radial-line-box-intersection

Arguments: point box &optional pos-to-return &key margin

Returns the point on the edge of a box where it is crossed by a line that passes through the center of the box and through an arbitrary point. Returns nil if the box center and the arbitrary point are the same.

The first argument is a position object for the arbitrary point, and the second argument is a box object. Returns a position object or nil.

If pos-to-return is nil or unspecified, then a new position object is created and returned. Otherwise pos-to-return should be a position object, which will be modified and returned.

The margin argument is somewhat obscure. If specified, it should be a real number indicating a distance. If the real intersection point is less than this distance from a corner of the box, then the returned point will be adjusted so that it is this distance from the corner. (This is used by one application to avoid drawing a connecting line to a rounded corner of a rounded box.) If you specify a value for margin, you must also specify a value for the pos-to-return optional argument. Specify nil if you do not want to specify a position.

As usual, position and box objects should contain integers. See make-position and make-box. See also radial-line-box-intersection-x-y and radial-line-circle-intersection.

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Created 2012.5.30.