24. 幕外渲染

  通常在图形编程中，需要能够将场景渲染到纹理上，并且针对某些目的可以使用这些纹理，例如，也许想在游戏中有一个安全监视器，游戏中间有一个电视屏可以在不同的位置显示游戏的状态。这对游戏玩家而言，非常有用，并且可以在很大程度上提高程序的真实感。

 同样，也可以使用幕外渲染技术创建映射、阴影映射纹理和大量如场地模糊、夜视、热视力、高精度动态范围渲染等许多不同的后处理效果。在Direct3D中实现幕外渲染需要创建幕外表面，并对其渲染而不是渲染到常用的后台缓存中。然后将渲染结果存储在贴在幕外表面上的纹理图像中。并且可以像所有常用的纹理图像一样使用。要增加幕外渲染功能，只要创建一个LPDIRECT3DSURFACE9对象，并在想要渲染该对象时，切换到该对象即可。Direct3D在内部可以处理这一切工作，所以尤其是和OpenGL中的幕外渲染相比，这里的工作量要少很多。

 为了创建幕外表面，就必须创建一个想要保存渲染结果的LPDIRECT3DTEXTURE9对象。这通过调用D3DXCreateTexture()函数就可以实现。该函数的参数有pDevice、纹理的宽度(Width)和高度(Height)、纹理中Mipmap的个数(Miplevels)、指定纹理使用方式的标识符(Usage)、图像格式(Format)、描述放置图像的内存类(Pool)，以及正在创建的纹理对象(ppTexture)。

 有了创建好的纹理，就可以调用LPDIRECT3DTEXTURE9的成员函数GetSurfaceLevel()创建幕外表面。GetSurfaceLevel()函数是一个可以从纹理对象中获取表面对象的函数。该表面就是要渲染的表面，而纹理对象将场景存储为一幅图像，而不是将其显示在屏幕上。GetSurfaceLevel()函数原型如程序清单4.23所示。其中，Level是创建表面时的纹理资源级别。ppSurfaceLevel是正在创建的LPDIRECT3DSURFACE9对象。

       程序清单4.23 GetSurfaceLevel()函数原型

HRESULT GetSurfaceLevel(
    UINT Level,    // 创建表面时的纹理资源级别
    IDirect3DSurface9** ppSurfaceLevel   // 正在创建的LPDIRECT3DSURFACE9对象
    );

　　  有了这两个函数，就需要一个渲染用的幕外表面和纹理。要牢记：为了避免内存泄漏，在使用完对象后，一定要释放它们。到目前为止，这对所创建和使用的每个Direct3D对象都是如此。

 当要渲染表面时，有两件事要做。首先，需要一份后台缓存表面的副本，这样在处理完幕外表面时，可以返回正常的渲染。这通过使用Direct3D设备函数GetBackBuffer()就可以实现。GetBackBuffer()函数原型如程序清单4.24所示，其中iSwapChain是一个无符号整数，用于指定正在使用的交换链索引，BackBuffer是后台缓存索引，Type在Direct3D9.0中只能是D3DBACKBUFFER_TYPE_MODO，ppBackBuffer是保存后台缓存的LPDIRECT3DSURFACE9对象。

       程序清单4.24 GetBackBuffer()函数原型

HRESULT GetBackBuffer(
  UINT  iSwapChain,         // 指定正在使用的交换链索引
  UINT BackBuffer,          // 后台缓存索引
  D3DBACKBUFFER_TYPE Type,
  IDirect3DSurface9 ** ppBackBuffer  // 保存后台缓存的LPDIRECT3DSURFACE9对象
);
    

　　   其次，当有了后台缓存后，就可以转换到想要用的所有渲染目标，这只需调用Direct3D设备函数SetRenderTarget()即可。SetRenderTarget()函数原型如程序清单4.25所示，其中RenderTargetIndex是渲染目标的索引，pRenderTarget代表要绘制的LPDIRECT3DSURFACE9对象。

HRESULT SetRenderTarget(

  DWORD RenderTargetIndex,            // 渲染目标的索引

  IDirect3DSurface9 * pRenderTarget   // 要绘制的LPDIRECT3DSURFACE9对象

);

