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();
}

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here