深度测试与alpha混合（2）
前面的一些例子中，已经遇到了alpha值的概念，如结构体D3DCOLORVALUE中的成员变量a，但它并没有被使用，因为还没有激活alpha混合（alpha blending），并且alpha总是被赋值为1.0f（默认渲染状态下alpha为1.0f，表示完全不透明）。
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 = (RGBsrc * Ksrc) OP (RGBdst * Kdst)
其中color表示alpha混合后的颜色值，RGBsrc表示源颜色值，即将要绘制的图元的颜色值；Ksrc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RGBsrc相乘。RGBdst表示目标颜色值，即当前颜色缓冲区中的颜色值，Kdst表示目标混合系数，可以是属于枚举D3DBLEND的任意值，用来和RGBdst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Ksrc赋值为D3DBLEND_SRCALPHA，即当前绘制像素的alpha值；把Kdst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 = (RGBsrc * Ksrc) + (RGBdst * Kdst)
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 (Rs, Gs, Bs, As).
D3DBLEND_INVSRCCOLOR
Blend factor is (1 - Rs, 1 - Gs, 1 - Bs, 1 - As).
D3DBLEND_SRCALPHA
Blend factor is (As, As, As, As).
D3DBLEND_INVSRCALPHA
Blend factor is ( 1 - As, 1 - As, 1 - As, 1 - As).
D3DBLEND_DESTALPHA
Blend factor is (Ad Ad Ad Ad).
D3DBLEND_INVDESTALPHA
Blend factor is (1 - Ad 1 - Ad 1 - Ad 1 - Ad).
D3DBLEND_DESTCOLOR
Blend factor is (Rd, Gd, Bd, Ad).
D3DBLEND_INVDESTCOLOR
Blend factor is (1 - Rd, 1 - Gd, 1 - Bd, 1 - Ad).
D3DBLEND_SRCALPHASAT
Blend factor is (f, f, f, 1); where f = min(As, 1 - Ad).
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 - As, 1 - As, 1 - As, 1 - As), and destination blend factor is (As, As, As, As); 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.
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
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计算机技术与应用 → 『 C/C++编程思想 』 → 深度测试与alpha混合
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2009/3/9 9:59:00
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深度测试与alpha混合(5638字) － 卷积内核，2009年3月9日
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