AlphaBlend：Wince 5以后有带支持这个函数.仔细查看这个函数的时候我们发现这个函数有两种的用法，最简单的就是设定统一的透明色，一个就是利用位图的Alpha通道，来进行叠加。这里我们主要说的就是这个利用位图的Alpha通道来透明位图：

下面的这个是来自MSDN:

 

This function displays bitmaps that have transparent or semitransparent pixels.

BOOL AlphaBlend(

  HDC           
hdcDest

,

  int           
nXOriginDest

,

  int           
nYOriginDest

,

  int           
nWidthDest

,

  int           
nHeightDest

,

  HDC           
hdcSrc

,

  int           
nXOriginSrc

,

  int           
nYOriginSrc

,

  int           
nWidthSrc

,

  int           
nHeightSrc

,

  BLENDFUNCTION 
blendFunction


);

Parameters

hdcDest

[in] Handle to the destination device context.

nXOriginDest

[in] Specifies the x-coordinate, in logical units, of the upper-left corner of the destination rectangle.

nYOriginDest

[in] Specifies the y-coordinate, in logical units, of the upper-left corner of the destination rectangle.

nWidthDest

[in] Specifies the width, in logical units, of the destination rectangle. This value cannot be negative.

nHeightDest

[in] Specifies the height, in logical units, of the destination rectangle. This value cannot be negative.

hdcSrc

[in] Handle to the source device context.

nXOriginSrc

[in] Specifies the x-coordinate, in logical units, of the upper-left corner of the source rectangle.

nYOriginSrc

[in] Specifies the y-coordinate, in logical units, of the upper-left corner of the source rectangle.

nWidthSrc

[in] Specifies the width, in logical units, of the source rectangle. This value cannot be negative.

nHeightSrc

[in] Specifies the height, in logical units, of the source rectangle. This value cannot be negative.

blendFunction

[in] A BLENDFUNCTIONstructure that specifies the alpha-blending function for source and destination bitmaps, a global alpha value to be applied to the entire source bitmap, and format information for the source bitmap. The source and destination blend functions are currently limited to AC_SRC_OVER.

Return Values

If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE.

To get extended error information, call GetLastError. GetLastErrorcan return the following value.

Remarks

If the source rectangle and destination rectangle are different sizes, the source bitmap is stretched to match the destination rectangle. If the SetStretchBltModefunction is used, the iStretchModevalue is automatically converted to COLORONCOLOR for this function.

The destination coordinates are transformed by using the transformation currently specified for the destination device context. The source coordinates are transformed by using the transformation currently specified for the source device context.

If the source device context identifies an enhanced metafile device context, an error occurs (and the function returns FALSE).

If destination and source bitmaps do not have the same color format, AlphaBlendconverts the source bitmap to match the destination bitmap.

AlphaBlenddoes not support mirroring.   If either the width or height of the source or destination is negative, the call to AlphaBlendwill fail.

When rendering to a printer, first call GetDeviceCapswith SHADEBLENDCAPS to determine if the printer supports blending with AlphaBlend . Note that, for a display DC, all blending operations are supported and these flags represent whether the operations are accelerated.

The source rectangle must lie completely within the source surface, otherwise an error occurs and the function returns FALSE.

The SourceConstantaAlphamember of the BLENDFUNCTIONstructure specifies an alpha transparency value to be used on the entire source bitmap. The SourceConstantAlphavalue is combined with any per-pixel alpha values. If SourceConstantAlphais 0, it is assumed that the image is transparent. To use only per-pixel alpha values, set the SourceConstantAlphavalue to 255 to indicate that the image is opaque.

Requirements

OS Versions:Microsoft® Windows CE® 5.0 and later.
Header:Windows.h.
Link Library:Coredll.lib.

 

 

 

 

 

 

 

This structure controls blending by specifying the blending functions for source and destination bitmaps.

typedef struct _BLENDFUNCTION {
BYTE  
BlendOp

;
BYTE  
BlendFlags

;
BYTE  
SourceConstantAlpha

;
BYTE  
AlphaFormat

;


}BLENDFUNCTION, *PBLENDFUNCTION, *LPBLENDFUNCTION;

Members

BlendOp

Specifies the source blend operation. Currently, the only source and destination blend operation that has been defined is AC_SRC_OVER. For details, see the following Remarks section.

BlendFlags

Must be zero.

SourceConstantAlpha

Specifies an alpha transparency value to be used on the entire source bitmap. The SourceConstantAlphavalue is combined with any per-pixel alpha values in the source bitmap. If you set SourceConstantAlphato 0, it is assumed that your image is transparent. When you only want to use per-pixel alpha values, set the SourceConstantAlphavalue to 255 (opaque) .

AlphaFormat

This member controls the way the source and destination bitmaps are interpreted. The following table shows the value for AlphaFormat .

Value	Description
AC_SRC_ALPHA	This flag is set when the bitmap has an Alpha channel (that is, per-pixel alpha). Because this API uses premultiplied alpha, the red, green and blue channel values in the bitmap must be premultiplied with the alpha channel value. For example, if the alpha channel value is x, the red, green and blue channels must be multiplied by x and divided by 0xff before the call.

Remarks

When the AlphaFormatparameter is AC_SRC_ALPHA, the source bitmap must be 32 bpp. If it is not, the AlphaBlend function will fail.

 

 

 

其实关键的一点就是如何使得我们得到的装载进入到得位图。

 

下面以MFC程序来示例：

 

       HDC hm_DC0=::GetDC(this->m_hWnd);
//装载前置32B RGBA位图
HBITMAP hBmp_Front0=::SHLoadDIBitmap(fileName2);//文件路径

BITMAP bitmap;
GetObject(hBmp_Front0,sizeof(BITMAP),(LPVOID)&bitmap); //获取信息
byte *p=(byte*)bitmap.bmBits;

HDC DirectDC=CreateCompatibleDC(NULL);

HGDIOBJ PreviousObject=SelectObject(DirectDC, hBmp_Front0);

BLENDFUNCTION blend;
blend.AlphaFormat=AC_SRC_ALPHA;
blend.BlendFlags=0;
blend.BlendOp=AC_SRC_OVER;
blend.SourceConstantAlpha=255;

if(!AlphaBlend(hm_DC0,0,0,256,245,DirectDC,0,0,256,245,blend))
{
MessageBox(L"Faild",L"AlphaBlend",MB_OK);

}

//释放资源
DeleteDC(DirectDC);
DeleteObject(hBmp_Front0);
DeleteDC(hm_DC0);