Fast update of bitmap from polar coordinate data

[boops boops]

Hi chriscross,

Unfortunately I was mistaken about the XNA example. I found the demo that Jenner made (it was from this thread, post#10) but unfortunately the link is now dead. I dug out the code but I think it will take a lot of work to convert it to something you can use; there is too much I don't know about XNA.

Still, I made my own attempt at a "radar" screen which you might find interesting for comparison. With an angular resolution of 1 degree it takes me about 3-4 seconds to do a full 360 degree sweep. It uses random data for each echo line.

It seems that most of the time goes into updating the bitmap with the new line (not the rendering after all). I almost doubled the performance up by drawing 2 lines in each pass, and the speed could be further improved by increasing that to 3 or more.

I widened the line bitmap to 3 pixels to provide a kind of anti-aliasing. (The middle line has the data at full opacity, and the first and third lines the same data partly transparent). It seems this costs little extra time and the resulting scan is much smoother.

If you want to try it you can just copy the code below into a default form. There's no need to do anything in the designer. You can resize the form to enlarge the scan. The timing results are shown in the Output window.

Public Class Form1

	Private WithEvents tim As New Timer With {.Interval = 15}
	Private WithEvents pictureBox1 As New PictureBox
	Private sw As Stopwatch

	Private echo1() As Integer 'echo data array 1
	Private echo2() As Integer 'echo data array 2
	Private echoResolution As Integer	'was "scale"
	Private angleResolution As Single = 1
	Private rnd As New Random
	Private scanBitmap As Bitmap 'was "bm"
	Private echoLine1 As Bitmap
	Private echoLine2 As Bitmap
	Private angle As Single

	Private refreshInterval As Integer = 1
	Private fadeInterval As Integer = 4
	Private refreshCount As Integer
	Private fadeCount As Integer

	'setup in the load event:
	Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
		Me.ClientSize = New Size(600, 600)
		PictureBox1.BackColor = Color.FromArgb(0, 50, 20)
		pictureBox1.Dock = DockStyle.Fill
		Me.Controls.Add(pictureBox1)
		tim.Start()
	End Sub

	'some setup in the SizeChanged event, to allow user resizing
	Private Sub PictureBox1_SizeChanged(sender As Object, e As System.EventArgs) Handles pictureBox1.SizeChanged
		scanBitmap = New Bitmap(pictureBox1.Width, pictureBox1.Height)
		echoResolution = pictureBox1.Width \ 2 - 5
		ReDim echo1(echoResolution - 1)
		ReDim echo2(echoResolution - 1)
		echoLine1 = New Bitmap(echoResolution, 3)
		echoLine2 = New Bitmap(echoResolution, 3)
	End Sub

	Private Sub PictureBox1_Paint(sender As Object, e As System.Windows.Forms.PaintEventArgs) Handles pictureBox1.Paint

		'draw a sweep indicator line
		e.Graphics.SmoothingMode = Drawing2D.SmoothingMode.AntiAlias
		Dim centre As New Point(pictureBox1.Width \ 2, pictureBox1.Height \ 2)
		Using mtx As New Drawing2D.Matrix
			mtx.RotateAt(angle + 0.2F, centre)
			e.Graphics.Transform = mtx
			Using pn As New Pen(Color.LightGreen, 1)
				e.Graphics.DrawLine(pn, centre, New Point(pictureBox1.Width - 5, centre.Y))
			End Using
		End Using

	End Sub

	Private Sub Timer1_Tick(sender As Object, e As System.EventArgs) Handles tim.Tick
		'update the bitmap and depending on the refreshinterval, put it in the picture box
		UpdateScanBitmap()
		refreshCount = (refreshCount + 1) Mod refreshInterval
		If refreshCount = 0 Then pictureBox1.Image = scanBitmap
		fadeCount = (fadeCount + 1) Mod fadeInterval
		If fadeCount = 0 Then scanBitmap = FadeImage(scanBitmap, 98)
	End Sub

	'Update the scan:
	Private Sub UpdateScanBitmap()
		If sw Is Nothing Then sw = Stopwatch.StartNew

		'create a set of simulated data for the echo line:
		For d As Integer = 0 To echo1.Count - 1
			echo1(d) = rnd.Next(0, 255)
			echo2(d) = rnd.Next(0, 255)
		Next

		'conver the data to a line bitmap:
		echoLine1 = GetEchoBitmap(echo1, echoLine1)
		echoLine2 = GetEchoBitmap(echo2, echoLine2)

		'draw the echo lines on the scan
		Using g As Graphics = Graphics.FromImage(scanBitmap)
			Dim centre As New Point(pictureBox1.Width \ 2, pictureBox1.Height \ 2)
			Using mtx As New Drawing2D.Matrix
				mtx.RotateAt(angle, centre)	'
				g.Transform = mtx 'rotate the Graphics
				g.DrawImageUnscaled(echoLine1, centre.X, centre.Y - 1)
				mtx.RotateAt(angleResolution, centre)
				g.Transform = mtx
				g.DrawImageUnscaled(echoLine2, centre.X, centre.Y - 1)
			End Using
		End Using

		'update the angle 
		angle = (angle + 2 * angleResolution) Mod 360.0F

		'provide feedback in the Output window:
		If angle < angleResolution Then
			angle = angle Mod 360.0F
			Console.WriteLine("360 deg. sweep time = " & ((sw.ElapsedMilliseconds / 1000).ToString("N4")))
			sw = Stopwatch.StartNew
		End If

	End Sub

	Private Function GetEchoBitmap(echo As Integer(), echoLine As Bitmap) As Bitmap
		Using fp As New FastPix(echoLine)
			Dim pixels() As Integer = fp.PixelArray
			For radius As Integer = 0 To echoResolution - 1
				Dim data As Integer = echo(radius) * 256
				pixels(radius) = &HAF000000 + data
				pixels(radius + echoResolution) = &HFF000000 + data
				pixels(radius + 2 * echoResolution) = &HAF000000 + data
			Next
		End Using
		Return echoLine
	End Function

	Private Function FadeImage(bmp As Bitmap, opacityPercent As Double) As Bitmap
		'set the opacity of an image:
		If opacityPercent >= 0 AndAlso opacityPercent <= 100 Then
			Dim alphaRatio = opacityPercent / 100
			Using fp As New FastPix(bmp)
				Dim pixels() = fp.PixelArray
				For i As Integer = 0 To pixels.Count - 1
					Dim pixel = pixels(i)
					Dim alpha As Integer = (pixel >> 24) And &HFF
					alpha = CInt(alphaRatio * alpha) << 24
					pixels(i) = pixel And &HFFFFFF Or alpha
				Next
			End Using
			Return bmp
		Else
			Console.WriteLine("FadeImage: opacityPercent must be in the range 0-100.")
			Return Nothing
		End If
	End Function

End Class

BB