OK, I have the solution now. 1) Starting at point X,Y draw a circle of radius B (B = buffering dist)
2) Construct the visibility polygon from X,Y limited by the circle. 3) The visibility polygon will be effectively a list of rays from X,Y to points on the obstacles. Collect the list of points where the rays touch an obstacle at a line segment end-point - including where they touch tangentially and may carry on past that edge point to another obstacle. I.e. one ray may generate more than one point. 4) At each of those points at a distance D from X,Y, recursively invoke this algorithm with the distance parameter reset to B-D, until the remaining distance is zero. The resulting polygon will consist of straight line segments (from obstacle edges facing the source points, and projections past edges) plus circular arcs where the circles expire due to having reached the maximum buffering distance without having touched any object. So the only polygon operation really needed to implement this is the construction of the simple distance-limited visibility polygon, which is done by finding the objects within range B from X,Y and enumerating the end-points of the segments composing that object facing X,Y. The recursion takes care of the complexity of shapes and the union of all the partial results creates the final buffered object. I hope I managed to explain that OK. If not, let me know and I'll try to generate some graphics to make it more clear. G
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