Changing the pixel color in a VB.net form? - vb.net

Question

How would I change the colour of individual pixels in a VB.NET form?
Thanks.

Answer 1

A hard requirement for Winforms is that you should be able to redraw the form whenever Windows asks it to. Which will happen when you minimize and restore the window. Or on older versions of Windows when you move another window across yours.
So just setting pixels on the window isn't good enough, you are going to lose them all when the window redraws. Instead use a bitmap. An additional burden is that you are going to have to keep the user interface responsive so you need to do your calculations on a worker thread. The BackgroundWorker is handy to get that right.
One way to do this is to use two bitmaps, one you fill in the worker and another that you display. Every, say, one row of pixels make a copy of the in-work bitmap and pass that to ReportProgress(). Your ProgressChanged event then disposes the old bitmap and stores the new passed one and calls Invalidate to force a repaint.

Answer 2

You might benefit from these resources: Setting background color of a form
DeveloperFusion forum , and extracting pixel color

Answer 3

Here's some demo code. It's slow to repaint, for the reasons Hans mentioned. A simple way to speed it up would be to only recalculate the bitmap after a delay.
Public Class Form1
Private Sub Form1_Paint(sender As Object, e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
'create new bitmap
If Me.ClientRectangle.Width <= 0 Then Exit Sub
If Me.ClientRectangle.Height <= 0 Then Exit Sub
Using bmpNew As New Bitmap(Me.ClientRectangle.Width, Me.ClientRectangle.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb)
'draw some coloured pixels
Using g As Graphics = Graphics.FromImage(bmpNew)
For x As Integer = 0 To bmpNew.Width - 1
For y As Integer = 0 To bmpNew.Height - 1
Dim intR As Integer = CInt(255 * (x / (bmpNew.Width - 1)))
Dim intG As Integer = CInt(255 * (y / (bmpNew.Height - 1)))
Dim intB As Integer = CInt(255 * ((x + y) / (bmpNew.Width + bmpNew.Height - 2)))
Using penNew As New Pen(Color.FromArgb(255, intR, intG, intB))
'NOTE: when the form resizes, only the new section is painted, according to e.ClipRectangle.
g.DrawRectangle(penNew, New Rectangle(New Point(x, y), New Size(1, 1)))
End Using
Next y
Next x
End Using
e.Graphics.DrawImage(bmpNew, New Point(0, 0))
End Using
End Sub
Private Sub Form1_ResizeEnd(sender As Object, e As System.EventArgs) Handles Me.ResizeEnd
Me.Invalidate() 'NOTE: when form resizes, only the new section is painted, according to e.ClipRectangle in Form1_Paint(). We invalidate the whole form here to form an entire form repaint, since we are calculating the colour of the pixel from the size of the form. Try commenting out this line to see the difference.
End Sub
End Class