天行健 君子當自強而不息

前面的一些例子中，已經遇到了alpha值的概念，如結構體D3DCOLORVALUE中的成員變量a，但它并沒有被使用，因為還沒有激活alpha混合（alpha blending），并且alpha總是被賦值為1.0f（默認渲染狀態下alpha為1.0f，表示完全不透明）。

 

alpha混合原理

在前面介紹的示例程序中，繪制圖形的顏色總是替換當前顏色緩沖區中存在的顏色，這樣后面的物體總是覆蓋在原有的物體上。但是當想要繪制類似于玻璃、水等具有透明效果的物體時，這種方法顯然滿足不了要求。通過定義一個表示物體半透明度的alpha值和一個半透明計算公式，可以將要繪制的物體顏色與顏色緩沖區中存在的顏色相混合，從而繪制出具有半透明效果的物體。Direct3D計算alpha顏色混合的方法如下：

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

其中color表示alpha混合后的顏色值，RGB_src表示源顏色值，即將要繪制的圖元的顏色值；K_src表示源混合系數，通常賦值為表示半透明程度的alpha值，也可以是屬于枚舉類型D3DBLEND的任意值，用來和RGB_src相乘。RGB_dst表示目標顏色值，即當前顏色緩沖區中的顏色值，K_dst表示目標混合系數，可以是屬于枚舉D3DBLEND的任意值，用來和RGB_dst相乘。OP表示源計算結果與顏色緩沖區計算結果的混合方法，默認狀態下OP為D3DBLEND_ADD，即源計算結果與顏色緩沖區計算結果相加。

圖形顯示中，對alpha混合最普遍的用法是：把K_src賦值為D3DBLEND_SRCALPHA，即當前繪制像素的alpha值；把K_dst賦值為D3DBLEND_INVSRCALPHA，即1減去當前繪制像素的alpha值；把OP賦值為D3DBLEND_ADD，使源計算結果和顏色緩沖區計算結果相加，這樣一來，alpha混合顏色的公式變為：

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

上面的設置可以較好地模擬大多數半透明物體的效果。

 

啟用alpha混合

想要繪制半透明物體，首先需要激活Direct3D的alpha混合運算，調用Direct3D渲染狀態設置函數IDirect3DDevice9:::SetRenderState()，將第一個參數設置為D3DRS_ALPHABLENDENABLE，第二個參數設置為TRUE，可以激活alpha混合，代碼如下：

g_device->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
 

設置alpha混合系數

在上面介紹的alpha混合原理中提到的源混合系數和目標混合系數，也是通過Direct3D渲染狀態設置函數IDirect3DDevice9::SetRenderState()設置的。若將第一個參數設置為D3DRS_SRCBLEND，則用于設置源混合系數，若將第一個參數設置為D3DRS_DESTBLEND，則用于設置目標混合系數，第二個參數可以設置為D3DBLEND枚舉常量，各具體枚舉常量的含義如下：

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 DestBlendCaps members 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 and d 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.

設置alpha混合系數的代碼示例如下：

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

 

設置alpha混合方法

alpha混合方法指定源顏色和目標顏色的混合方法，通過Direct3D渲染狀態設置函數IDirect3DDevice9::SetRenderState()設置，其中第一個參數設置為 D3DRS_BLENDOP，第二個參數設置為D3DBLENDOP枚舉常量，各常量的含義如下：

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：

該示例程序模擬了直升飛機螺旋槳的半透明效果。在程序的初始化階段，載入Heli.x文件，它是一個包含直升飛機的三維模型文件，其中螺旋槳的材質漫反射屬性為（R, G, B, A） = (0.183700; 0.183700; 0.183700; 0.500000; )，可以用文本方式打開Heli.x查看它的材質屬性，Heli.x是一個文本格式的.x文件。

示例程序中沒有設置alpha混合方法，所以應用程序將采用默認的alpha混合方法D3DBLEND_ADD，即將源計算結果與顏色緩沖區計算結果相加。由于直升機螺旋槳的材質的alpha值為0.5f，因此它的最終顏色就是50%的玻璃顏色加上50%的背景顏色。

按下數字鍵"1"，激活alpha混合。

 

按下數字鍵"0"，禁用alpha混合。

 

設置alpha混合方法為D3DBLENDOP_SUBTRACT的效果圖，啟用alpha混合時。

g_device->SetRenderState(D3DRS_BLENDOP, D3DBLENDOP_SUBTRACT);

 

設置以下代碼時的效果圖，可以看到直升機變亮了，啟用alpha混合：

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

 

設置以下代碼時的效果圖，可以看到直升機變暗了，啟用alpha混合：

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

 

可以根據需要自己修改設置，以實現不同的顏色混合效果。

 

由于alpha混合是當前繪制的像素顏色與顏色緩沖區中存在的顏色的混合運算，因此，在繪制半透明物體前，必須保證位于半透明物體后的物體先于半透明物體繪制，也就是說，先繪制不透明物體，再繪制半透明物體。

有些計算機硬件由于功能的限制，可能不支持某些混合方法，這時Direct3D會自動使用alpha混合方法D3DBLENDOP_ADD。

 

源程序：

 

下載示例工程