Fast "for each pixel in bitmap" loop - vb.net

Question

I'm writing a Visual Basic application that takes a screenshot of the desktop and crops it down to a 200px by 200px image around the center of the screen. One part of the application would iterate through each pixel and check if the RGB of that pixel is a certain color (this is meant to take under a second for it to be efficient), and unfortunately Bitmap.Getpixel is not doing me any good whether or not It's being loaded into the memory via Bitmap.Lock or not.
Is there a faster (almost instantaneous) way of doing so? Thanks.

Answer 1

Sure there is. Typically what you do is :
for each pixel
Get device contex
Read Pixel
Release device contex (unless you want memory leak)
For this to work you need few external windows library calls, ex :
[DllImport("user32.dll")]
static extern IntPtr GetDC(IntPtr hwnd);
[DllImport("user32.dll")]
static extern Int32 ReleaseDC(IntPtr hwnd, IntPtr hdc);
[DllImport("gdi32.dll")]
static extern uint GetPixel(IntPtr hdc, int nXPos, int nYPos);
static public System.Drawing.Color getPixelColor(int x, int y) {
IntPtr hdc = GetDC(IntPtr.Zero);
uint pixel = GetPixel(hdc, x, y);
ReleaseDC(IntPtr.Zero, hdc);
Color color = Color.FromArgb((int)(pixel & 0x000000FF),
(int)(pixel & 0x0000FF00) >> 8,
(int)(pixel & 0x00FF0000) >> 16);
return color;
}
It would be much better to
GetDC
for each pixel
read pixel and store value
ReleaseDC
However I have found that get pixel method itself is slow. Therefore to get better performance just grab the entire screen into a bitmap and get the pixels from there.
Here is some sample code in c#, you can convert it in VB.net if you want using online converters:
var maxX=200;
var maxY=200;
var screensize = Screen.PrimaryScreen.Bounds;
var xCenterSub100 = (screensize.X-maxX)/2;
var yCenterSub100 = (screensize.Y-maxY)/2;
Bitmap hc = new Bitmap(maxX, maxY);
using (Graphics gf = Graphics.FromImage(hc)){
gf.CopyFromScreen(xCenterSub100, yCenterSub100, 0, 0, new Size(maxX, maxY), CopyPixelOperation.SourceCopy);
//...
for (int x = 0; x < maxX; x++){
for (int y = 0; y < maxY; y++){
var pColor = hc.GetPixel(x, y);
//do something with the color...
}
}
}
In Vb.net (using http://converter.telerik.com/) :
Dim maxX = 200
Dim maxY = 200
Dim screensize = Screen.PrimaryScreen.Bounds
Dim xCenterSub100 = (screensize.X - maxX) / 2
Dim yCenterSub100 = (screensize.Y - maxY) / 2
Dim hc As New Bitmap(maxX, maxY)
Using gf As Graphics = Graphics.FromImage(hc)
gf.CopyFromScreen(xCenterSub100, yCenterSub100, 0, 0, New Size(maxX, maxY), CopyPixelOperation.SourceCopy)
'...
For x As Integer = 0 To maxX - 1
For y As Integer = 0 To maxY - 1
Dim pColor = hc.GetPixel(x, y)
'do something with the color...
Next
Next
End Using
With c# on my old computer i got around 30 fps, run time is about 35ms. There are faster ways, but they start to abuse several things to get that speed. Note that you do not use the getPixelColor, it is here just for reference. You instead use the screen scraped image method.

Answer 2

If you don't wish to resort to p/invoke, you can use the LockBits method. This code sets each component of a 200 x 200 area at the center of a bitmap in a PictureBox to a random value. It runs in about 100 milliseconds (not counting the refresh of the PictureBox).
EDIT: I realized you were trying to read pixels, so I added a line to show how to do that.
Private Sub DoGraphics()
Dim x As Integer
Dim y As Integer
'PixelSize is 4 bytes for a 32bpp Argb image.
'Change this value appropriately
Dim PixelSize As Integer = 4
Dim rnd As New Random()
'This code uses a bitmap that is loaded in a picture box.
'Any bitmap should work.
Dim bm As Bitmap = Me.PictureBox1.Image
'lock an area of the bitmap for editing that is 200 x 200 pixels in the center.
Dim bmd As BitmapData = bm.LockBits(New Rectangle((bm.Width - 200) / 2, (bm.Height - 200) / 2, 200, 200), System.Drawing.Imaging.ImageLockMode.ReadOnly, bm.PixelFormat)
'loop through the locked area of the bitmap.
For y = 0 To bmd.Height - 1
For x = 0 To bmd.Width - 1
'Get the various pixel locations This calculation is for a 32bpp Argb bitmap
Dim blue As Integer = (bmd.Stride * y) + (PixelSize * x)
Dim green As Integer = blue + 1
Dim red As Integer = green + 1
Dim alpha As Integer = red + 1
'Set each component of the pixel to a random rgb value.
'There are 4 bytes that make up each pixel (32bpp Argb)
Marshal.WriteByte(bmd.Scan0, red, CByte(rnd.Next(0, 256)))
Marshal.WriteByte(bmd.Scan0, blue, CByte(rnd.Next(0, 256)))
Marshal.WriteByte(bmd.Scan0, green, CByte(rnd.Next(0, 256)))
Marshal.WriteByte(bmd.Scan0, alpha, 255)
'Use the ReadInt32() method to read back the entire pixel
Dim intColor As Integer = Marshal.ReadInt32(bmd.Scan0)
If intColor = Color.Blue.ToArgb() Then
'The pixel is blue
Else
'The pixel is not blue
End If
Next
Next
'Important!
bm.UnlockBits(bmd)
Me.PictureBox1.Refresh()
End Sub