深度测试与alpha混合（4）

材质alpha

顶点alpha是没有使用光照和材质的情况，如果对场景内的物体添加光照和材质而没有添加纹理时，顶点alpha值取决于材质属性中漫反射颜色的alpha系数和灯光颜色中的alpha系数，顶点alpha值是根据光照计算得到的。顶点光照计算是分别针对红、绿、蓝和alpha进行的，其中alpha光照计算的结果就是顶点的alpha值。有了顶点的alpha值就可根据着色模式计算出每个像素的alpha值，第一个示例程序就是材质alpha的例子。

纹理alpha

当对物体表面使用了纹理之后，像素的alpha值就是纹理alpha混合之后的值，所以这又取决于纹理的alpha混合方法，纹理alpha混合方法决定了纹理alpha混合之后的alpha值是取自材质，还是取自纹理，或者取自二者的某种运算。像素alpha值的具体计算过程是这样的，首先得到顶点alpha值，顶点alpha值可能是直接指定的，也可能是光照计算得到，然后根据着色模式对顶点alpha值进行插值，得到的结果再根据纹理alpha混合方法和纹理采样得到的alpha值进行指定的运算，得到最终每个像素的alpha值。

示例程序中将一幅纹理应用到一个矩形表面，其中纹理alpha混合的设置如下：

g_device->SetTextureStageState(0, D3DTSS_ALPHAOP,	D3DTOP_MODULATE);
g_device->SetTextureStageState(0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE);
g_device->SetTextureStageState(0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE);

D3DTOP_MODULATE Multiply the components of the arguments.
SRGBA = Arg1 x Arg2

在示例程序中将矩形4个顶点的颜色值设置为0xFFFFFFFF，其alpha成分设置为ff，即alpha值为1.0f，所以纹理alpha混合的最终结果就是取纹理的alpha值。

sCustomVertex vertices[] =
{
{ -3,   -3,  0.0f,   0xffffffff,  0.0f, 1.0f},
{ -3,    3,  0.0f,   0xffffffff,  0.0f, 0.0f},
{  3,   -3,  0.0f,   0xffffffff,  1.0f, 1.0f},
{  3,    3,  0.0f,   0xffffffff,  1.0f, 0.0f}
}

示例程序在一个矩形表面贴了一颗树的纹理，在树的纹理中，没有树叶和树枝的地方alpha值为0，即完全透明；有树叶和树枝的地方alpha值为1，即完全不透明。所以通过alpha混合后，渲染的结果就像是一棵真的树。



按下数字键"1"，启用纹理alpha混合。



按下数字键"0"，禁用纹理alpha混合。



将顶点alpha由ff改为88后启用纹理混合的效果，可以看出纹理的颜色变暗了。

sCustomVertex vertices[] =
{
{ -3,   -3,  0.0f,   0x88ffffff,  0.0f, 1.0f},
{ -3,    3,  0.0f,   0x88ffffff,  0.0f, 0.0f},
{  3,   -3,  0.0f,   0x88ffffff,  1.0f, 1.0f},
{  3,    3,  0.0f,   0x88ffffff,  1.0f, 0.0f}
};

源程序：

#include <d3dx9.h>

#pragma warning(disable : 4127) // disable warning: conditional expression is constant

#define CLASS_NAME "GameApp"

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

typedef unsigned char uchar;

IDirect3D9* g_d3d;
IDirect3DDevice9* g_device;
IDirect3DVertexBuffer9* g_vertex_buffer;
IDirect3DTexture9* g_texture;

struct sCustomVertex
{
float x, y, z;
DWORD color;
float u, v;
};

#define D3DFVF_CUSTOM_VERTEX (D3DFVF_XYZ | D3DFVF_DIFFUSE | D3DFVF_TEX1)

void setup_matrices()
{
// setup world matrix
D3DXMATRIX mat_world;
D3DXMatrixIdentity(&mat_world);
g_device->SetTransform(D3DTS_WORLD, &mat_world);

// setup view matrix

D3DXVECTOR3 eye(0.0f, 0.0f, -10.0f);
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_vb()
{
if(FAILED(D3DXCreateTextureFromFile(g_device, "tree.tga", &g_texture)))
{
MessageBox(NULL, "Can not load texture file tree.tga!", "ERROR", MB_OK);
return false;
}

sCustomVertex vertices[] =
{
{ -3, -3, 0.0f, 0xffffffff, 0.0f, 1.0f},
{ -3, 3, 0.0f, 0xffffffff, 0.0f, 0.0f},
{ 3, -3, 0.0f, 0xffffffff, 1.0f, 1.0f},
{ 3, 3, 0.0f, 0xffffffff, 1.0f, 0.0f}

/*
{ -3, -3, 0.0f, 0x88ffffff, 0.0f, 1.0f},
{ -3, 3, 0.0f, 0x88ffffff, 0.0f, 0.0f},
{ 3, -3, 0.0f, 0x88ffffff, 1.0f, 1.0f},
{ 3, 3, 0.0f, 0x88ffffff, 1.0f, 0.0f}
*/
};

g_device->CreateVertexBuffer(sizeof(vertices), 0, D3DFVF_CUSTOM_VERTEX, D3DPOOL_DEFAULT, &g_vertex_buffer, NULL);

void* ptr;

g_vertex_buffer->Lock(0, 0, &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_vb())
return false;

setup_matrices();

g_device->SetRenderState(D3DRS_LIGHTING, FALSE);

g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
g_device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
g_device->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);

g_device->SetTextureStageState(0, D3DTSS_COLOROP, D3DTOP_SELECTARG1);
g_device->SetTextureStageState(0, D3DTSS_COLORARG1, D3DTA_TEXTURE);

g_device->SetTextureStageState(0, D3DTSS_ALPHAOP, D3DTOP_MODULATE);
g_device->SetTextureStageState(0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE);
g_device->SetTextureStageState(0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE);

return true;
}

void cleanup()
{
release_com(g_vertex_buffer);
release_com(g_texture);
release_com(g_device);
release_com(g_d3d);
}

void render()
{
g_device->Clear(0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB(55, 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 48: // press key "0", disable alpha blend.
g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, FALSE);
break;

case 49: // press key "1", enable alpha blend.
g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
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, 600, 500,
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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