Why is this code so slow??!?

[mickn66]

I have a program that is supposed to take a 300 by 200 pixel image and examine it one 5 by 5 block at a time. However, it is extremely slow. The following is the code that does the looping ('spotsize' is 5):

 

Dim x, y As Integer

For y = 0 To pct1.Height - spotsize Step spotsize

For x = 0 To pct1.Width - spotsize Step spotsize

spot.X = x

spot.Y = y

DrawSpot()

Next x

Next y

MsgBox("done")

 

"DrawSpot" is the function that analyzes each spot and that's where the trouble is, because without it, the above code takes about 1 second to run. Drawspot does 3 things. It draws a red rectangle around the spot in pct1, it draws the spot onto pct2, which is 50x50 and set to strectchimage mode, thereby showing a close up of spot, and it analyzes the brightness of spot (not in this order).

 

Here is the code for DrawSpot():

 

Private Sub DrawSpot()

 

Dim biggy As New Rectangle(0, 0, 50, 50) ' rectangle for pct2

Dim g2 As Graphics = pct2.CreateGraphics ' to draw on pct2

Dim bitty As New Bitmap(pct1.Image) ' source image of pct1

Dim g As Graphics = Graphics.FromHwnd(pct1.Handle) ' to draw on pct1

 

 

g2.DrawImage(bitty, biggy, spot, GraphicsUnit.Pixel)

 

'calculate brightness of spot before drawing rectangle

Dim x, y As Integer

Dim aColor As Color

Dim total As Double = 0

Dim counter As Integer = 0

For x = spot.Left To spot.Right

For y = spot.Top To spot.Bottom

aColor = bitty.GetPixel(x, y)

total = total + aColor.GetBrightness

counter = counter + 1

Next y

Next x

lblBrightness.Text = CStr(total / counter)

 

'draw the rectangle on Spot on pct1

Dim myPen As New Pen(Color:=Color.Red, Width:=2)

g.DrawRectangle(myPen, spot)

End Sub

 

I don't understand why it slows it down as much as it does. Have I done something horribly stupid? Is there a faster way? Thanks!

 

Mick

[aewarnick]

Either use global variables for the variables that never change

OR

Don't use DrawSpot and put all the code within the loop, declaring the variables that never change before the loop.

 

You are creating new variables each loop and that will really slow the program down. Especially when it is a Bitmap:

 

Dim biggy As New Rectangle(0, 0, 50, 50) ' rectangle for pct2

Dim g2 As Graphics = pct2.CreateGraphics ' to draw on pct2

Dim bitty As New Bitmap(pct1.Image) ' source image of pct1

Dim g As Graphics = Graphics.FromHwnd(pct1.Handle) ' to draw on pct1

[mickn66]

Thank you. I was just beginning to suspect it was all the dim statements, since I've commented out almost everything else and it's still so slow! I'm going to try global variables first.

[mutant]

If you want maximum speed you can get drawing then you would need to use the paint event of the object you want to draw on and use the Graphics object thats passed into the event in PaintEventArgs, and then set the style for it: Double Buffering, All Painting in WM Paint, and user Paint.

[mickn66]

I'm not sure I understand. What do you mean I can "get drawing"? And "Double Buffering, All Painting in WM Paint, and user Paint" - This sounds like something I should know by now :) But I don't.

 

I sure would like maximum speed though! Any more information you can give me would be appreciated. Thanks

[aewarnick]

He is talking about putting your code in Form.OnPaint method and Double Buffing the Form so that there is no flicker.

 

Put the equivalent of this C# code in your Form's constructor:

this.SetStyle(ControlStyles.DoubleBuffer, true);
this.SetStyle(ControlStyles.UserPaint, true);
this.SetStyle(ControlStyles.AllPaintingInWmPaint, true);

 

To call OnPaint call, Invalidate()

In Invalidate you can draw only the pixel that will be changed instead of drawing the whole form over again by sending a Rectangle to Invalidate of the area that will be changed. It will be much much faster that way.