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I'm experimenting with a vector based graphics style with objects represented as series of line segments with a given width(it would probably be easier to think of these as rectangles). The problem is that these segments are connected at the center and leave a gap (shown below). I've determined that the most efficient way to cover this gap is simply to cover it with a triangle, and since I'm working in OpenGL, all I need are the points of the two points that don't overlap with the other rectangle, the third point being the center point where the two line segments(rectangles) are connected. How can I determine which points I need to use for the triangle, given that I have all of the points from both rectangles?

Example

EDIT: I will also accept alternative solutions, as long as they cover up that gap.

EDIT 2: Nevermind, I solved it. I'll post code once I have better Internet connection.

Henry Conklin
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  • I think this is what you're looking for: http://stackoverflow.com/questions/687173/how-do-i-render-thick-2d-lines-as-polygons – Rahul Banerjee Mar 28 '13 at 01:50
  • That is basically the same problem, and I didn't see that before posting, but the answers there are a bit too theoretical for me. – Henry Conklin Mar 28 '13 at 23:59

2 Answers2

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Maybe I'm misunderstanding the question... but if you zoom in on the top corner of your red pentagon, you get something like this, am I right?

junction between two polygon edges

where A and B are nodes on the rectangle for edge1 and C and D are nodes on the rectangle for edge2. You say you already know these coordinates. And from what you say, the edges meet at the centre, which is halfway between A and B, and also halfway between C and D. So call this point X, and you can calculate its coordinates easily I guess.

So all you need to do is draw the missing triangle AXC, right? So one way would be to determine that A and C are on the "outside" of the polygon (and therefore need filling) and B and D are on the "inside" and therefore don't. But it's probably easier to just draw both, as it doesn't hurt. So if you fill AXC and BXD, you'd get this:

enter image description here

Penfold
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  • I've tried that, but I'm running into a problem with some ambiguity in my preceding code so I'm not entirely able to differentiate between A and B or C and D, so I was hoping to find a more general solution. – Henry Conklin Mar 29 '13 at 00:08
  • To specify, A is not necessarily paired with C and B is not necessarily paired with D, but when I draw all four possible triangles it looks strange. – Henry Conklin Mar 29 '13 at 00:13
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The solution I found assumes that there are 3 basic cases:

First, the three unique center points for the two rectangle proceed upward (positive y direction) so the gap is either on the left or right of the connection. In my code, I had the corner points of the rectangle organized by their orientation to the left or right of the center point, so if the bottom rectangle's left point is below the top rectangle's left point, then the gap is between the left points of the two rectangles, otherwise the gap is between the right points.

Second, the three unique center points have a maximum at the center most of the center points, so the gap is on the top. The gap is then between the two points with the maximum y values.

Third, the three unique center points have a minimum at the center most of the center points, so the gap is on the bottom. The gap is then between the two points with the minimum y values.

[I'll post pictures of the example cases if it is requested]

Henry Conklin
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