Shapes in shapefiles all come with their bbox precomputed, there must be a similar reason.ġ) I was skeptical too initially about implementing the w-n algo for one-one checks, but if you put actual numbers in front of your data (Ntrees*Nexcl*Nnodes) and divide by common flops you’ll probably end-up in the order of the second. “(Realistically a better implementation is probably to have polygonal exclusion zones with an AABB saved around them as an early-exit test.)” (Realistically a better implementation is probably to have polygonal exclusion zones with an AABB saved around them as an early-exit test.) In the case of an airport being excluded from a DSF, most of the trees are outside the airport, and on average half of them will be outside the first segment of the polygon’s contour, letting us skip a lot of checking a lot of the time. On paper it’s O(N) just like the winding rule, and it’s more restricted, but it has one really nice feature: if you are outside of ANY edge, you are outside the polygon and you can -stop the check early-.įor something like a DSF where exclusion zones are small and most stuff is outside the polygon, the early exit is a pretty nice win. In a convex polygon test, you test whether the entity is on the left side of your polygon edges – if you are inside -every- edge, you are inside. There is another option: convex polygon exclusion zones. One other note: a number of DSF shapes -are not points-, so the point-in-winding test doesn’t do us a lot of good – it’s only useful for objects (where it is reasonably cheap) and trees (where it’s cheap by the tree but since there are a huge number of trees, it’s not cheap compared to what we do now). But it is also true that users will be able to make arbitrarily complex polygons – WED makes it cheap to really make a complicated exclusion zone. There is no question that with if the user inputs a large number of AABBs (to try to simulate a complex shape) then the shear number of AABBs makes things expensive. In other words, AABB exclusion zones exclude many trees with a single very cheap test polygons exclude a single tree with a significantly more expensive test. Okay – you’ve now made each tree test “N” times more expensive where N is the number of sides of the exclusion zone.īy comparison, with the box rasterizer, we simply _skip rasterizing entire scanline segments_ when a box excludes it. So you want to go exclude with a polygon. This AABB restriction is important for ensuring that the cost of forest rasterization doesn’t become amplified by the subdivision of the final output mesh into “buckets” for scenery paging and scrolling. The forest code works by rasterizing an arbitrary polygon with holes _restricted_ to within an axis aligned bounding box. Here’s an example of why it’s not as easy as you think it is: /cgit/cgit.cgi/xptools.git/tree/src/Utils/CompGeomDefs2.h#n1676

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