 至此，就得到该演示程序的全部内容。有了这4个函数，就包含了所要渲染的全部内容。既然已经了解到这一点，接下来就可以开发一个程序，实现幕外渲染功能。

#include<d3d9.h>
#include<d3dx9.h>

#define WINDOW_CLASS    "UGPDX"
#define WINDOW_NAME     "Off-Screen Rendering"
#define WINDOW_WIDTH    640
#define WINDOW_HEIGHT   480

// Function Prototypes...
bool InitializeD3D(HWND hWnd, bool fullscreen);
bool InitializeObjects();
void RenderScene();
void Shutdown();


// Direct3D object and device.
LPDIRECT3D9 g_D3D = NULL;
LPDIRECT3DDEVICE9 g_D3DDevice = NULL;

// Matrices.
D3DXMATRIX g_projection;
D3DXMATRIX g_ViewMatrix;

// stD3DVertex buffer to hold the square's geometry.
LPDIRECT3DVERTEXBUFFER9 g_VertexBuffer = NULL;

// g_Teapot object and its material.
LPD3DXMESH g_Teapot = NULL;
D3DMATERIAL9 g_Material;
float g_RotationAngle = 0.0f;

// Light object.
D3DLIGHT9 g_Light;

// Back buffer, offscreen texture, offscreen surface.
LPDIRECT3DSURFACE9 g_BackSurface = NULL;
LPDIRECT3DTEXTURE9 g_SurfaceTexture = NULL;
LPDIRECT3DSURFACE9 g_OffScreenSurface = NULL;


// A structure for our custom vertex type
struct stD3DVertex
{
    float x, y, z;
    float tu, tv;
};

// Our custom FVF, which describes our custom vertex structure
#define D3DFVF_VERTEX (D3DFVF_XYZ | D3DFVF_TEX1)


LRESULT WINAPI MsgProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch(msg)
    {
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
        break;

    case WM_KEYUP:
        if(wParam == VK_ESCAPE) PostQuitMessage(0);
        break;
    }

    return DefWindowProc(hWnd, msg, wParam, lParam);
}


int WINAPI WinMain(HINSTANCE hInst, HINSTANCE prevhInst, LPSTR cmdLine, int show)
{
    // Register the window class
    WNDCLASSEX wc = { sizeof(WNDCLASSEX), CS_CLASSDC, MsgProc, 0L, 0L,
        GetModuleHandle(NULL), NULL, NULL, NULL, NULL,
        WINDOW_CLASS, NULL };
    RegisterClassEx(&wc);

    // Create the application's window
    HWND hWnd = CreateWindow(WINDOW_CLASS, WINDOW_NAME, WS_OVERLAPPEDWINDOW,
        100, 100, WINDOW_WIDTH, WINDOW_HEIGHT,
        GetDesktopWindow(), NULL, wc.hInstance, NULL);

    // Initialize Direct3D
    if(InitializeD3D(hWnd, false))
    {
        // Show the window
        ShowWindow(hWnd, SW_SHOWDEFAULT);
        UpdateWindow(hWnd);

        // Enter the message loop
        MSG msg;
        ZeroMemory(&msg, sizeof(msg));

        while(msg.message != WM_QUIT)
        {
            if(PeekMessage(&msg, NULL, 0U, 0U, PM_REMOVE))
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
            else
                RenderScene();
        }
    }

    // Release any and all resources.
    Shutdown();

    // Unregister our window.
    UnregisterClass(WINDOW_CLASS, wc.hInstance);
    return 0;
}


bool InitializeD3D(HWND hWnd, bool fullscreen)
{
    D3DDISPLAYMODE displayMode;

    // Create the D3D object.
    g_D3D = Direct3DCreate9(D3D_SDK_VERSION);
    if(g_D3D == NULL) return false;

    // Get the desktop display mode.
    if(FAILED(g_D3D->GetAdapterDisplayMode(D3DADAPTER_DEFAULT, &displayMode)))
        return false;

    // Set up the structure used to create the D3DDevice
    D3DPRESENT_PARAMETERS d3dpp;
    ZeroMemory(&d3dpp, sizeof(d3dpp));

    if(fullscreen)
    {
        d3dpp.Windowed = FALSE;
        d3dpp.BackBufferWidth = WINDOW_WIDTH;
        d3dpp.BackBufferHeight = WINDOW_HEIGHT;
    }
    else
        d3dpp.Windowed = TRUE;
    d3dpp.SwapEffect = D3DSWAPEFFECT_DISCARD;
    d3dpp.BackBufferFormat = displayMode.Format;
    d3dpp.BackBufferCount = 1;
    d3dpp.EnableAutoDepthStencil = TRUE;
    d3dpp.AutoDepthStencilFormat = D3DFMT_D16;

