最近點采樣
最近點采樣是4種過濾方式中速度最快但效果最差的過濾方式。Direct3D計算得到的紋理元素地址通常是一個浮點數(shù)值，而非整數(shù)的紋理下標(biāo)值，當(dāng)使用最近點采樣時，Direct3D會復(fù)制與這個浮點值地址最接近的整數(shù)地址的紋理元素的顏色。
設(shè)置最近點采樣的具體方法如下：調(diào)用IDirect3DDevice9::SetSamplerState()，可分別設(shè)置紋理過濾的放大過濾器和縮小過濾器。將第一個參數(shù)設(shè)置為紋理過濾器關(guān)聯(lián)的紋理層序號（0~7）。如果要設(shè)置放大過濾器，第二個參數(shù)設(shè)為D3DSAMP_MAGFILTER，如果要設(shè)置縮小過濾器，第二個參數(shù)設(shè)為D3DSAMP_MINFILTER。第三個參數(shù)可設(shè)為表示最近點采樣的枚舉常量D3DTEXF_POINT。下列代碼將紋理層0的紋理過濾方式設(shè)置為最近點采樣。
g_device->SetSamplerState(0,
D3DSAMP_MAGFILTER, D3DTEXF_POINT);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_POINT);
如果紋理的大小和屏幕圖元的實際大小將近，那么采用最近點采樣方法對圖像質(zhì)量的影響不大。但是，如果大小相差太多，就會降低圖像精度，從而影響圖像質(zhì)量，出現(xiàn)色塊或閃爍的失真現(xiàn)象。
線性紋理過濾
線性紋理過濾是目前使用最廣泛的紋理過濾方法。它與最近點采樣相比，能有效地提高圖像的顯示質(zhì)量，并且對系統(tǒng)性能影響不大。線性紋理過濾取得與計算得到的紋理元素的浮點地址最接近的上、下、左、右4個紋理元素，對這4個紋理元素進行加權(quán)平均，得到最終顯示的顏色值。
與設(shè)置最近點采樣的方法相似，調(diào)用函數(shù)IDirect3DDevice9::SetSamplerState()設(shè)置線性紋理過濾，所不同的是第三個參數(shù)設(shè)置為D3DTEXF_LINEAR。下面的代碼將紋理層0的放大和縮小過濾器設(shè)置為線性紋理過濾。
g_device->SetSamplerState(0,
D3DSAMP_MAGFILTER, D3DTEXF_LINEAR);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_LINEAR);
因為是在單一紋理層上的線性過濾，而且是x、y方向上的線性過濾，所以稱為雙線性紋理過濾。目前大多數(shù)顯卡都為線性紋理過濾進行了優(yōu)化，所以使用線性紋理過濾一方面可以獲得較好的圖形質(zhì)量，另一方面對程序性能影響不大。
各項異性紋理過濾
當(dāng)三維物體表面與投影平面不平行時，它在屏幕上的投影會有拉長或扭曲，這種現(xiàn)象稱為各項異性（anisotropy）。當(dāng)一個各向異性圖元的像素映射到紋理元素時，它的形狀發(fā)生扭曲。Direct3D根據(jù)屏幕像素反向轉(zhuǎn)換到紋理元素的延長度，決定各項異性程度。
要使用各項異性紋理過濾，還應(yīng)當(dāng)設(shè)置最大各項異性程度值。通過將函數(shù)IDirect3DDevive9::SetSamplerState()的第一個參數(shù)設(shè)為紋理層索引，第二個參數(shù)設(shè)為D3DSAMP_MAXANISOTOPY，第三個參數(shù)設(shè)為大于1的任何值，可以完成最大各項異性程度值的設(shè)置。下面的示例代碼指定了最大各項異性值為4。
g_device->SetSamplerState(0,
D3DSAMP_MAGFILTER, D3DTEXF_ANISOTROPIC);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_ANISOTROPIC);
g_device->SetSamplerState(0, D3DSAMP_MAXANISOTROPY, 4);
最大各項異性程度值D3DSAMP_MAXANISOTROPY為1時，表示禁用各項異性過濾。一般說來，其值越大，圖像效果越好，計算量越大，速度越慢。需要注意的是，在設(shè)置最大各項異性之前，應(yīng)調(diào)用IDirect3D9::GetDeviceCaps()函數(shù)，查詢當(dāng)前設(shè)備支持的Direct3D特性，獲取當(dāng)前設(shè)備支持的最大各項異性度的取值范圍，具體代碼如下：
DWORD get_max_anisotropy(IDirect3DDevice9* device)
{
D3DCAPS9 caps;
device->GetDeviceCaps(&caps);
return caps.MaxAnisotropy;
}
紋理過濾方式示例程序
按下數(shù)字鍵“1”使用最近點采樣紋理過濾方式，按下數(shù)字鍵“2”使用線性紋理過濾方式，按下數(shù)字鍵“3”使用各項異性紋理過濾方式。
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;
IDirect3DVertexBuffer9* g_vertex_buffer;
IDirect3DTexture9* g_texture;
struct sCustomVertex
{
float x, y, z;
float u, v;
};
#define D3DFVF_CUSTOM_VERTEX (D3DFVF_XYZ | D3DFVF_TEX1)
