math stuff : converting from circle to ellipse

Discussion in 'Game Development (Technical)' started by svero, May 19, 2011.

  1. svero

    Moderator Original Member Indie Author

    Jul 27, 2004
    Likes Received:
    So... I have a 2d collision routine to intersect a circle and line segment. Broadly this boils down to the pseudocode...

    point = Get Closest Point On Line from circle
    if(distance(point to circle-center)<=circle radius) { ok collides! }

    Now what I want to do is convert that so it works with a center and a "radius vector" instead of a radius, so I can collide with ellipses instead of circles. What is the best/fastest way to do this?
  2. Wyzfen

    Wyzfen New Member

    Mar 22, 2011
    Likes Received:
    What if you transform the line to be relative to the ellipse, then scale everything to make the ellipse a circle, then test against the circle.

    Assuming ellipse is at [Ex,Ey], with radii Rx, Ry, aligned along axes (if rotated, you'll need to rotate the line too)
    1. create a new line; each end does (x,y) -> ((x-Ex) * Ry/Rx, y - Ey)
    2. get the closest point of that line to circle, centred at [0, 0], with radius Ry.

    hope that helps,
    #2 Wyzfen, May 19, 2011
    Last edited: May 20, 2011

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