    // Create the D3DDevice
    if(FAILED(g_D3D->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hWnd,
        D3DCREATE_SOFTWARE_VERTEXPROCESSING, &d3dpp, &g_D3DDevice)))
    {
        return false;
    }

    // Initialize any objects we will be displaying.
    if(!InitializeObjects()) return false;

    return true;
}


bool InitializeObjects()
{
    // Setup the g_Light source.
    g_Light.Type = D3DLIGHT_DIRECTIONAL;
    g_Light.Direction = D3DXVECTOR3(0.0f, 0.0f, 1.0f);
    g_Light.Diffuse.r = 1;
    g_Light.Diffuse.g = 1;
    g_Light.Diffuse.b = 1;
    g_Light.Diffuse.a = 1;
    g_Light.Specular.r = 1;
    g_Light.Specular.g = 1;
    g_Light.Specular.b = 1;
    g_Light.Specular.a = 1;

    g_D3DDevice->SetLight(0, &g_Light);
    g_D3DDevice->LightEnable(0, TRUE);

    // Setup the material properties for the teapot.
    ZeroMemory(&g_Material, sizeof(D3DMATERIAL9));
    g_Material.Diffuse.r = g_Material.Ambient.r = 0.6f;
    g_Material.Diffuse.g = g_Material.Ambient.g = 0.6f;
    g_Material.Diffuse.b = g_Material.Ambient.b = 0.7f;
    g_Material.Specular.r = 0.4f;
    g_Material.Specular.g = 0.4f;
    g_Material.Specular.b = 0.4f;
    g_Material.Power = 8.0f;

    // Create the teapot.
    if(FAILED(D3DXCreateTeapot(g_D3DDevice, &g_Teapot, NULL)))
        return false;


    // Create the texture that will be rendered to.
    if(FAILED(D3DXCreateTexture(g_D3DDevice, WINDOW_WIDTH,
        WINDOW_HEIGHT, 1, D3DUSAGE_RENDERTARGET, D3DFMT_A8R8G8B8,
        D3DPOOL_DEFAULT, &g_SurfaceTexture))) return false;

    // Set texture to offscreen surface. 将纹理与幕外表面关联起来
    g_SurfaceTexture->GetSurfaceLevel(0, &g_OffScreenSurface);


    // Square that will be textured with offscreen rendering data.
    stD3DVertex square[] =
    {
        {-1, 1,  -1, 0, 0}, {1, 1,  -1, 1, 0},
        {-1, -1, -1, 0, 1}, {1, -1, -1, 1, 1}
    };

    g_D3DDevice->CreateVertexBuffer(4 * sizeof(stD3DVertex), 0,
        D3DFVF_VERTEX, D3DPOOL_DEFAULT,
        &g_VertexBuffer, NULL);

    void *pData = NULL;
    g_VertexBuffer->Lock(0, sizeof(square), (void**)&pData, 0);
    memcpy(pData, square, sizeof(square));
    g_VertexBuffer->Unlock();


    // Set the image states to get a good quality image.
    g_D3DDevice->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_LINEAR);
    g_D3DDevice->SetSamplerState(0, D3DSAMP_MAGFILTER, D3DTEXF_LINEAR);

    // Set default rendering states.
    g_D3DDevice->SetRenderState(D3DRS_LIGHTING, TRUE);
    //让逆时针的三角形显示出来
    g_D3DDevice->SetRenderState(D3DRS_CULLMODE, D3DCULL_CCW);
    //启用深度缓存
    g_D3DDevice->SetRenderState(D3DRS_ZENABLE, TRUE);


    // Set the projection matrix.
    D3DXMatrixPerspectiveFovLH(&g_projection, 45.0f,
        WINDOW_WIDTH/WINDOW_HEIGHT, 0.1f, 1000.0f);
    g_D3DDevice->SetTransform(D3DTS_PROJECTION, &g_projection);


