waiting for a pixel to change

[sdlangers]

Hi All,

 

I was wondering if anyone knew a good way to detect a change in a pixel.

 

i.e. I have code that uses bitblt to capture a pixel on the screen as follows:

 

   Public Function checkPixel(ByVal X As Integer, ByVal Y As Integer, ByVal col() As Color, ByVal errRange As Integer) As Boolean
       Dim imgPixel As Image
       Dim bmpPixel As Bitmap
       Dim gr As Graphics
       Dim dcDest As IntPtr, dcSource As IntPtr
       Dim i As Integer, j As Integer

       ' initialize image
       imgPixel = New Bitmap(1, 1)
       ' get handle to new image
       gr = Graphics.FromImage(imgPixel)
       dcDest = gr.GetHdc()
       ' get handle to desktop
       dcSource = GetWindowDC(GetDesktopWindow())
       ' copy pixel to image
       BitBlt(dcDest, 0, 0, 1, 1, dcSource, X, Y, SRCCOPY)
       ' release handle
       gr.ReleaseHdc(dcDest)

       ' process image
       bmpPixel = New Bitmap(imgPixel)

       ' Check all colors in array
       For i = 0 To UBound(col)
           If isColorEqual(bmpPixel.GetPixel(0, 0), col(i), errRange) Then
               bmpPixel = Nothing
               System.GC.Collect()
               checkPixel = True
               Exit Function
           End If
       Next


       bmpPixel = Nothing
       System.GC.Collect()
       checkPixel = False

   End Function

 

and what i've been doing is simply putting this in a while loop and passing it the color of the pixel im looking for it to change too and waiting for it to return true.

 

It seems there should be a smoother/easier way that would be easier on resources ? :)

 

any thoughts/ideas would be great

 

also any advise on memory management/clean up techniques in vb.net would ge great (i know c# is better for this, but the app is in vb.net)

 

 

thanks

[Volte]

There's no handler to do it, really... I think your way is probably the only decent way to do it (unless you want to try Threading, but I don't think that will be necessary).

 

BTW, those GC.Collect()'s aren't needed. The GC won't collect objects until you are completely done with them, so it won't matter. You need to use the 'Dispose' method of the objects before the garbage collector will touch them (if you don't use it, the GC will pick them up when the object is destroyed).

[sdlangers]

thanks volte force

[sdlangers]

also, the while loop i have calling that basically runs continuously until the pixel changes - i have a delay on the thread in the loop and an application.doevents

 

are there any other good ways/techniques to keep CPU % down?

 

thanks again

[Volte]

When a loop is running continuously, your CPU % will most likely jump to near 100%. There really isn't any way to avoid that. It may not affect performance, but it will kick the CPU usage gauge up.

 

You could make a second thread in your application and put the loop code in that. Then you wouldn't require a DoEvents, so it may be quicker.

[sdlangers]

ok - i think the way i have it should do then

 

on to the next and hopefully final problem..

 

the progam i wrote runs perfectly for about 2-3 hours and the memory and CPU% stay low throughout.

 

but then after a couple of hours, the API calls (getdesktopwindow and/or bitblt) return an

 

'invalid parameter'

 

error or sometimes 'out of memory error'

 

any ideas what could be causing that?

[Volte]

It sounds like you are probably not cleaning up all of your GDI objects. Make sure you call DeleteObject on all objects you create (pens, brushes, etc), and DeleteDC on all the DCs that you create (not the ones you get by way of GetDC or whatever, thought you should call ReleaseDC() on those to release them).

[sdlangers]

thanks for all your help volte force..

 

its basically just that function that i listed above at the start of thread.

 

is there anywhere in that code where i should be using deleteobject or deleteDC ?

 

thanks again

[Volte]

Well, if you are talking strictly about the code you pasted above, it doesn't look like there are any GDI objects being left in memory.

 

You might try calling ReleaseDC on that dcSource object though. I'm not sure if not releasing DCs will cause Out of Memory errors or not.

[divil]

Couldn't you use the GetPixel api instead? That would avoid having to create bitmaps and use bitblt, you could just read the pixel straight from the screen.

[sdlangers]

thanks divil - that sounds like itd be better on resources etc..

 

but now i cant get that working right :(

 

ive a feeling its something to do with the API declarations - the types of the parameters maybe?

 

heres the code...

 

   ' GDI Functions
   Public Declare Function GetPixel Lib "gdi32" Alias "GetPixel" (ByVal hdc As IntPtr, ByVal x As Integer, ByVal y As Integer) As Integer

   ' User32 Functions
   Public Declare Function GetDesktopWindow Lib "user32.dll" Alias "GetDesktopWindow" () As IntPtr
   Public Declare Function GetWindowDC Lib "user32.dll" Alias "GetWindowDC" (ByVal ptr As Int32) As IntPtr
   Public Declare Function ReleaseDC Lib "user32.dll" Alias "ReleaseDC" (ByVal hWnd As IntPtr, ByVal hDc As IntPtr) As IntPtr

   Public Function checkPixel(ByVal X As Integer, ByVal Y As Integer) As Integer
       Dim hDC As IntPtr
       ' Get Desktop hDC
       hDC = GetWindowDC(GetDesktopWindow().ToInt32)
       ' Get Pixel
       checkPixel = GetPixel(hDC, X, Y)
       ' Release Desktop hDC
       ReleaseDC(GetDesktopWindow(), hDC)
   End Function

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       Dim c As New Color()
       Dim i As Integer
       ' get color integer value
       i = checkPixel(100, 50)
       ' convert to color and display
       c = Color.FromArgb(i)
       MsgBox("Color = " & CStr(i) & " - " & c.R & ", " & c.G & ", " & c.B)
   End Sub

 

thanks for yer help guys