Keith Hair

Scripting is fun like any other hobby

May 27 2009

Knowing if a Point is behind a Line

Posted by: Keith H in ActionScript 3, SciFi, tags: Behind, Line, Points, Test

Yea yea, I know Battlestar Galactica is over, but today I thought of the "Daybreak part 1" episode of BSG.
Hera, a little girl has been kidnapped by the Cylons. As part of a desperate plan to save Hera, Admiral Adama draws a red line on the deck, and asks for those willing to risk their lives to save her, do so by crossing the line.

I had a similar problem also with my scripts. I wanted to simply know if a Point was behind a Line or not...

 
/*-----------------------------------------------------------------
Returns true if Point P is behind line defined by Points C and D.
 ----------------------------------------------------------------*/
function isBehindLine(P:Point,C:Point,D:Point):Boolean
{
	var a:Number=Point.distance(P,C.add(Point.polar(1,Math.atan2(C.y - D.y,C.x - D.x) + Math.PI / 2)));
	var b:Number=Point.distance(P,C.add(Point.polar(-1,Math.atan2(C.y - D.y,C.x - D.x) + Math.PI / 2)));
	return a < b;
}

This entry was posted on Wednesday, May 27th, 2009 at 5:15 pm and is filed under ActionScript 3, SciFi.You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

2 Responses to “Knowing if a Point is behind a Line”

Keith Hair » Know where a Point is relevant to a Segment says: May 28th, 2009 at 3:35 am

[...] « Knowing if a Point is behind a Line May 28 2009 [...]
John Mark Isaac Madison says: August 15th, 2012 at 3:21 pm

Optimizations:
1. Store Math.PI in a class variable.
Ex: private const MATH_PI:Number = Math.PI;

2.Calculate Math.atan2 only ONCE since you are using it with identical values on
both lines. This means you are doing two unneccessary subtractions and 1 unneccessary
call to Math.atan2. Math library calls are infamously slow.
Ex: var deltaAtan:Number = Math.atan2(C.y-D.y, C.x-D.x);

3. Whenever you can MULTIPLY rather than DIVIDE:
Multiply by .5 rather than divide by 2.

4. EDIT while writing you optimized code:
Store Math.PI / 2 in a constant variable.

function isBehindLine(P:Point,C:Point,D:Point):Boolean
{
var halfPI :Number = Math.PI * 0.5;
var deltaAtan:Number = Math.atan2(C.y – D.y,C.x – D.x );

var a:Number=Point.distance(P,C.add(Point.polar( 1,deltaAtan + halfPI )));
var b:Number=Point.distance(P,C.add(Point.polar(-1,deltaAtan + halfPI )));
return a < b;
}