    // Define camera information.
    D3DXVECTOR3 cameraPos(0.0f, 0.0f, -4.0f);
    D3DXVECTOR3 lookAtPos(0.0f, 0.0f, 0.0f);
    D3DXVECTOR3 upDir(0.0f, 1.0f, 0.0f);

    // Build view matrix.
    D3DXMatrixLookAtLH(&g_ViewMatrix, &cameraPos,
        &lookAtPos, &upDir);

    return true;
}


void RenderScene()
{
    // Get a copy of the back buffer.
    g_D3DDevice->GetBackBuffer(0, 0, D3DBACKBUFFER_TYPE_MONO, &g_BackSurface);

    // Prepare to draw to the offscreen surface.
    g_D3DDevice->SetRenderTarget(0, g_OffScreenSurface);

    // Clear the offscreen surface.
    g_D3DDevice->Clear(0, NULL, D3DCLEAR_TARGET/* | D3DCLEAR_ZBUFFER*/, D3DCOLOR_XRGB(200,200,200), 1.0f, 0);

    // Begin the scene.  Start rendering.
    g_D3DDevice->BeginScene();

    // Turn on lighting.
    g_D3DDevice->SetRenderState(D3DRS_LIGHTING, TRUE);

    // Set projection.
    g_D3DDevice->SetTransform(D3DTS_PROJECTION, &g_projection);

    // Create rotation matrix to rotate teapot.
    D3DXMATRIXA16 w;
    D3DXMatrixRotationY(&w, g_RotationAngle);
    g_D3DDevice->SetTransform(D3DTS_WORLD, &w);

    // Add to the rotation.
    g_RotationAngle += 0.02f;
    if(g_RotationAngle >= 360) g_RotationAngle = 0.0f;

    // Apply the view (camera).
    g_D3DDevice->SetTransform(D3DTS_VIEW, &g_ViewMatrix);

    // Set the material and draw the Teapot.
    g_D3DDevice->SetMaterial(&g_Material);
    g_D3DDevice->SetTexture(0, NULL);
    g_Teapot->DrawSubset(0);

    // End the scene.  Stop rendering.
    g_D3DDevice->EndScene();


    // Switch back to our back buffer.
    g_D3DDevice->SetRenderTarget(0, g_BackSurface);

    // Clear the back buffer.
    g_D3DDevice->Clear(0, NULL, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER,
        D3DCOLOR_XRGB(0,0,0), 1.0f, 0);

    // Begin the scene.  Start rendering.
    g_D3DDevice->BeginScene();

    // Turn off lighting. Don't need it.
    g_D3DDevice->SetRenderState(D3DRS_LIGHTING, FALSE);

    // Set projection.
    g_D3DDevice->SetTransform(D3DTS_PROJECTION, &g_projection);

    // Rotate just a little to see this is a flat surface.
    D3DXMatrixRotationY(&w, 120);
    g_D3DDevice->SetTransform(D3DTS_WORLD, &w);

    // Apply the view (camera).
    g_D3DDevice->SetTransform(D3DTS_VIEW, &g_ViewMatrix);

    g_D3DDevice->SetTexture(0, g_SurfaceTexture);
    g_D3DDevice->SetStreamSource(0, g_VertexBuffer, 0, sizeof(stD3DVertex));
    g_D3DDevice->SetFVF(D3DFVF_VERTEX);
    g_D3DDevice->DrawPrimitive(D3DPT_TRIANGLESTRIP,  0, 2);

    // End the scene.  Stop rendering.
    g_D3DDevice->EndScene();

    // Display the scene.
    g_D3DDevice->Present(NULL, NULL, NULL, NULL);
}


void Shutdown()
{
    if(g_D3DDevice != NULL) g_D3DDevice->Release();
    g_D3DDevice = NULL;

    if(g_D3D != NULL) g_D3D->Release();
    g_D3D = NULL;

    if(g_VertexBuffer != NULL) g_VertexBuffer->Release();
    g_VertexBuffer = NULL;

    if(g_Teapot != NULL) g_Teapot->Release();
    g_Teapot = NULL;

    if(g_BackSurface != NULL) g_BackSurface->Release();
    g_BackSurface = NULL;

    if(g_SurfaceTexture != NULL) g_SurfaceTexture->Release();
    g_SurfaceTexture = NULL;

    if(g_OffScreenSurface != NULL) g_OffScreenSurface->Release();
    g_OffScreenSurface = NULL;
}