HOWTO: Platform Independent Alpha Blending (303916)
The information in this article applies to:
- Microsoft Platform Software Development Kit (SDK) 1.0
This article was previously published under Q303916 SUMMARY
The AlphaBlend function is not supported on Microsoft Windows 95 or on versions of Microsoft Windows NT prior to Microsoft Windows 2000. Applications that require the AlphaBlend function must provide their own implementations.
The source code in the "More Information" section of this article provides an example of a roll-your-own approach to implementing the most often used capabilities of the AlphaBlend function in a platform independent way. Applications can call the DoAlphaBlend function (illustrated in the sample code) with almost the same parameters that they can pass into the AlphaBlend function. The one difference is that, instead of taking a BLENDFUNCTION as the last parameter, DoAlphaBlend instead takes a source weighting value. The weighting value can be between 0 and 255 and the weighting value determines how the source and destination are mixed.
The formula for the weighting is:
dest' = ((weighting * source) + ((255-weighting) * dest)) / 256
By applying the formula, you can see that a value of 255 completely favors the source and a value of 0 completely favors the destination. A value of 127 provides a 50/50 mix of the source and destination.
MORE INFORMATIONSample Code
/*********************************************************************\
* *
* Create24BPPDIBSection(HDC, int, int); *
* *
* Purpose: *
* Creates a 24 bit-per-pixel DIB section with the specified *
* width and height. *
* *
\*********************************************************************/
HBITMAP Create24BPPDIBSection(HDC hDC, int iWidth, int iHeight)
{
BITMAPINFO bmi;
HBITMAP hbm;
LPBYTE pBits;
// Initialize header to 0s.
ZeroMemory(&bmi, sizeof(bmi));
// Fill out the fields you care about.
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = iWidth;
bmi.bmiHeader.biHeight = iHeight;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 24;
bmi.bmiHeader.biCompression = BI_RGB;
// Create the surface.
hbm = CreateDIBSection(hDC, &bmi, DIB_RGB_COLORS, &pBits, NULL, 0);
return(hbm);
}
/*********************************************************************\
* *
* BitmapsCompatible(LPBITMAP, LPBITMAP) *
* *
* Purpose: *
* Compares two bitmap structures to see if the surfaces are *
* the same width, height, and pixel format. *
* *
\*********************************************************************/
BOOL BitmapsCompatible(LPBITMAP lpbm1, LPBITMAP lpbm2)
{
if (lpbm1->bmBitsPixel != lpbm2->bmBitsPixel) return FALSE;
if (lpbm1->bmPlanes != lpbm2->bmPlanes) return FALSE;
if (lpbm1->bmWidth != lpbm2->bmWidth) return FALSE;
if (lpbm1->bmHeight != lpbm2->bmHeight) return FALSE;
return TRUE;
}
/*********************************************************************\
* *
* BOOL BlendImages(HBITMAP, HBITMAP, HBITMAP, DWORD); *
* *
* Purpose: *
* Blends two source images into a destination image *
* using a specified weighting value for the first source *
* image. *
* *
* Notes: *
* All HBITMAP parameters must be 24 bit-per-pixel DIB sections. *
* All HBITMAP parameters must have the same widths and heights. *
* Weighting values must be in the range of 0 .. 255. *
* *
\*********************************************************************/
BOOL BlendImages(HBITMAP hbmSrc1, HBITMAP hbmSrc2, HBITMAP hbmDst, DWORD dwWeight1)
{
BITMAP bmSrc1, bmSrc2, bmDst;
RGBTRIPLE *lprgbSrc1, *lprgbSrc2, *lprgbDst;
DWORD dwWidthBytes, dwWeight2;
int x, y;
// Only values between 0 and 255 are valid.
if (dwWeight1 > 255) return FALSE;
// Get weighting value for second source image.
dwWeight2 = 255-dwWeight1;
// Get information about the surfaces you were passed.
if (!GetObject(hbmSrc1, sizeof(BITMAP), &bmSrc1)) return FALSE;
if (!GetObject(hbmSrc2, sizeof(BITMAP), &bmSrc2)) return FALSE;
if (!GetObject(hbmDst, sizeof(BITMAP), &bmDst)) return FALSE;
// Make sure you have data that meets your requirements.
if (!BitmapsCompatible(&bmSrc1, &bmSrc2)) return FALSE;
if (!BitmapsCompatible(&bmSrc1, &bmDst)) return FALSE;
if (bmSrc1.bmBitsPixel != 24) return FALSE;
if (bmSrc1.bmPlanes != 1) return FALSE;
if (!bmSrc1.bmBits || !bmSrc2.bmBits || !bmDst.bmBits) return FALSE;
dwWidthBytes = bmDst.bmWidthBytes;
// Initialize the surface pointers.
lprgbSrc1 = bmSrc1.bmBits;
lprgbSrc2 = bmSrc2.bmBits;
lprgbDst = bmDst.bmBits;
for (y=0; y<bmDst.bmHeight; y++) {
for (x=0; x<bmDst.bmWidth; x++) {
lprgbDst[x].rgbtRed = (BYTE)((((DWORD)lprgbSrc1[x].rgbtRed * dwWeight1) +
((DWORD)lprgbSrc2[x].rgbtRed * dwWeight2)) >> 8);
lprgbDst[x].rgbtGreen = (BYTE)((((DWORD)lprgbSrc1[x].rgbtGreen * dwWeight1) +
((DWORD)lprgbSrc2[x].rgbtGreen * dwWeight2)) >> 8);
lprgbDst[x].rgbtBlue = (BYTE)((((DWORD)lprgbSrc1[x].rgbtBlue * dwWeight1) +
((DWORD)lprgbSrc2[x].rgbtBlue * dwWeight2)) >> 8);
}
// Move to next scan line.
