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  Calculating line intersection for a screen view  (Read 790 times)
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Offline wjp

Senior Newbie




Java games rock!


« Posted 2005-04-12 07:53:54 »

Hi,

I have an issue when calculating the perspective view in my game. Most of the time it's fine, but sometimes I end up with a very large x or y value (-ve or +ve) and due to the limitations of java it flips the coordinates and I end up with a line drawing in the wrong direction.

What I would like to calculate (as part of my screen projection method) is if the point is outside the screen (lets say 600,800) I'd like to calculate the point on edge of the screen, so that the points are always between zero and the max dimension of the screen.

I thought about trying to calculate the line intersection depending which quadrant it is, but this seems messy. Is there a better way to do this?

Here's my code for reference:

 public Point getScreenProjection(Coordinates a)
  {

   double f = (viewPoint.z - screenPlane.z) / (viewPoint.z - a.z);
   Point p = new Point();
   p.x = (int)(a.x * f + dimensions.x * .5);
   p.y = (int)(dimensions.y * .5 - a.y * f) + (int)(m_yoffset);
   return p;
  }



The point 'dimensions' is the dimension of the screen view (ie 600,800).
Offline wjp

Senior Newbie




Java games rock!


« Reply #1 - Posted 2005-04-12 14:19:56 »

Hi,

just to add - what I'm essentially after is Clipping Operations.

A clipping algorithm to  identify portions of the picture that lie inside or outside of a specified region (ie clip window).

   * Point Clipping - checking whether a single point is within the clip window
   * Line Clipping  - eliminate lines entirely outside the clip window, and calculates new endpoints for lines only partially within the clip window
   * Polygon Clipping  - calculate the boundary(s) for that part of a polygon which overlaps the clip window
Offline CaptainJester

JGO Knight


Medals: 12
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Make it work; make it better.


« Reply #2 - Posted 2005-04-13 11:24:26 »

Instead of using Point, try Point2D.Double.  This should increase the available precision.  Point only uses int for the x,y value.

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Offline wjp

Senior Newbie




Java games rock!


« Reply #3 - Posted 2005-04-18 14:59:59 »

Hi,

I used a Cohen-Sutherland algorithm to clip all the points in my polygons. This has worked perfectly. However I realise the issue is in fact the getScreenProjection algorithm.

This issue occurs the the point that needs to be mapped is either very close to the viewPoint or behind it. This gives a very laarge factor. This simple fix is to have a large viewPoint - however you then don't get the perspective look I'm after - you get a rather flat view.
Any suggestions or ideas? what do you do?



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