void setup_matrices()
{
// build world matrix
D3DXMATRIX mat_world;
D3DXMatrixIdentity(&mat_world);
g_device->SetTransform(D3DTS_WORLD, &mat_world);
// setup view matrix
D3DXVECTOR3 eye(0.0f, 0.0f, -8.0f);
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, 100.0f);
g_device->SetTransform(D3DTS_PROJECTION, &mat_proj);
}
bool init_graphics()
{
if(FAILED(D3DXCreateTextureFromFile(g_device, "texture.jpg", &g_texture)))
{
MessageBox(NULL, "Create texture failed!", "ERROR", MB_OK);
return false;
}
sCustomVertex vertices[] =
{
{ -3, -3, 0.0f, 0.0f, 1.0f},
{ -3, 3, 0.0f, 0.0f, 0.0f},
{ 3, -3, 0.0f, 1.0f, 1.0f},
{ 3, 3, 0.0f, 1.0f, 0.0f }
};
g_device->CreateVertexBuffer(sizeof(vertices), 0, D3DFVF_CUSTOM_VERTEX, D3DPOOL_MANAGED, &g_vertex_buffer, NULL);
void* ptr;
g_vertex_buffer->Lock(0, 0, (void**)&ptr, 0);
memcpy(ptr, vertices, sizeof(vertices));
g_vertex_buffer->Unlock();
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;
if(FAILED(g_d3d->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hwnd, D3DCREATE_SOFTWARE_VERTEXPROCESSING,
&d3dpp, &g_device)))
{
return false;
}
if(! init_graphics())
return false;
setup_matrices();
g_device->SetRenderState(D3DRS_LIGHTING, FALSE);
return true;
}
void cleanup()
{
release_com(g_texture);
release_com(g_vertex_buffer);
release_com(g_device);
release_com(g_d3d);
}
void render()
{
g_device->Clear(0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB(5, 5, 5), 1.0f, 0);
g_device->BeginScene();
g_device->SetTexture(0, g_texture);
g_device->SetStreamSource(0, g_vertex_buffer, 0, sizeof(sCustomVertex));
g_device->SetFVF(D3DFVF_CUSTOM_VERTEX);
g_device->DrawPrimitive(D3DPT_TRIANGLESTRIP, 0, 2);
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 VK_ESCAPE:
DestroyWindow(hwnd);
break;
case 49: // press key "1", use nearest point texture filter mode
g_device->SetSamplerState(0, D3DSAMP_MAGFILTER, D3DTEXF_POINT);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_POINT);
break;
case 50: // press key "2", use linear texture filter mode
g_device->SetSamplerState(0, D3DSAMP_MAGFILTER, D3DTEXF_LINEAR);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_LINEAR);
break;
case 51: // press key "3", use anisotropy texture filter mode
g_device->SetSamplerState(0, D3DSAMP_MAGFILTER, D3DTEXF_ANISOTROPIC);
g_device->SetSamplerState(0, D3DSAMP_MINFILTER, D3DTEXF_ANISOTROPIC);
g_device->SetSamplerState(0, D3DSAMP_MAXANISOTROPY, 8);
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, 800, 600,
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;
}