AlphaBlending

• Download demo project - 16.4 Kb

Introduction

This code snippet shows how to alpha-blend two bitmaps by using the linear interpolation formula.

Background

• Linear interpolation formula:

  Let 0 <= a < 1, RGBblended = a * RGBfirst + (1 - a) * RGBsecond

• Using GetBitmapBits and SetBitmapBits by Amol Kakhandki

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();
}