为3D场景增加雾会得到更真实的渲染效果。雾可以增加大量真实感，尤其对地形、树林地区以及幽灵般的户外背景。同样，雾可以很好地限制观察者对环境的可见度。现实中并不存在无限的视觉距离。事实上，这就是虚拟环境的一个镜像。通过限制观察距离，同样可以限制发送给管道的多边形数目。这对于提高渲染时间更有用。如果读者还记得PS游戏机上的第一个游戏《寂静岭》(Silient Hill)，该游戏物景中有许多雾，很难看到外面的事物，除非是事物就在游戏玩家的面前，就可以很清楚地了解这一点。
Direct3D中有两类硬件雾可用：顶点雾和像素雾。顶点雾是按照每个顶点来计算雾的数据，而像素雾根据像素级别计算雾的数据。由于通常像素比顶点要多得多，因此两者相比而言，像素雾的渲染速度较慢。同样，顶点雾提供了很好的视觉效果。有了顶点雾，就可以在表面上逐点内插处理雾的信息。
使用雾就要使用RenderScene()函数。需要设置雾和使用雾的代码非常简短，演示程序也十分简单。
CHAPTER6文件夹中的Fog演示程序就像Antialiasing演示程序一样代码很少。在Fog演示程序中正方形充当大地，并启用雾。除了要设置雾和绘制的几何图形数据之外，Fog演示程序是第4章中Texture Mapping演示程序的改进版。如果读者想跟随本书一起学习，那么可以从Texture Mapping演示程序中提取代码，修改InitializeObjects()函数以使用雾，并修改纹理正方形的大小和方位。程序清单6.3给出了完整的InitializeObjects()函数。
bool InitializeObjects()
{
D3DCAPS9 caps;
g_D3DDevice->GetDeviceCaps(&caps);
// Start and end distance of the fog.
float start = 2, end = 8;
// Set fog properties.
// 设置雾的相关属性
g_D3DDevice->SetRenderState(D3DRS_FOGENABLE, true); // 启用雾效果
// 设置要渲染的雾的期望颜色(这里为灰色)
g_D3DDevice->SetRenderState(D3DRS_FOGCOLOR,
D3DCOLOR_XRGB(128, 128, 128));
// 使用顶点雾
g_D3DDevice->SetRenderState(D3DRS_FOGVERTEXMODE, D3DFOG_LINEAR);
// 指定雾的开始距离和终点距离
g_D3DDevice->SetRenderState(D3DRS_FOGSTART, *(DWORD*)(&start));
g_D3DDevice->SetRenderState(D3DRS_FOGEND, *(DWORD*)(&end));
// Pixel Fog 像素雾
//g_D3DDevice->SetRenderState(D3DRS_FOGTABLEMODE, D3DFOG_LINEAR);
// Can only use if hardware supports it.
// 如果支持，则启用基于距离的雾
if(caps.RasterCaps & D3DPRASTERCAPS_FOGRANGE)
g_D3DDevice->SetRenderState(D3DRS_RANGEFOGENABLE, true);
// Object
stD3DVertex objData[] =
{
{-2.5f, -0.5f, -6.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 1.0f},
{2.5f, -0.5f, -6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 1.0f},
{2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},
{2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},
{-2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 0.0f},
{-2.5f, -0.5f, -6.0f, 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, "ground.bmp",
&g_Texture) != D3D_OK) return false;
// 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, -3.0f);
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;
}
代码中，从InitializeObjects()函数开始，先获取设备性能。稍后将用其测试硬件是否支持D3DRS_RANGEFOGENABLE以便使用硬件雾范围。之后，通过几次调用SetRenderState()函数设置雾。第一种状态D3DRS_FOGENABLE用于在API中启用雾效果。第二种状态D3DRS_FOGCOLOR用于设置要渲染的雾的期望颜色。第三种状态D3DRS_FOGVERTEXMODE指明了正在使用顶点雾。使用像素雾将需要使用D3DRS_FOGTABLEMODE而不是D3DRS_FOGVERTEXMODE。第四种和第五种状态D3DRS_FOGSTART和D3DRS_FOGEND指定雾的开始距离和终点距离。这就会让Direct3D知道雾在观察者前面的开始距离以及最远处的终点位置。令人感兴趣的最后一行代码是最后的渲染状态D3DRS_RANGEFOGENABLE，它将启用基于距离的雾，该值在变换和光照阶段由Direct3D计算得到。基于距离的雾只能在支持它的硬件上使用，不支持它的硬件上当然就不能使用。所以本书使用设备性能来测试运行演示程序的硬件是否支持它。
完整代码：
#include <d3d9.h>
#include <d3dx9.h>
#pragma comment(lib, "d3d9.lib")
#pragma comment(lib, "d3dx9.lib")
#define WINDOW_CLASS "UGPDX"
#define WINDOW_NAME "Fog"
#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()
{
D3DCAPS9 caps;
g_D3DDevice->GetDeviceCaps(&caps);
// Start and end distance of the fog.
