Anyone know how to detect if a Point(x, y) is in a Line(x1, y1, x2, y2) or not?
Quiet not good at math please...
This is a discussion on Need help about point and line within the Game Programming forums, part of the General Programming Boards category; Anyone know how to detect if a Point(x, y) is in a Line(x1, y1, x2, y2) or not? Quiet not ...
Anyone know how to detect if a Point(x, y) is in a Line(x1, y1, x2, y2) or not?
Quiet not good at math please...
Last edited by audinue; 01-01-2009 at 11:04 AM.
Just GET it OFF out my mind!!
If the slope from (x1,y1) to (x,y) is the same as the slope from (x1,y1) to (x2,y2). Breaking out the formula for slope gives (x-x1)*(y2-y1) == (x2-x1)*(y-y1).
Works like a charm, thanks~! You are smart tabstob! Thanks!
http://audinue.navhost.com/LineTest.html
Code:var line = {x1:10, y1:10, x2:100, y2:100}; _root.lineStyle(1); _root.moveTo(line.x1, line.y1); _root.lineTo(line.x2, line.y2); onEnterFrame = function () { var point = {x:_root._xmouse, y:_root._ymouse}; hit_mc._visible = (point.x-line.x1)*(line.y2-line.y1) == (line.x2-line.x1)*(point.y-line.y1); };
Last edited by audinue; 01-01-2009 at 11:16 AM.
Just GET it OFF out my mind!!
You also need to check that x >= x1 && x <= x2 to make sure it is on the line.
The slope extends to +/- infinity, so you need to test for more than just being on the slope.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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Salem's point is that if you want if you want the point to be on the line segment strictly between (x1, y1) and (x2, y2) [as opposed to the line between the points that goes forever], then you need to check that x is between x1 and x2 (i.e., that (x1-x)*(x2-x) < 0).
What the...Code:boolean i s H i t (Point point, Line line) { return ((point.getX() >= line.getX1()) && (point.getX() <= line.getX2()) && (point.getY() >= line.getY1()) && (point.getY() <= line.getY2())) && ((point.getX() - line.getX1()) * (line.getY2() - line.getY1()) == (line.getX2() - line.getX1()) * (point.getY() - line.getY1())); }
Just GET it OFF out my mind!!
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
Heres a version I came up with:
Code:int PointIsOnLine(int x1, int y1, int x2, int y2, int x3, int y3) { if(x1 == x2 && y1 == y2) //Not a line return(x1 == x3 && y1 == y3); if(x1 == x2) //No gradient return (y3 == y1); if(y1 == y2) //Infinite gradient return (x3 == x1); float gradient = (float)(y2 - y1) / (float)(x2 - x1); int x01 = x1 - ((float)y1 / gradient +0.5); int x02 = x3 - ((float)y3 / gradient +0.5); return (!(x02-x01)); } int PointIsInLine(int x1, int y1, int x2, int y2, int x3, int y3) { if(! PointIsOnLine(x1, y1, x2, y2, x3, y3)) return 0; if(x3 > MAX(x1, x2) || x3 < MIN(x1, x2)) return 0; return 1; }
Hrm. So if your line is from (0, 0) to (100, 1)... There are absolutely no points which are on that line which have integer coordinates, except for the endpoints.
So using this sort of algorithm, you'd decide that nothing at all ever intersects that line, even though it's over 100 units long. Using integer coordinates isn't really the best choice for this sort of thing I think
Code://try //{ if (a) do { f( b); } while(1); else do { f(!b); } while(1); //}
Yeah, the co-ords get rounded:
I was using integer maths for this as I was checking pixel values in 2D to see if they sat on a line. I guess generally it would be better to do everything in floating point - that would very simple modification.Code:int x01 = x1 - ((float)y1 / gradient +0.5); int x02 = x3 - ((float)y3 / gradient +0.5);
Edit: although actually it looks as if its rounding the wrong way, lol. Perhaps it should have been:
this might fix an old bug I never worked out...Code:int x01 = x1 - ((float)y1 / gradient) +0.5; int x02 = x3 - ((float)y3 / gradient) +0.5;
Last edited by mike_g; 01-02-2009 at 05:04 PM.
But if you do it with floating point, you'll want to do a delta comparison, since floating points rarely compare exactly equal, even when they should.
All the buzzt!
CornedBee
"There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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Yeah, thats one of the reasons I have an irrational fear of using floats, lol.