深度测试与alpha混合（5）

透过那些透明度非常高的物体看其他物体，例如透过几乎完全透明的玻璃看其他物体，会感到玻璃好像不存在，在三维图形程序中渲染时就可以不渲染这些透明度非常高的物体，从而可以提高渲染速度，这可以通过alpha测试来实现。

alpha测试根据当前像素是否满足alpha测试条件（即是否达到一定的透明度）来控制是否绘制该像素，图形程序应用alpha测试可以有效地屏蔽某些像素颜色。与alpha混合相比，alpha测试不将当前像素的颜色与颜色缓冲区中像素的颜色混合，像素要么完全不透明，要么完全透明。由于无需进行颜色缓冲区的读操作和颜色混合，因此alpha测试在速度上要优于alpha混合。

alpha测试通过激活渲染状态D3DRS_ALPHATESTENABLE来设置，示例代码如下：

g_device->SetRenderState(D3DRS_ALPHATESTENABLE, TRUE);

渲染状态D3DRS_ALPHAREF用来设置alpha测试的参考值，alpha测试函数比较当前绘制的像素的alpha值和参考值，如果返回TRUE，则通过测试并绘制像素，反之不予绘制。参考值的取值范围是0x00000000到0x000000ff。

渲染状态D3DRS_ALPHAFUNC用来设置alpha测试函数，alpha测试函数属于D3DCMPFUNC枚举类型，默认状态为D3DCMP_ALWAYS。下列代码设置alpha测试函数为D3DCMP_GREATER，表示测试点像素的alpha值大于设置的alpha参考值时则返回TRUE：

g_device->SetRenderState(D3DRS_ALPHATESTENABLE, TRUE);
g_device->SetRenderState(D3DRS_ALPHAREF, 0x00000081);
g_device->SetRenderState(D3DRS_ALPHAFUNC, D3DCMP_GREATER);

已知蓝色玻璃的alpha值为浮点值0.5f，等价于两位16进制数0x80，小于程序中设置的alpha测试参考值0x81，并且alpha测试函数被设为D3DCMP_GREATER，所以蓝色玻璃的颜色不会被绘制出来。

按下数字键"1"，启用alpha测试。

按下数字键"0"，禁用alpha测试。

源程序：

#include <d3dx9.h>

#pragma warning(disable : 4127)

#define CLASS_NAME    "GameApp"

#define release_com(p)    do { if(p) { (p)->Release(); (p) = NULL; } } while(0)

IDirect3D9*                g_d3d;
IDirect3DDevice9*        g_device;

ID3DXMesh*                g_mesh;
D3DMATERIAL9*            g_mesh_materials;
IDirect3DTexture9**        g_mesh_textures;
DWORD                    g_num_materials;

void setup_world_matrix()
{
    D3DXMATRIX mat_world;
    D3DXMatrixRotationY(&mat_world, timeGetTime() / 1000.0f);
    g_device->SetTransform(D3DTS_WORLD, &mat_world);
}

void setup_view_proj_matrices()
{
    // setup view matrix

    D3DXVECTOR3 eye(0.0f, 15.0f, -20.0f);
    D3DXVECTOR3 at(0.0f,  0.0f,   0.0f);
    D3DXVECTOR3 up(0.0f,  1.0f,   0.0f);

    D3DXMATRIX mat_view;
    D3DXMatrixLookAtLH(&mat_view, &eye, &at, &up);
    g_device->SetTransform(D3DTS_VIEW, &mat_view);

    // setup projection matrix
    D3DXMATRIX mat_proj;
    D3DXMatrixPerspectiveFovLH(&mat_proj, D3DX_PI/4, 1.0f, 1.0f, 500.0f);
    g_device->SetTransform(D3DTS_PROJECTION, &mat_proj);
}

bool init_geometry()
{
    ID3DXBuffer* material_buffer;

    /*
     D3DXLoadMeshFromXA(
        LPCSTR pFilename, 
        DWORD Options, 
        LPDIRECT3DDEVICE9 pD3DDevice, 
        LPD3DXBUFFER *ppAdjacency,
        LPD3DXBUFFER *ppMaterials, 
        LPD3DXBUFFER *ppEffectInstances, 
        DWORD *pNumMaterials,
        LPD3DXMESH *ppMesh);
    */

    if(FAILED(D3DXLoadMeshFromX("heli.x", D3DXMESH_SYSTEMMEM, g_device, NULL, &material_buffer, NULL,
                                &g_num_materials, &g_mesh)))
    {
        MessageBox(NULL, "Could not find heli.x", "ERROR", MB_OK);
        return false;
    }

    D3DXMATERIAL* xmaterials = (D3DXMATERIAL*) material_buffer->GetBufferPointer();

    g_mesh_materials = new D3DMATERIAL9[g_num_materials];
    g_mesh_textures     = new IDirect3DTexture9*[g_num_materials];

    for(DWORD i = 0; i < g_num_materials; i++)
    {
        g_mesh_materials[i] = xmaterials[i].MatD3D;

