On Thu, Oct 12, 2023 at 4:25 AM Graham Toal <gt...@gtoal.com> wrote:
> You need a variation of the shortest path algorithm; it's basically a > breadth-first flood fill, which is usually implemented by a raster > algorithm, however it *is* possible to come up with something less > expensive (but more complex to code) by using a ray-tracing algorithm (or > more specifically 'shadow casting'). I can't see an easy way to do it > using standard polygon operations though. > I've been thinking more about this problem and I think it might be possible on polygons. Some years back several of us at the MIT "Scratch" site wrote different ways of shadow casting (see https://scratch.mit.edu/projects/237786845/ for an example - click and drag the lightbulb around) not all of which were raster-based. Search the site for "pen shadow(s)" to see other examples. The missing polygon primitive needed is to construct a visibility polygon ( https://en.wikipedia.org/wiki/Visibility_polygon ), after which you would recurse down all the lines extending from the point of view with some sort of continuously varying buffering that extended the visibility by the remaining unexplored distance. One experiment I did on constructing the visibility polygon was to do a binary chop on all the obstacle polygons to determine their leftmost and rightmost extents relative to the viewpoint - try this demo to see that work: https://scratch.mit.edu/projects/115648650/ - I think it is doable in GEOS but not trivial. If GEOS is used in any programming classes I think the problem would make a good class project for kids! (The kids at MIT who worked on the shadow casting were mostly 12 - 16 yr olds) G
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