20. 纹理混合透明度设置

在将图像施加到表面时，并非图像的所有部分都是可见的。例如，如果有一幅叶子的图像，并且只想看到图像中实际构成叶子的那一部分内容，而不是空的场景，就需要使用名为“透明度”的技术，以帮助实现预期效果。有了透明度，就可以设置图像的一些像素可见，而另一些像素不可见。

        使用RGBA纹理图像中的Alpha成分或顶点颜色的alpha成分可以控制透明度。同样，可以将像素设为半透明，而不是可见或不可见。这意味着图像的某些部分在某种程度上是可见的，而某种程度上像玻璃一样是可以看穿的。这对于像菜单或GUI对象、窗口、X射线视觉等这样的东西而言非常有用。控制图像的alpha通道或是顶点颜色的alpha通道可以控制显示给终端用户的物体透明度。为了在Direct3D中使用透明度，只要启用混合模式渲染对象即可，这样Direct3D就会知道基于Alpha通道的混合结果。

为了设置透明度，首先必须启动设置，为要混合的源对象设置一个参数，为正在渲染用的目的缓存设置一个参数。本例中的目的缓存是后台缓存。若要为纹理加载图像就必须有一个alpha通道，或是使用D3DCOLOR_ARGB宏而不是D3DCOLOR_XRGB，以便在顶点结构中使用alpha。

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

#define WINDOW_CLASS    "UGPDX"
#define WINDOW_NAME     "Alpha Transparency"
#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;

// Vertex buffer to hold the geometry.
LPDIRECT3DVERTEXBUFFER9 g_VertexBuffer = NULL;

// Holds a texture image.
LPDIRECT3DTEXTURE9 g_Texture = NULL;

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

// Our custom FVF, which describes our custom vertex structure
#define D3DFVF_VERTEX (D3DFVF_XYZ | D3DFVF_DIFFUSE | 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;

    // 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()
{
    // Fill in our structure to draw an object.
    // x, y, z, color, texture coords.
    stD3DVertex objData[] =
    {
        {-0.3f, -0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 1.0f},
        {0.3f, -0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 1.0f},
        {0.3f, 0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},

        {0.3f, 0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},
        {-0.3f, 0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 0.0f},
        {-0.3f, -0.4f, 0.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 1.0f}
    };

    // Create the vertex buffer.
    if(FAILED(g_D3DDevice->CreateVertexBuffer(sizeof(objData), 0,
        D3DFVF_VERTEX, D3DPOOL_DEFAULT, &g_VertexBuffer, NULL))) return false;

    // Fill the vertex buffer.
    void *ptr;
    if(FAILED(g_VertexBuffer->Lock(0, sizeof(objData), (void**)&ptr, 0))) return false;
    memcpy(ptr, objData, sizeof(objData));
    g_VertexBuffer->Unlock();


    // Load the texture image from file.
    if(D3DXCreateTextureFromFile(g_D3DDevice, "decal.tga", &g_Texture) != D3D_OK)
        return false;

    // 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, FALSE);
    g_D3DDevice->SetRenderState(D3DRS_CULLMODE, D3DCULL_NONE);


    // 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, -1.0f);
    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()
{
    // Clear the backbuffer.
    g_D3DDevice->Clear(0, NULL, D3DCLEAR_TARGET,D3DCOLOR_XRGB(255,0,0), 1.0f, 0);

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

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

    /*
    设置三种不同的渲染状态，设置和启用透明度功能。第一个函数设置的渲染状态是
    D3DRS_ALPHABLENDENABLE，将其设置为TRUE，从而启动Direct3D Alpha混合。
    第二个函数将正在渲染的对象的源混合操作(D3DRS_SRCBLEND)设置为源对象的Alpha通道，
    因为这里想使用图像或顶点颜色的Alpha值来控制后台中表面的混合。最后一个(D3DRS_DESTBLEND)是
    当前屏幕的内容，设置用来使用源对象的逆alpha。这意味着Direct3D会在将两个结果混合到一起时
    从Alpha成分中提取一个值。

    */
    // Set alpha transparency on for the texture image.
    g_D3DDevice->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
    g_D3DDevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
    g_D3DDevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);

    // Draw square.
    g_D3DDevice->SetTexture(0, g_Texture);
    g_D3DDevice->SetStreamSource(0, g_VertexBuffer, 0, sizeof(stD3DVertex));
    g_D3DDevice->SetFVF(D3DFVF_VERTEX);
    g_D3DDevice->DrawPrimitive(D3DPT_TRIANGLELIST, 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_Texture != NULL) g_Texture->Release();
    g_Texture = NULL;
}

　　

// Set alpha transparency on for the texture image.
// 启用D3D Alpha混合运算
g_D3DDevice->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
// 设置源混合因子
g_D3DDevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
// 设置目标混合因子
g_D3DDevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);
    

　　调用三个函数，设置三种不同的渲染状态，设置和启用透明度功能。第一个函数设置的渲染状态是D3DRS_ALPHABLENDENABLE，将其设置为TRUE，从而启动Direct3D Alpha混合。第二个函数将正在渲染的对象的源混合因子(D3DRS_SRCBLEND)设置为源对象的Alpha通道，因为这里想使用图像或顶点颜色的Alpha值来控制后台中表面的混合。最后一个(D3DRS_DESTBLEND)是当前屏幕的内容，设置用来使用源对象的逆alpha。这意味着Direct3D会在将两个结果混合到一起时从Alpha成分中提取一个值。