float start = 2, end = 8;
// Set fog properties.
g_D3DDevice->SetRenderState(D3DRS_FOGENABLE, true);
g_D3DDevice->SetRenderState(D3DRS_FOGCOLOR, D3DCOLOR_XRGB(128, 128, 128));
g_D3DDevice->SetRenderState(D3DRS_FOGVERTEXMODE, D3DFOG_LINEAR);
g_D3DDevice->SetRenderState(D3DRS_FOGSTART, *(DWORD*)(&start));
g_D3DDevice->SetRenderState(D3DRS_FOGEND, *(DWORD*)(&end));
// Pixel Fog
//g_D3DDevice->SetRenderState(D3DRS_FOGTABLEMODE, D3DFOG_LINEAR);
// Can only use if hardware supports it.
if(caps.RasterCaps & D3DPRASTERCAPS_FOGRANGE)
g_D3DDevice->SetRenderState(D3DRS_RANGEFOGENABLE, true);
// Object
stD3DVertex objData[] =
{
{-2.5f, -0.5f, -6.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 1.0f},
{2.5f, -0.5f, -6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 1.0f},
{2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},
{2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 1.0f, 0.0f},
{-2.5f, -0.5f, 6.0f, D3DCOLOR_XRGB(255,255,255), 0.0f, 0.0f},
{-2.5f, -0.5f, -6.0f, 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, "ground.bmp",
&g_Texture) != D3D_OK) return false;
// 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, -3.0f);
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(128,128,128), 1.0f, 0);
// Begin the scene. Start rendering.
g_D3DDevice->BeginScene();
// Apply the view (camera).
g_D3DDevice->SetTransform(D3DTS_VIEW, &g_ViewMatrix);
// 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;
}
在Direct3D 中，雾化是通过将景物颜色与雾的颜色，以随物体到观察点距离增加而衰减的混合因子混合而实现的。
两种雾化方法：顶点雾化和像素雾化。
三种雾化公式：线性雾化，指数雾化，指数平方雾化。
两种雾化处理：基于深度的雾化处理和基于范围的雾化处理。基于深度是指两个点之间的深度（Z）差值，基于范围则是两点间的直线距离。Direct3D默认的是基于深度的雾化。可设置基于范围的雾化，但要先检测设备是否支持:
g_pd3dDevice->GetDeviceCaps(&staps);
if ( stCaps.RasterCaps & D3DPASTERCAPS_FOGRANGE )
return TRUE;
Direct3D默认禁用雾化效果，可以激活雾化：g_pd3dDevice->SetRendState( D3DRS_FOGENABLE, true );
设置雾的颜色 g_pd3dDevice->SetRendState( D3DRS_FOGCOLOR, 0xffffffff); //白色
根据按键消息进行多种雾化效果的切换
if (gInputSystem->KeyDown(DIK_V))
{
//禁用雾化
g_pd3dDevice->SetRenderState( D3DRS_FOGENABLE,FALSE );
}
if (gInputSystem->KeyDown(DIK_M))
{
g_pd3dDevice->SetRenderState( D3DRS_FOGENABLE,TRUE );
//设置雾化混合因子计算公式 : 指数雾化
g_pd3dDevice->GetDevice()->SetRenderState(D3DRS_FOGTABLEMODE, D3DFOG_EXP);
//设置雾的浓度
static float fogDensity = 0.005f;
g_pd3dDevice->SetRenderState(D3DRS_FOGDENSITY, *(DWORD*)&fogDensity);
}
if (gInputSystem->KeyDown(DIK_N))
{
g_pd3dDevice->SetRenderState( D3DRS_FOGENABLE,TRUE );
//激活基于范围的雾化
g_pd3dDevice->SetRenderState( D3DRS_RANGEFOGENABLE,TRUE );
//设置雾化混合因子计算公式 : 线性雾化
g_pd3dDevice->SetRenderState(D3DRS_FOGTABLEMODE, D3DFOG_LINEAR);
//设置线性雾化开始位置和结束位置
static float fogStart = 20;
static float fogEnd = 200;
g_pd3dDevice->SetRenderState(D3DRS_FOGSTART,*(DWORD*)&fogStart);
g_pd3dDevice->SetRenderState(D3DRS_FOGEND,*(DWORD*)&fogEnd);
}
if (gInputSystem->KeyDown(DIK_B))
{
g_pd3dDevice->SetRenderState( D3DRS_FOGENABLE,TRUE );
//设置雾化混合因子计算公式 : 指数平方雾化
g_pd3dDevice->SetRenderState(D3DRS_FOGTABLEMODE, D3DFOG_EXP2);
//设置雾的浓度
static float fogDensity = 0.01f;
g_pd3dDevice->SetRenderState(D3DRS_FOGDENSITY, *(DWORD*)&fogDensity);
}
其中设置雾化混合因子计算公式时，SetRenderState的第一个参数可以是：
D3DRS_FOGTABLEMODE(像素雾化) 或 D3DRS_FOGVERTEXMODE(顶点雾化)