alpha混合原理（2）

alpha混合原理

在前面介绍的示例程序中，绘制图形的颜色总是替换当前颜色缓冲区中存在的颜色，这样后面的物体总是覆盖在原有的物体上。但是当想要绘制类似于玻璃、水等具有透明效果的物体时，这种方法显然满足不了要求。通过定义一个表示物体半透明度的alpha值和一个半透明计算公式，可以将要绘制的物体颜色与颜色缓冲区中存在的颜色相混合，从而绘制出具有半透明效果的物体。Direct3D计算alpha颜色混合的方法如下：

color = (RGB_src * K_src) OP (RGB_dst * K_dst)

其中color表示alpha混合后的颜色值，RGB_src表示源颜色值，即将要绘制的图元的颜色值；K_src表示源混合系数，通常赋值为表示半透明程度的alpha值，也可以是属于枚举类型D3DBLEND的任意值，用来和RGB_src相乘。RGB_dst表示目标颜色值，即当前颜色缓冲区中的颜色值，K_dst表示目标混合系数，可以是属于枚举D3DBLEND的任意值，用来和RGB_dst相乘。OP表示源计算结果与颜色缓冲区计算结果的混合方法，默认状态下OP为D3DBLEND_ADD，即源计算结果与颜色缓冲区计算结果相加。

图形显示中，对alpha混合最普遍的用法是：把K_src赋值为D3DBLEND_SRCALPHA，即当前绘制像素的alpha值；把K_dst赋值为D3DBLEND_INVSRCALPHA，即1减去当前绘制像素的alpha值；把OP赋值为D3DBLEND_ADD，使源计算结果和颜色缓冲区计算结果相加，这样一来，alpha混合颜色的公式变为：

color = (RGB_src * K_src) + (RGB_dst * K_dst)

上面的设置可以较好地模拟大多数半透明物体的效果。

 

启用alpha混合

想要绘制半透明物体，首先需要激活Direct3D的alpha混合运算，调用Direct3D渲染状态设置函数IDirect3DDevice9:::SetRenderState()，将第一个参数设置为D3DRS_ALPHABLENDENABLE，第二个参数设置为TRUE，可以激活alpha混合，代码如下：

g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
 

设置alpha混合系数

在上面介绍的alpha混合原理中提到的源混合系数和目标混合系数，也是通过Direct3D渲染状态设置函数IDirect3DDevice9::SetRenderState()设置的。若将第一个参数设置为D3DRS_SRCBLEND，则用于设置源混合系数，若将第一个参数设置为D3DRS_DESTBLEND，则用于设置目标混合系数，第二个参数可以设置为D3DBLEND枚举常量，各具体枚举常量的含义如下：

Defines the supported blend mode.

typedef enum D3DBLEND
{
D3DBLEND_ZERO = 1,
D3DBLEND_ONE = 2,
D3DBLEND_SRCCOLOR = 3,
D3DBLEND_INVSRCCOLOR = 4,
D3DBLEND_SRCALPHA = 5,
D3DBLEND_INVSRCALPHA = 6,
D3DBLEND_DESTALPHA = 7,
D3DBLEND_INVDESTALPHA = 8,
D3DBLEND_DESTCOLOR = 9,
D3DBLEND_INVDESTCOLOR = 10,
D3DBLEND_SRCALPHASAT = 11,
D3DBLEND_BOTHSRCALPHA = 12,
D3DBLEND_BOTHINVSRCALPHA = 13,
D3DBLEND_BLENDFACTOR = 14,
D3DBLEND_INVBLENDFACTOR = 15,
D3DBLEND_SRCCOLOR2 = 16,
D3DBLEND_INVSRCCOLOR2 = 17,
D3DBLEND_FORCE_DWORD = 0x7fffffff,
} D3DBLEND, *LPD3DBLEND; 

Constants

D3DBLEND_ZERO

Blend factor is (0, 0, 0, 0).

D3DBLEND_ONE

Blend factor is (1, 1, 1, 1).

D3DBLEND_SRCCOLOR

Blend factor is (R_s, G_s, B_s, A_s).

