C# intersects problem

SoapSurgeon · 23 Mar 2006 at 14:55

Hi,

I have two rectangles with coords as follows:

Rectangle 1:
Top left = (10, 0 )
bottom Right = (621, -457)

Rectangle 2:
Top left = (0, 0)
bottom right = (1024, -1024)

now that appears to me as if they are intersecting, however, when i do the intersectsWith method of either rectangle, passing in the other rectangle as a parameter it returns false.

Am i doing something wrong or is it a bug?

UPDATE: It seems to be a problem with having a minus height for my rectangles, is there any way round this?

happytechie · 23 Mar 2006 at 15:19

I would have said that rectangle 1 is completely enclosed by rect2?

have you tried moving the origin so that all the x,y coords are positive?

HT

Lagz · 23 Mar 2006 at 15:20

Its very unlikely to be a bug, the libraries are excellent.

Are you sure you are creating the rectangles properly? Maybe you are thinking the constructor is
public Rectangle ( int x1, int y1, int x2, int y2)
when infact it is
public Rectangle ( int x, int y, int width, int height )
.
I would guess that width and height always have to be positive and that x and y must specify the top left hand corner of the rectangle. Only a guess though.

Haircut · 23 Mar 2006 at 15:35

If it is a problem with having negative co-ordinates in there then you could try applying a matrix transform to the graphics object to generate a co-ordinate system where you can use positive values for the points of your rectangles?

Haircut · 23 Mar 2006 at 18:29

OK, I'm home from work now so I fired up Reflector and it turns out this is the code for the IntersectsWith method

Code:

public bool IntersectsWith(Rectangle rect)
{
      if (((rect.X < (this.X + this.Width)) && (this.X < (rect.X + rect.Width))) && (rect.Y < (this.Y + this.Height)))
      {
            return (this.Y < (rect.Y + rect.Height));
      }
      return false;
}

In your case as you have a negative height the third condition [rect.Y < (this.Y + this.Height)] isn't being satisfied.

I suppose you can either create a new class which inherits from Rectangle and override the method if you really are insistent on a negative height

Also, if you are using the following to create your rectangles:

Code:

Rectangle r = new Rectangle(10, 0, 521, -457)

You should be able to replace it with

Code:

Rectangle r = new Rectangle(10, 457, 521, 457)

and the method will work. I would recommend this as it makes no sense to have a negative height.

PS If any of you don't know about Reflector and are .NET programmers I strongly urge you to download it (it's free too) as it is very helpful for giving an insight into the framework.
http://www.aisto.com/roeder/dotnet/

SoapSurgeon · 23 Mar 2006 at 19:04

yeah, i worked out it was a problem with negative height/width values so ive converted them all to positive, just had to rework a bit of code so wondered if there was another way round it, being lazy!!

thanks for the info and ill be sure to down load that proggy

cheers,

matt