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。