Check Intersection Between Line and Rotated Rectangle

Question

I have try many things without finding the good solution so here I am.
In my game (2D) I have to check collision with all my Object (house, garage..) which are image inside Rotated Rectangle, between a ray from a Point A to Point B.

I'm using Xna and there some code:

public void Update(List<Obstacle> Lob, DragObj Ldo)
{
    bool inter = false;
    Point A;
    Point B;

    A = new Point((int)pos.X, (int)pos.Y);
    B = new Point((int)Ldo.Position.X, (int)Ldo.Position.Y);
    for (int j = 0; j < Lob.Count(); j++) 
    {
        if (inter = interclass.LineIntersectsRect(A, B, Lob[j].Shape)) // I have this for the moment, Shape is the rectangle but not rotated )
        {
            inter = true;
            islight = false;
        }
        else
        {
            inter = false;
        }
    }
}

So to solve my problem, whether I find a solution to have a rotatedRectangle Object with a method to check collision with line. Whether totally something else, maybe only check collision between yy straight and each rotated Rectangle Axis?

Thanks for your advices.

Rotated the line will change my 2D space organisation, doesn't it ? I'm gonna try to rotate my 2 points, but its wont be easy to implement in my current code. — Gabson, Nov 27 '13 at 13:47
but you can imagine it right? if you take both together and rotate so the rectangle is straight, you can use the regular axis-aligned rectangle function — MickLH, Nov 27 '13 at 14:24
Y I have draw it to imagine it and its ok. But It has a strange behavior. Intersection is working but not in the right place, i also tried with radians angle. I use this to rotate my point : In 2D, you make (X,Y) from (x,y) with a rotation by angle t so: X = x cos t - y sin t Y = x sin t + y cos t — Gabson, Nov 27 '13 at 14:50

score 0 · Answer 1 · answered Nov 28 '13 at 11:36

0

I have solver this problem by checking intersection between my line and each side of the rotated Rectangle (I have to rotate each Line-side of the rectangle first). I will post the little algo soon.

answered Nov 28 '13 at 11:36

Gabson

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score 0 · Answer 2 · answered Jan 18 '19 at 21:30

I don't know C# but... There is this algorithm that can get the closest point on a line from a point. (Note that the closestPointOnLine function is not my code)

var closestPointOnLine = function(line1, point1)
{ 
    var A1 = line1.y2 - line1.y1;
    var B1 = line1.x1 - line1.x2;

    var C1 =  A1 * line1.x1 + B1 * line1.y1;
    var C2 = -B1 * point1.x + A1 * point1.y;

    var det = A1 * A1 + B1 * B1;

    var cx = 0;
    var cy = 0;

    if(det !== 0)
    { 
        cx = ((A1 * C1 - B1 * C2) / det);
        cy = ((A1 * C2 + B1 * C1) / det);
    }else{
        cx = point1.x;
        cy = point1.y;
    }

    return {
        x : constrain(cx, Math.min(line1.x1, line1.x2), Math.max(line1.x1, line1.x2)),
        y : constrain(cy, Math.min(line1.y1, line1.y2), Math.max(line1.y1, line1.y2)),
    }; 
};

Before we go any further let's make it clear that: Our line is:

var lineToTest = {
    x1: someNumber
    y1: someNumber,
    x2: someNumber,
    y2: someNumber
};

And that our rotated rectangle contains:

var rectToTest = {
    points: [
       {
          x : someNumber,
          y : someNumber,
       },
       {
          x : someNumber,
          y : someNumber,
       },
       {
          x : someNumber,
          y : someNumber,
       },
       {
          x : someNumber,
          y : someNumber,
       },
    ],
};

We then take the lineToTest and take it's first point and begin using the closestPointOnLine to get a point on a line of the rectToTest, We then check if that point is touching the lineToTest, if it is not repeat for the other lines in the rectangle.

Now I don't actually know the code to check if a point is touching a line: But it might go something like this:

function isLineTouchingPoint(line1, point)
{
     //Other code here
     //You'll have to use trigonometry for this one though

     return boolean;
}

Now you could convert this code into C# to get it to work.

Check Intersection Between Line and Rotated Rectangle

2 Answers2