C++: 2D Alpha Blend Q:

[Halsafar]

Alright I wrote a very basic AlphaBlending technique using Dx7 Surfaces and some Win32API: GetPixel/SetPixel.

Check it out:

//-----------------
//Alpha Blend -- DC Basic
//-----------------
void DDSurface7::AlphaBlendDC(LPDIRECTDRAWSURFACE7 &Destination, USINT AlphaValue)
{
    COLORREF SrcRGB, DestRGB;
    BYTE r,g,b;
    HDC hdcDestination=0, hdcAlpha=0;

    while (AlphaValue > 255) AlphaValue -= 255;

    Destination->GetDC(&hdcDestination);
    m_AlphaSurface->GetDC(&hdcAlpha);

    for (USINT y = 0; y < m_Height; y++)
    {
        for (USINT x = 0; x < m_Width; x++)
        {
            SrcRGB = GetDCPixel(x,y);
            DestRGB = GetPixel(hdcDestination, x,y);

            r = (AlphaValue * ( GetRValue(SrcRGB) + 256 - GetRValue(DestRGB) )) / 256 + GetRValue(DestRGB) - AlphaValue;
            g = (AlphaValue * ( GetGValue(SrcRGB) + 256 - GetGValue(DestRGB) )) / 256 + GetGValue(DestRGB) - AlphaValue;
            b = (AlphaValue * ( GetBValue(SrcRGB) + 256 - GetBValue(DestRGB) )) / 256 + GetBValue(DestRGB) - AlphaValue;
            
            SetPixel( hdcAlpha,x,y,RGB(r,g,b) );
        }
    }

    Destination->ReleaseDC(hdcDestination);
    m_AlphaSurface->ReleaseDC(hdcAlpha);
} 

All I did was get that formula for calculating the finalpixel value...from one of ElectroMans posts on the subject.

Now what this does essentially is:
Loops through row by row.
Gets the pixel from the surface in the class and the destination surface passed.
Calculates the proper alpha value.
Sets the pixel in a new surface which is then bltfast to the screen later.


My problem is:
I know 2D Alpha is naturally slow, but jeez, doing it every frame takes at least 10 seconds a frame on my comp for a picture the size of the screen.
This is super slow...
It works...works fine. I get a nice alpha blend for translucency.

All I really need it for is screen fading...Nothing more.