
		if (FAILED(mGame->Direct3DDevice()->CreateBuffer(&vertexBufferDesc, &vertexSubResourceData, &mVertexBuffer)))
		{
			throw GameException("ID3D11Device::CreateBuffer() failed.");
		}

		UINT indices[] =
		{
			0, 1, 2,
			0, 2, 3
		};

		mIndexCount = ARRAYSIZE(indices);

		D3D11_BUFFER_DESC indexBufferDesc;
		ZeroMemory(&indexBufferDesc, sizeof(indexBufferDesc));
		indexBufferDesc.ByteWidth = sizeof(UINT)* mIndexCount;
		indexBufferDesc.Usage = D3D11_USAGE_IMMUTABLE;
		indexBufferDesc.BindFlags = D3D11_BIND_INDEX_BUFFER;

		D3D11_SUBRESOURCE_DATA indexSubResourceData;
		ZeroMemory(&indexSubResourceData, sizeof(indexSubResourceData));
		indexSubResourceData.pSysMem = indices;
		if (FAILED(mGame->Direct3DDevice()->CreateBuffer(&indexBufferDesc, &indexSubResourceData, &mIndexBuffer)))
		{
			throw GameException("ID3D11Device::CreateBuffer() failed.");
		}
	}

	void FullScreenQuad::Draw(const GameTime& gameTime)
	{
		assert(mPass != nullptr);
		assert(mInputLayout != nullptr);

		ID3D11DeviceContext* direct3DDeviceContext = mGame->Direct3DDeviceContext();
		direct3DDeviceContext->IASetPrimitiveTopology(D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST);
		direct3DDeviceContext->IASetInputLayout(mInputLayout);

		UINT stride = sizeof(VertexPositionTexture);
		UINT offset = 0;
		direct3DDeviceContext->IASetVertexBuffers(0, 1, &mVertexBuffer, &stride, &offset);
		direct3DDeviceContext->IASetIndexBuffer(mIndexBuffer, DXGI_FORMAT_R32_UINT, 0);

		if (mCustomUpdateMaterial != nullptr)
		{
			mCustomUpdateMaterial();
		}

		mPass->Apply(0, direct3DDeviceContext);

		direct3DDeviceContext->DrawIndexed(mIndexCount, 0, 0);
	}
}

首先分析FullScreenQuad::Initialize()函数，该函数的实现与之前示例中的代码基本相同，主要不同的地方是4个vertices所处的空间位置。在screen space中，（-1, -1）表示屏幕的左上角，（1, 1）表示右下角。因为vertices的坐标位置已经是处于screen space中，所以在vertext shader中不需要对这些坐标执行变换。
接下来，分析FullScreen::Draw()函数。除了执行std::function类型的mCustomUpdateMaterial()语句之外，在Draw()函数没有特别需要说明的地方。其中std::function<T>类是一个函数对象（或函数），并提供了公有的operator()函数（重载()运算符）。使用这种方式，看起来就像是调用了一个命名为mCustomUpdateMaterial的函数；实际上，只是调用了std::function<T>::operation()运算符重载函数，真正的目的是用于回调函数或lambda表达式。
最后，需要注意的是四边形的vertices由一个position和texture coordinates表示。这种表示方式限制了在FullScreenQuad类中能够使用的material种类，但是该依然具有多种应用。只需要对该类做一点点扩展，就可以根据具体的material动态的创建vertex buffer。