Programmatically Masking an Image

[YoungBuck]

OK here is a function I've written to create a mask of any image that is currently stored in a Device Context and it works wonderfully besides being slow as hell....

Public Sub MaskTo(ByVal phDC As Long, pTranspColor As Long)
 Dim lX As Long, lY As Long

 For lX = 0 To lngWdth
    For lY = 0 To lngHgt
        
        If GetPixel(lngDC, lX, lY) = pTranspColor Then
            
            SetPixelV phDC, lX, lY, vbBlack
            
        Else
            
            SetPixelV phDC, lX, lY, vbWhite
            
        End If
        
    Next lY
 Next lX
 
End Sub

What I'm wondering is if there is a better way of doing this as this method is slow and tedious, it seems I've seen this done a more efficient ways but I cannot seem to find the examples anywhere, so if anybody can get me on the right track it would be much appreciated. Thanks in advance!

[HarryW]

Ouch, all those Get & SetPixels! It's painful just seeing them... can't you get an array of RGB values or something?

[YoungBuck]

Not quite sure what that would accomplish HarryW, can you elaborate a lil' bit?

[HarryW]

Well I mean it would be much much quicker if you could deal with the image as an array of RGB values, instead of GetPixel and SetPixel all the time. I only use DirecDraw for graphics stuff, and I also only really use C++, so I'm used to having everything in memory and... well.... quicker

[YoungBuck]

Well essentially this MaskTo function should only be called once and draws a mask to a seperate DC offscreen and then all the Blting stuff is done to make the image transparent, so in other words it would seem like a waste of memory to store the entire image as an array of pixels when this array should only be accessed once per image. I'm probably just being too picky but 8 - 10 seconds to make a mask of each image just seems like to much too me.