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Math Help - Reflecting a circle off another circle

  1. #1
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    Reflecting a circle off another circle

    I am working on a computer game and I need to correctly reflect a ball off a circle object. I am trying to do it as a line and circle intersection. I know the intersect point of the line (ball path) and the circle. Now I want to rotate the ending point of the ball path about the intersection point to get the correct angle of reflection.

    The following are known:

    ball current x
    ball current y
    ball end x
    ball end y
    ball radius

    circle center x
    circle center y
    circle radius

    intersection point of ball path and circle x and y

    I know I need to find the angle of incidence between the tangent line and the incoming ball path which will also equal my angle of reflection. I think once I know those two angles I can subtract them from 180 to get my rotation angle then rotate my end point about the angle of intersection by that amount. I just don't know how.

    I have attached a pic. Again, I know the two end points of my line segment, the point of intersection and the radius of the circle.

    I am ultimately trying to get the point marked by the open circle, so I need to know the angle between the ball path and the norm, which is the blue line.

    Thanks.
    Attached Thumbnails Attached Thumbnails Reflecting a circle off another circle-untitled.jpg  
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