GetPixel optimization - weird results [help]

[abuckau907]

So, my program uses a few windows api's to read pixel colors from the monitor. I made a function to get the color of 1 pixel, and to get an area defined by a rectangle. Accidently I re-wrote the 'get an area' code, both versions worked..1 is obviously better, or so I thought.

    Public Function modPublic.GetPixelColor(ByVal x As Int32, ByVal y As Int32) As Color
        Dim hWnd As IntPtr
        Dim hDC As IntPtr
        hWnd = GetDesktopWindow()
        hDC = GetWindowDC(hWnd)

        Dim lColor As Int32 = GetPixel(hDC, x, y)
        ReleaseDC(hWnd, hDC)

        Return ColorTranslator.FromWin32(lColor)
    End Function

So, that's the code to get 1 specific pixel --> and when I wrote the first version of 'GetRectArea' ..I just called GetPixelColor() a bunch of times --> it works, but it makes LOTS of calls to GetDesktopWindow and GetWindowDC + ReleaseDC.

Here is the bad version of the 'getRegion' func.

    Public Shared Function GetScreenArea(ByVal topLeft As Point, ByVal bottomRight As Point) As Bitmap
        Dim boxWidth As Int32 = bottomRight.X - topLeft.X ' no error checkecking: assumes br.x is > tl.x && br.y is > tl.y
        Dim boxHeight As Int32 = bottomRight.Y - topLeft.Y

        Dim rtnBmp As New Bitmap(boxWidth, boxHeight)
        ''scans vertically: top-to-bottom, left-to-right

        For column As Int32 = 0 To boxWidth - 1
            For row As Int32 = 0 To boxHeight - 1
                rtnBmp.SetPixel(column, row, modPublic.GetPixelColor(topLeft.X + column, topLeft.Y + row))
            Next
        Next

        Return rtnBmp
    End Function

again..it works, but modPublic.GetPixColor keeps calling the api's when we don't need to (This is because GetArea is functionally different than GetPixel) so..I re-wrote GetArea w/ a..I hate to re-write code, but it's essentially the GetPixel code, w/ a loop - but it doesn't release the ptrs (and therefor doesn't have to call the api's over and over again) until it's done.

    Public Function GetPixelArea(ByVal tl As Point, ByVal br As Point) As Bitmap
        Dim hWnd As IntPtr
        Dim hDC As IntPtr
        hWnd = GetDesktopWindow
        hDC = GetWindowDC(hWnd)
        Dim tmpColor As Color
        '''''''''''''''''''''''''''''''''''''
        Dim areaWidth As Int32 = br.X - tl.X
        Dim areaHeight As Int32 = br.Y - tl.Y
        Dim rtnBtmp As New Bitmap(areaWidth, areaHeight)
        'scans top-to-bottom, left-to-right
        For xx As Int32 = 0 To areaWidth - 1
            For yy As Int32 = 0 To areaHeight - 1
                tmpColor = ColorTranslator.FromWin32(GetPixel(hDC, tl.X + xx, tl.Y + yy))
                rtnBtmp.SetPixel(xx, yy, tmpColor)
            Next
        Next
        ReleaseDC(hWnd, hDC)
        Return rtnBtmp
    End Function

If you had to pick a function..you'd think GetPixelArea would be faster than GetScreenArea ..but it's not :S I don't understand..it's virtually the exact same code...except the 'slower' version makes less api calls :S so..this truly confuses me..any advice would be greatly appreciated.


I added some timing code to see how much slower/faster they are:

1:57:43 PM->modPublic.ImageScan: 390ms 'Should only call GetWindow etc 1x
1:57:57 PM->wowGUI.GetScreenArea : 265ms 'Should call GetWindow etc proportional to img size: 300,300 = 90,000x!

2:00:55 PM->modPublic.ImageScan: 328ms
2:01:06 PM->wowGUI.GetScreenArea : 500ms

2:02:18 PM->modPublic.ImageScan: 656ms
2:02:28 PM->wowGUI.GetScreenArea : 406ms

I'm guessing they change so much because I'm only on a Pentium4 2.7 w/ 1gb ram - wow,msvs,movie running..so ram is kinda tight :P
Eithor way GetScreenArea is on average a little faster than GetPixelArea -- i would have NEVER expected that..what could be causing?