Is the behaviour of DrawLine predictable with out of range parameters?

Question

Can DrawLine handle coordinates outside the defined area?

For example myGraphics.DrawLine(MyPen, -20, -80, 20, 90);

I would expect this to produce a line correctly as though it had used an infinite canvas but plotting only the section within my graphic.

My code is as follows. I am plotting movement from coordinates recorded in a database. Occasionally the subject moves further than expected, beyond the edges of my bitmap. I do not check for this occurrence as I was relying on DrawLine to handle it.

Bitmap Border = new Bitmap(5000, 5000);
Border.SetResolution(254, 254);

Graphics MyGraphics= Graphics.FromImage(Border);
Pen MyPen = new Pen(Color.Black, 1);


for (Int32 Point = 1; Point <= Points; Point++)
{
   XCoord2 = XCoord1;
   YCoord2 = YCoord1;
   XCoord1 = *READ FROM DATABASE*
   YCoord1 = *READ FROM DATABASE*

   if (Point > 1)
   {
        MyGraphics.DrawLine(MyPen, XCoord1, YCoord1, XCoord2, YCoord2);
   }
}

In reality, my plots work most of the time. However I do get an occasional graphic with missing lines or with an obscure line originating from a strange coordinate.

In summary, should the behaviour of DrawLine predictable with unusual parameters. Should I introduce some trigonometry to force the plots to always be within my grid?

Unless you are using very large coordinates (which could cause overflow or rounding issues) you should be OK I think. GDI has all sorts of viewport and window support which means handling this stuff should be a doddle for the implementers. I've never had any problems with the clipping. — Matthew Watson, Mar 28 '13 at 21:43
@AshBurlaczenko As the question says "my plots work most of the time". I have spent hours trying to track down why I have an occasional failure. When I scan through my 1000-2000 coordinates, nothing seems unusual, which is why I am now suspecting DrawLine. I came here in the hope that someone might confirm that. — MortimerCat, Mar 28 '13 at 21:59
All GDI+ drawing is done internally with *float*. Rounding errors are common. — Hans Passant, Mar 28 '13 at 23:56

score 0 · Answer 1 · edited May 23 '17 at 12:21

The actual limits are a billion positive or negative see this past question (that used .net )

What are the hard bounds for drawing coordinates in GDI+?

My guess is that your database pulls are wrong, this can happen if you are using a string data storage and forcing that to be parsed.

Add a thread.sleep() and have it Debug.WriteLine the new pulls (or just breakpoint things), likely a value is getting in there that is either odd or getting parsed oddly

score 0 · Accepted Answer · answered Mar 29 '13 at 13:46

After more experimentation, I finally cured my problem with...

SolidBrush WhiteBrush = new SolidBrush(Color.White);
myGraphics.FillRectangle(WhiteBrush,0,0,5000,5000);

i.e. I gave my graphics a solid white background before I drew any lines. Before I was drawing a black line on a NULL background. I have no idea why this would affect anything, but it did!

Is the behaviour of DrawLine predictable with out of range parameters?

2 Answers2