D3DBLEND_INVSRCCOLOR

Blend factor is (1 - R_s, 1 - G_s, 1 - B_s, 1 - A_s).

D3DBLEND_SRCALPHA

Blend factor is (A_s, A_s, A_s, A_s).

D3DBLEND_INVSRCALPHA

Blend factor is ( 1 - A_s, 1 - A_s, 1 - A_s, 1 - A_s).

D3DBLEND_DESTALPHA

Blend factor is (A_d A_d A_d A_d).

D3DBLEND_INVDESTALPHA

Blend factor is (1 - A_d 1 - A_d 1 - A_d 1 - A_d).

D3DBLEND_DESTCOLOR

Blend factor is (R_d, G_d, B_d, A_d).

D3DBLEND_INVDESTCOLOR

Blend factor is (1 - R_d, 1 - G_d, 1 - B_d, 1 - A_d).

D3DBLEND_SRCALPHASAT

Blend factor is (f, f, f, 1); where f = min(A_s, 1 - A_d).

D3DBLEND_BOTHSRCALPHA

Obsolete. Starting with DirectX 6, you can achieve the same effect by setting the source and destination blend factors to D3DBLEND_SRCALPHA and D3DBLEND_INVSRCALPHA in separate calls.

D3DBLEND_BOTHINVSRCALPHA

Source blend factor is (1 - A_s, 1 - A_s, 1 - A_s, 1 - A_s), and destination blend factor is (A_s, A_s, A_s, A_s); the destination blend selection is overridden. This blend mode is supported only for the D3DRS_SRCBLEND render state.

D3DBLEND_BLENDFACTOR

Constant color blending factor used by the frame-buffer blender. This blend mode is supported only if D3DPBLENDCAPS_BLENDFACTOR is set in the SrcBlendCaps or DestBlendCaps members of D3DCAPS9.

D3DBLEND_INVBLENDFACTOR

Inverted constant color-blending factor used by the frame-buffer blender. This blend mode is supported only if the D3DPBLENDCAPS_BLENDFACTOR bit is set in the SrcBlendCaps or DestBlendCapsmembers of D3DCAPS9.

D3DBLEND_SRCCOLOR2

Blend factor is (PSOutColor[1]_r, PSOutColor[1]_g, PSOutColor[1]_b, not used). See Render Target Blending.

Differences between Direct3D 9 and Direct3D 9Ex:

This flag is available in Direct3D 9Ex only.

D3DBLEND_INVSRCCOLOR2

Blend factor is (1 - PSOutColor[1]_r, 1 - PSOutColor[1]_g, 1 - PSOutColor[1]_b, not used)). See Render Target Blending.

Differences between Direct3D 9 and Direct3D 9Ex:

This flag is available in Direct3D 9Ex only.

D3DBLEND_FORCE_DWORD

Forces this enumeration to compile to 32 bits in size. Without this value, some compilers would allow this enumeration to compile to a size other than 32 bits. This value is not used.

Remarks

In the preceding member descriptions, the RGBA values of the source and destination are indicated by the s andd subscripts.

The values in this enumerated type are used by the following render states:

• D3DRS_DESTBLEND
• D3DRS_SRCBLEND
• D3DRS_DESTBLENDALPHA
• D3DRS_SRCBLENDALPHA

See D3DRENDERSTATETYPE

Render Target Blending

Direct3D 9Ex has improved text rendering capabilities. Rendering clear-type fonts would normally require two passes. To eliminate the second pass, a pixel shader can be used to output two colors, which we can call PSOutColor[0] and PSOutColor[1]. The first color would contain the standard 3 color components (RGB). The second color would contain 3 alpha components (one for each component of the first color).

These new blending modes are only used for text rendering on the first render target.