lprgbSrc1 = (RGBTRIPLE *)((LPBYTE)lprgbSrc1 + dwWidthBytes);
lprgbSrc2 = (RGBTRIPLE *)((LPBYTE)lprgbSrc2 + dwWidthBytes);
lprgbDst = (RGBTRIPLE *)((LPBYTE)lprgbDst + dwWidthBytes);
}
return TRUE;
}
/*********************************************************************\
* *
* BOOL DoAlphaBlend() *
* *
* Purpose: *
* Captures a copy of the source and destination areas and *
* alpha blends them into a memory surface that it displays *
* into the destination area. * * *
* Notes: *
* Takes the same parameters as the AlphaBlend function except *
* that the last parameter is a source weighting value rather *
* than a BLENDFUNCTION structure. *
* *
\*********************************************************************/
BOOL DoAlphaBlend(
HDC hdcDest, // Handle to destination DC.
int nXOriginDest, // X-coord of upper-left corner.
int nYOriginDest, // Y-coord of upper-left corner.
int nWidthDest, // Destination width.
int nHeightDest, // Destination height.
HDC hdcSrc, // Handle to source DC.
int nXOriginSrc, // X-coord of upper-left corner.
int nYOriginSrc, // Y-coord of upper-left corner.
int nWidthSrc, // Source width.
int nHeightSrc, // Source height.
DWORD dwSourceWeight) // Source weighting (between 0 and 255).
{
HDC hdcSrc1 = NULL;
HDC hdcSrc2 = NULL;
HDC hdcDst = NULL;
HBITMAP hbmSrc1 = NULL;
HBITMAP hbmSrc2 = NULL;
HBITMAP hbmDst = NULL;
BOOL bReturn;
// Create surfaces for sources and destination images.
hbmSrc1 = Create24BPPDIBSection(hdcDest, nWidthDest,nHeightDest);
if (!hbmSrc1) goto HANDLEERROR;
hbmSrc2 = Create24BPPDIBSection(hdcDest, nWidthDest,nHeightDest);
if (!hbmSrc2) goto HANDLEERROR;
hbmDst = Create24BPPDIBSection(hdcDest, nWidthDest,nHeightDest);
if (!hbmDst) goto HANDLEERROR;
// Create HDCs to hold our surfaces.
hdcSrc1 = CreateCompatibleDC(hdcDest);
if (!hdcSrc1) goto HANDLEERROR;
hdcSrc2 = CreateCompatibleDC(hdcDest);
if (!hdcSrc2) goto HANDLEERROR;
hdcDst = CreateCompatibleDC(hdcDest);
if (!hdcDst) goto HANDLEERROR;
// Prepare the surfaces for drawing.
SelectObject(hdcSrc1, hbmSrc1);
SelectObject(hdcSrc2, hbmSrc2);
SelectObject(hdcDst, hbmDst);
SetStretchBltMode(hdcSrc1, COLORONCOLOR);
SetStretchBltMode(hdcSrc2, COLORONCOLOR);
// Capture a copy of the source area.
if (!StretchBlt(hdcSrc1, 0,0,nWidthDest,nHeightDest,
hdcSrc, nXOriginSrc, nYOriginSrc, nWidthSrc, nHeightSrc,
SRCCOPY))
goto HANDLEERROR;
// Capture a copy of the destination area.
if (!StretchBlt(hdcSrc2, 0,0,nWidthDest,nHeightDest,
hdcDest, nXOriginDest, nYOriginDest, nWidthDest, nHeightDest,
SRCCOPY))
goto HANDLEERROR;
// Blend the two source areas to create the destination image.
bReturn = BlendImages(hbmSrc1, hbmSrc2, hbmDst, dwSourceWeight);
// Clean up objects you do not need any longer.
// You cannot delete an object that's selected into an
// HDC so delete the HDC first.
DeleteDC(hdcSrc1);
DeleteDC(hdcSrc2);
DeleteObject(hbmSrc1);
DeleteObject(hbmSrc2);
// Display the blended (destination) image to the target HDC.
if (bReturn) {
BitBlt(hdcDest, nXOriginDest, nYOriginDest, nWidthDest, nHeightDest,
hdcDst, 0,0, SRCCOPY);
}
// Clean up the rest of the objects you created.
DeleteDC(hdcDst);
DeleteObject(hbmDst);
return bReturn;
HANDLEERROR:
if (hdcSrc1) DeleteDC(hdcSrc1);
if (hdcSrc2) DeleteDC(hdcSrc2);
if (hdcDst) DeleteDC(hdcDst);
if (hbmSrc1) DeleteObject(hbmSrc1);
if (hbmSrc2) DeleteObject(hbmSrc2);
if (hbmDst) DeleteObject(hbmDst);
return FALSE;
}
| Modification Type: | Minor | Last Reviewed: | 4/4/2006 |
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| Keywords: | kbDSWGDI2003Swept kbhowto KB303916 |
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