Why does GetPixel stop working in my program?

Question

Hi All
I have a strange problem. I am using GetPixel to perform a screen scrape which takes about half an hour to get all the data I need once a week. At least it WOULD take half an hour if it ever finished! It always crashes after about 20 minutes, but I can then
restart from the point of the crash and it completes fine.
Here is my code:
Do
lColor = GetPixel(DC, x, y)
If lColor >= 0 Then Exit Do
DC = GetDC(0)
Loop
I know that GetPixel / DC can sometimes drop out and return -1 hence the above loop, but it seems that after about 8 million calls (about 20 minutes in my program) it just starts returning zero every single time. This causes an exception elsewhere in my program
but the underlying problem is GetPixel returning zero. I have watched this happen on several occasions now and the pixels being interrogated are most definitely not black and the screensaver has not come on ar anything like that.
Does anybody have any ideas why this might be happening and more importantly, how I can fix it?
Many thanks
Rich

Answer 1

Can I ask you if you are getting more than one pixel -at a time-? If you are, then I have some good news for you, you can do this considerably faster if you were to use BitMap.Lockbits. The reason being that each time you call the GetPixel method, internally
lockbits gets called to first lock the resource, and then again to unlock it, all for a single GetPixel operation. Implementing your own routine to gather pixels can be considerably faster, but admittedly more difficult. Below the following example will show
you how to get large collections of pixels with a single call to Bitmap.Lock, and Bitmap.Unlock, which will DRASTICALLY increase your overall performance. Please be advised that this example only supports 32 bit per pixel images, and if you wish to add support,
you would have to first understand how this works.
Example:
Option Strict On
Option Explicit On
Option Infer Off
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
PictureBox1.Width = 101
PictureBox1.Height = 101
Dim bmp As New Bitmap(PictureBox1.Width, PictureBox1.Height)
Dim g As Graphics = Graphics.FromImage(bmp)
Dim rTopLeft As New Rectangle(0, 0, 50, 50)
Dim rTopRight As New Rectangle(50, 0, 50, 50)
Dim rBottomLeft As New Rectangle(0, 50, 50, 50)
Dim rBottomRight As New Rectangle(50, 50, 50, 50)
g.FillRectangle(Brushes.Red, rTopLeft)
g.FillRectangle(Brushes.Blue, rTopRight)
g.FillRectangle(Brushes.Yellow, rBottomLeft)
g.FillRectangle(Brushes.Green, rBottomRight)
PictureBox1.Image = bmp
Dim getPixel1 As New Point(0, 0)
Dim getPixel2 As New Point(50, 0)
Dim getPixel3 As New Point(0, 50)
Dim getPixel4 As New Point(50, 50)
Dim pixelsToGet As New List(Of Point)
pixelsToGet.Add(getPixel1)
pixelsToGet.Add(getPixel2)
pixelsToGet.Add(getPixel3)
pixelsToGet.Add(getPixel4)
Dim pixels As List(Of Pixel) = GetPixels(pixelsToGet, bmp)
pbPix1.BackColor = pixels(0).Color
pbPix2.BackColor = pixels(1).Color
pbPix3.BackColor = pixels(2).Color
pbPix4.BackColor = pixels(3).Color
End Sub
Function GetPixels(pixelsToGet As List(Of Point), source As Bitmap) As List(Of Pixel)
Dim results As New List(Of Pixel)
Dim sourceRect As New Rectangle(0, 0, source.Width - 1, source.Height - 1)
Dim bmd As System.Drawing.Imaging.BitmapData = _
source.LockBits(sourceRect, Imaging.ImageLockMode.ReadOnly, source.PixelFormat)
Dim ptr As IntPtr = bmd.Scan0
Dim bytes As Integer = Math.Abs(bmd.Stride) * bmd.Height
Dim allPixelBytes(bytes - 1) As Byte
System.Runtime.InteropServices.Marshal.Copy(ptr, allPixelBytes, 0, bytes)
Dim bytesPerPixel As Integer = 0
Select Case bmd.PixelFormat
Case Imaging.PixelFormat.Format32bppArgb
bytesPerPixel = 4
Case Else
Throw New NotImplementedException("Currently only 32bits per pixel is supported!")
End Select
For currentIndex As Integer = 0 To allPixelBytes.Count - 1 Step bytesPerPixel
Select Case bmd.PixelFormat
Case Imaging.PixelFormat.Format32bppArgb
For Each Loc As Point In pixelsToGet
If PixelLocationToIndex(Loc, bmd.PixelFormat, source.Width) = currentIndex Then
Dim blueComponent As Byte = allPixelBytes(currentIndex)
Dim greenComponent As Byte = allPixelBytes(currentIndex + 1)
Dim redComponent As Byte = allPixelBytes(currentIndex + 2)
Dim alphaComponent As Byte = allPixelBytes(currentIndex + 3)
Dim pixelColor As Color = Color.FromArgb(alphaComponent, redComponent, greenComponent, blueComponent)
Dim pixel As New Pixel(Loc, pixelColor)
results.Add(pixel)
End If
Next
Case Else
Throw New NotImplementedException("Currently only 32bits per pixel is supported!")
End Select
Next
source.UnlockBits(bmd)
Return results
End Function
Public Function PixelLocationToIndex(loc As Point, pixelFormat As System.Drawing.Imaging.PixelFormat, bitmapWidth As Integer) As Integer
Dim bytesPerPixel As Integer = 0
Select Case pixelFormat
Case Imaging.PixelFormat.Format32bppArgb : bytesPerPixel = 4
Case Else : Throw New NotImplementedException("Currently only 32bits per pixel is supported!")
End Select
Dim bytesPerRowOfPixel As Integer = bytesPerPixel * bitmapWidth
Dim yPixels As Integer = bytesPerRowOfPixel * loc.Y
Dim xPixels As Integer = bytesPerPixel * loc.X
Return xPixels + yPixels
End Function
Class Pixel
Public Property Location As Point
Public Property Color As Color
Sub New(location As Point, color As Color)
Me.Location = location
Me.Color = color
End Sub
End Class
End Class
“If you want something you've never had, you need to do something you've never done.”
Don't forget to mark
helpful posts and answers
! Answer an interesting question? Write a
new article
about it! My Articles
*This post does not reflect the opinion of Microsoft, or its employees.