设置alpha混合系数的代码示例如下：

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

 

设置alpha混合方法

alpha混合方法指定源颜色和目标颜色的混合方法，通过Direct3D渲染状态设置函数IDirect3DDevice9::SetRenderState()设置，其中第一个参数设置为 D3DRS_BLENDOP，第二个参数设置为D3DBLENDOP枚举常量，各常量的含义如下：

Defines the supported blend operations. See Remarks for definitions of terms.

typedef enum D3DBLENDOP
{
D3DBLENDOP_ADD = 1,
D3DBLENDOP_SUBTRACT = 2,
D3DBLENDOP_REVSUBTRACT = 3,
D3DBLENDOP_MIN = 4,
D3DBLENDOP_MAX = 5,
D3DBLENDOP_FORCE_DWORD = 0x7fffffff,
} D3DBLENDOP, *LPD3DBLENDOP; 

Constants

D3DBLENDOP_ADD

The result is the destination added to the source. Result = Source + Destination

D3DBLENDOP_SUBTRACT

The result is the destination subtracted from to the source. Result = Source - Destination

D3DBLENDOP_REVSUBTRACT

The result is the source subtracted from the destination. Result = Destination - Source

D3DBLENDOP_MIN

The result is the minimum of the source and destination. Result = MIN(Source, Destination)

D3DBLENDOP_MAX

The result is the maximum of the source and destination. Result = MAX(Source, Destination)

D3DBLENDOP_FORCE_DWORD

Forces this enumeration to compile to 32 bits in size. Without this value, some compilers would allow this enumeration to compile to a size other than 32 bits. This value is not used.

Remarks

Source, Destination, and Result are defined as:

Term	Type	Description
Source	Input	Color of the source pixel before the operation.
Destination	Input	Color of the pixel in the destination buffer before the operation.
Result	Output	Returned value that is the blended color resulting from the operation.

This enumerated type defines values used by the following render states:

• D3DRS_BLENDOP
• D3DRS_BLENDOPALPHA

 

示例程序：

该示例程序模拟了直升飞机螺旋桨的半透明效果。在程序的初始化阶段，载入Heli.x文件，它是一个包含直升飞机的三维模型文件，其中螺旋桨的材质漫反射属性为（R, G, B, A） = (0.183700; 0.183700; 0.183700; 0.500000; )，可以用文本方式打开Heli.x查看它的材质属性，Heli.x是一个文本格式的.x文件。

示例程序中没有设置alpha混合方法，所以应用程序将采用默认的alpha混合方法D3DBLEND_ADD，即将源计算结果与颜色缓冲区计算结果相加。由于直升机螺旋桨的材质的alpha值为0.5f，因此它的最终颜色就是50%的玻璃颜色加上50%的背景颜色。

按下数字键"1"，激活alpha混合。

 

按下数字键"0"，禁用alpha混合。

 

设置alpha混合方法为D3DBLENDOP_SUBTRACT的效果图，启用alpha混合时。

g_device->SetRenderState(D3DRS_BLENDOP, D3DBLENDOP_SUBTRACT);

 

设置以下代码时的效果图，可以看到直升机变亮了，启用alpha混合：

g_device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ONE);
g_device->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_DESTCOLOR);

 

设置以下代码时的效果图，可以看到直升机变暗了，启用alpha混合：

g_device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ONE);
g_device->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVDESTCOLOR);

 

可以根据需要自己修改设置，以实现不同的颜色混合效果。

 

由于alpha混合是当前绘制的像素颜色与颜色缓冲区中存在的颜色的混合运算，因此，在绘制半透明物体前，必须保证位于半透明物体后的物体先于半透明物体绘制，也就是说，先绘制不透明物体，再绘制半透明物体。

有些计算机硬件由于功能的限制，可能不支持某些混合方法，这时Direct3D会自动使用alpha混合方法D3DBLENDOP_ADD。

 

 

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;

    

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

    //g_device->SetRenderState(D3DRS_SRCBLEND,         D3DBLEND_ONE);
    //g_device->SetRenderState(D3DRS_DESTBLEND,         D3DBLEND_INVDESTCOLOR);    

    //g_device->SetRenderState(D3DRS_BLENDOP,             D3DBLENDOP_SUBTRACT);        

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

    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 blend.
            g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, FALSE);
            break;

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

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

    return 0;
}