        // set ambient reflected coefficient, because .x file do not set it.
        g_mesh_materials[i].Ambient = g_mesh_materials[i].Diffuse;

        g_mesh_textures[i] = NULL;

        if(xmaterials[i].pTextureFilename != NULL && strlen(xmaterials[i].pTextureFilename) > 0)    
            D3DXCreateTextureFromFile(g_device, xmaterials[i].pTextureFilename, &g_mesh_textures[i]);    
    }

    material_buffer->Release();

    return true;
}

bool init_d3d(HWND hwnd)
{
    g_d3d = Direct3DCreate9(D3D_SDK_VERSION);

    if(g_d3d == NULL)
        return false;

    D3DPRESENT_PARAMETERS d3dpp;
    ZeroMemory(&d3dpp, sizeof(d3dpp));

    d3dpp.Windowed                    = TRUE;
    d3dpp.SwapEffect                = D3DSWAPEFFECT_DISCARD;
    d3dpp.BackBufferFormat            = D3DFMT_UNKNOWN;
    d3dpp.EnableAutoDepthStencil    = TRUE;
    d3dpp.AutoDepthStencilFormat    = D3DFMT_D16;

    if(FAILED(g_d3d->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hwnd, D3DCREATE_SOFTWARE_VERTEXPROCESSING,
                                  &d3dpp, &g_device)))
    {
        return false;
    }
    
    if(! init_geometry())
        return false;

    setup_view_proj_matrices();    

    g_device->SetRenderState(D3DRS_ZENABLE,            TRUE);
    g_device->SetRenderState(D3DRS_ZFUNC,            D3DCMP_LESS);
    g_device->SetRenderState(D3DRS_ZWRITEENABLE,    TRUE);

    g_device->SetRenderState(D3DRS_ALPHATESTENABLE, TRUE);
    g_device->SetRenderState(D3DRS_ALPHAREF,        0x00000081);
    g_device->SetRenderState(D3DRS_ALPHAFUNC,        D3DCMP_GREATER);    

    g_device->SetRenderState(D3DRS_AMBIENT, 0xFFFFBB55);
    
    return true;
}

void cleanup()
{
    delete[] g_mesh_materials;

    if(g_mesh_textures)
    {
        for(DWORD i = 0; i < g_num_materials; i++)
            release_com(g_mesh_textures[i]);

        delete[] g_mesh_textures;
    }
    
    release_com(g_mesh);
    release_com(g_device);
    release_com(g_d3d);
}

void render()
{
    g_device->Clear(0, NULL, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER, D3DCOLOR_XRGB(5, 5, 5), 1.0f, 0);

    g_device->BeginScene();

    setup_world_matrix();

    // render opaque object first
    for(DWORD i = 0; i < g_num_materials; i++)
    {
        if(g_mesh_materials[i].Diffuse.a == 1.0f)
        {
            g_device->SetMaterial(&g_mesh_materials[i]);
            g_device->SetTexture(0, g_mesh_textures[i]);

            g_mesh->DrawSubset(i);
        }        
    }        

    // render transparent object second
    for(DWORD i = 0; i < g_num_materials; i++)
    {
        if(g_mesh_materials[i].Diffuse.a != 1.0f)
        {
            g_device->SetMaterial(&g_mesh_materials[i]);
            g_device->SetTexture(0, g_mesh_textures[i]);

            g_mesh->DrawSubset(i);
        }        
    }        
    
    g_device->EndScene();

    g_device->Present(NULL, NULL, NULL, NULL);
}

LRESULT WINAPI WinProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch(msg)
    {
    case WM_KEYDOWN:
        switch(wParam)
        {
        case 48: // press key "0", disable alpha test.
            g_device->SetRenderState(D3DRS_ALPHATESTENABLE, FALSE);
            break;

        case 49: // press key "1", enable alpha test.
            g_device->SetRenderState(D3DRS_ALPHATESTENABLE, TRUE);
            break;

        case VK_ESCAPE:
            DestroyWindow(hwnd);
            break;
        }    

        break;

    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }

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

int WINAPI WinMain(HINSTANCE inst, HINSTANCE, LPSTR, INT)
{
    WNDCLASSEX wc;

    wc.cbSize            = sizeof(WNDCLASSEX);
    wc.style            = CS_CLASSDC;
    wc.lpfnWndProc        = WinProc;
    wc.cbClsExtra        = 0;
    wc.cbWndExtra        = 0;
    wc.hInstance        = inst;
    wc.hIcon            = NULL;
    wc.hCursor            = NULL;
    wc.hbrBackground    = NULL;
    wc.lpszMenuName        = NULL;
    wc.lpszClassName    = CLASS_NAME;
    wc.hIconSm            = NULL;

    if(! RegisterClassEx(&wc))
        return -1;

    HWND hwnd = CreateWindow(CLASS_NAME, "Direct3D App", WS_OVERLAPPEDWINDOW, 200, 100, 640, 480,
                             NULL, NULL, wc.hInstance, NULL);    

    if(hwnd == NULL)
        return -1;

    if(init_d3d(hwnd))
    {
        ShowWindow(hwnd, SW_SHOWDEFAULT);
        UpdateWindow(hwnd);

        MSG msg;
        ZeroMemory(&msg, sizeof(msg));

        while(msg.message != WM_QUIT)
        {
            if(PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
                
            render();
            Sleep(10);
        }
    }

    cleanup();
    UnregisterClass(CLASS_NAME, wc.hInstance);    

    return 0;
}

 

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