Introduction
This code snippet shows how to alpha-blend two bitmaps by using the linear interpolation formula.
Background
Using the code
Here is a function which can be used in OnPaint()
or OnDraw()
. Actually, in the sample project, I didn't prepare a function, I put the code directly in the OnPaint()
method. In this function, the new blended bitmap is placed on a second bitmap. You can implement the code and create a third bitmap for the new bitmap if you need.
The second parameter, double blend
, must be in [0,1). This number is for the first bitmap, and the second number is 1-blend
for the second bitmap.
void CAlphaBlendingDlg::AlphaBlend(CDC *pDC, double blend)
{
BITMAP bmpX,bmpY;
CBitmap bmp1,bmp2;
bmp2.LoadBitmap(IDB_BITMAP2);
bmp1.LoadBitmap(IDB_BITMAP1);
bmp1.GetBitmap(&bmpX);
UINT* bmpBuffer=(UINT*) GlobalAlloc(GPTR,
bmpX.bmWidthBytes * bmpX.bmHeight);
bmp1.GetBitmapBits(bmpX.bmWidthBytes * bmpX.bmHeight, bmpBuffer);
bmp2.GetBitmap(&bmpY);
UINT* bmpBuffer2=(UINT*) GlobalAlloc(GPTR,
bmpY.bmWidthBytes * bmpY.bmHeight);
bmp2.GetBitmapBits(bmpY.bmWidthBytes * bmpY.bmHeight, bmpBuffer2);
int nSize = bmpY.bmWidth * bmpY.bmHeight;
for (int i = 0; i < nSize; i++)
{
int abR = (int) (GetR(bmpBuffer[i]) * blend +
(1-blend) * GetR(bmpBuffer2[i]));
int abG = (int) (GetG(bmpBuffer[i]) * blend +
(1-blend) * GetG(bmpBuffer2[i]));
int abB = (int) (GetB(bmpBuffer[i]) * blend +
(1-blend) * GetB(bmpBuffer2[i]));
bmpBuffer2[i] = RGB(abB, abG, abR);
}
bmp2.SetBitmapBits(bmpX.bmWidthBytes * bmpX.bmHeight, bmpBuffer2);
CDC memdc;
memdc.CreateCompatibleDC(pDC);
memdc.SelectObject(bmp2);
pDC->BitBlt(10,10,bmpX.bmWidthBytes,
bmpX.bmHeight, &memdc, 0, 0, SRCCOPY);
GlobalFree((HGLOBAL)bmpBuffer);
GlobalFree((HGLOBAL)bmpBuffer2);
memdc.DeleteDC();
}