Alpha Blending a Bitmap 失败 原因

AlphaBlend中目标DC的矩形范围设置错误，应为0,0，nWidth，nHeight；

另外参考微软MSDN源码：

Alpha Blending a Bitmap

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The following code sample divides a window into three horizontal areas. Then it draws an alpha-blended bitmap in each of the window areas as follows:

In the top area, constant alpha = 50% but there is no source alpha.

In the middle area, constant alpha = 100% (disabled) and source alpha is 0 (transparent) in the middle of the bitmap and 0xff (opaque) elsewhere.

In the bottom area, constant alpha = 75% and source alpha changes.

void DrawAlphaBlend (HWND hWnd, HDC hdcwnd)
{
HDC hdc;               // handle of the DC we will create
BLENDFUNCTION bf;      // structure for alpha blending
HBITMAP hbitmap;       // bitmap handle
BITMAPINFO bmi;        // bitmap header
VOID *pvBits;          // pointer to DIB section
ULONG   ulWindowWidth, ulWindowHeight;      // window width/height
ULONG   ulBitmapWidth, ulBitmapHeight;      // bitmap width/height
RECT    rt;            // used for getting window dimensions
UINT32   x,y;          // stepping variables
UCHAR ubAlpha;         // used for doing transparent gradient
UCHAR ubRed;
UCHAR ubGreen;
UCHAR ubBlue;
float fAlphaFactor;    // used to do premultiply

// get window dimensions
GetClientRect(hWnd, &rt);

// calculate window width/height
ulWindowWidth = rt.right - rt.left;
ulWindowHeight = rt.bottom - rt.top;

// make sure we have at least some window size
if ((!ulWindowWidth) || (!ulWindowHeight))
return;

// divide the window into 3 horizontal areas
ulWindowHeight = ulWindowHeight / 3;

// create a DC for our bitmap -- the source DC for AlphaBlend
hdc = CreateCompatibleDC(hdcwnd);

// zero the memory for the bitmap info
ZeroMemory(&bmi, sizeof(BITMAPINFO));

// setup bitmap info
// set the bitmap width and height to 60% of the width and height of each of the three horizontal areas. Later on, the blending will occur in the center of each of the three areas.
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = ulBitmapWidth = ulWindowWidth - (ulWindowWidth/5)*2;
bmi.bmiHeader.biHeight = ulBitmapHeight = ulWindowHeight - (ulWindowHeight/5)*2;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32;         // four 8-bit components
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = ulBitmapWidth * ulBitmapHeight * 4;

// create our DIB section and select the bitmap into the dc
hbitmap = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, &pvBits, NULL, 0x0);
SelectObject(hdc, hbitmap);

// in top window area, constant alpha = 50%, but no source alpha
// the color format for each pixel is 0xaarrggbb
// set all pixels to blue and set source alpha to zero
for (y = 0; y < ulBitmapHeight; y++)
for (x = 0; x < ulBitmapWidth; x++)
((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0x000000ff;

bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 0x7f;  // half of 0xff = 50% transparency
bf.AlphaFormat = 0;             // ignore source alpha channel

if (!AlphaBlend(hdcwnd, ulWindowWidth/5, ulWindowHeight/5,
ulBitmapWidth, ulBitmapHeight,
hdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf))
return;                     // alpha blend failed

// in middle window area, constant alpha = 100% (disabled), source
// alpha is 0 in middle of bitmap and opaque in rest of bitmap
for (y = 0; y < ulBitmapHeight; y++)
for (x = 0; x < ulBitmapWidth; x++)
if ((x > (int)(ulBitmapWidth/5)) && (x < (ulBitmapWidth-ulBitmapWidth/5)) &&
(y > (int)(ulBitmapHeight/5)) && (y < (ulBitmapHeight-ulBitmapHeight/5)))
//in middle of bitmap: source alpha = 0 (transparent).
// This means multiply each color component by 0x00.
// Thus, after AlphaBlend, we have a, 0x00 * r,
// 0x00 * g,and 0x00 * b (which is 0x00000000)
// for now, set all pixels to red
((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0x00ff0000;
else
// in the rest of bitmap, source alpha = 0xff (opaque)
// and set all pixels to blue
((UINT32 *)pvBits)[x + y * ulBitmapWidth] = 0xff0000ff;
endif;

bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.AlphaFormat = AC_SRC_ALPHA;  // use source alpha
bf.SourceConstantAlpha = 0xff;  // opaque (disable constant alpha)

if (!AlphaBlend(hdcwnd, ulWindowWidth/5, ulWindowHeight/5+ulWindowHeight, ulBitmapWidth, ulBitmapHeight, hdc, 0, 0, ulBitmapWidth, ulBitmapHeight, bf))
return;

// bottom window area, use constant alpha = 75% and a changing
// source alpha. Create a gradient effect using source alpha, and
// then fade it even more with constant alpha
ubRed = 0x00;
ubGreen = 0x00;
ubBlue = 0xff;

for (y = 0; y < ulBitmapHeight; y++)
for (x = 0; x < ulBitmapWidth; x++)
{
// for a simple gradient, base the alpha value on the x
// value of the pixel
ubAlpha = (UCHAR)((float)x / (float)ulBitmapWidth * 255);
//calculate the factor by which we multiply each component
fAlphaFactor = (float)ubAlpha / (float)0xff;
// multiply each pixel by fAlphaFactor, so each component
// is less than or equal to the alpha value.
((UINT32 *)pvBits)[x + y * ulBitmapWidth]
= (ubAlpha << 24) |                       //0xaa000000
((UCHAR)(ubRed * fAlphaFactor) << 16) |  //0x00rr0000
((UCHAR)(ubGreen * fAlphaFactor) << 8) | //0x0000gg00
((UCHAR)(ubBlue   * fAlphaFactor));      //0x000000bb
}

bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.AlphaFormat = AC_SRC_ALPHA;   // use source alpha
bf.SourceConstantAlpha = 0xbf;   // use constant alpha, with
// 75% opaqueness

AlphaBlend(hdcwnd, ulWindowWidth/5,
ulWindowHeight/5+2*ulWindowHeight, ulBitmapWidth,
ulBitmapHeight, hdc, 0, 0, ulBitmapWidth,
ulBitmapHeight, bf);

// do cleanup
DeleteObject(hbitmap);
DeleteDC(hdc);

}