Commits on Mar 14, 2024

1. [encoding] Bump buffer estimation for segments …

   The segments (and segment counts) buffers contain intermediate data
   structures that associate lines (post-flatten, post-binning) with tiles.
   Their overall count depends on the number tile crossings for each line.
   
   These counts are now estimated as follows:
   
   1. Explict lines with known endpoints are estimated precisely by
      guessing the worst case rasterization of the hypothenuse (i.e. the
      sum of the tiling estimate for the x and y coordinates.
   
   2. Explicit lines with unknown endpoints (e.g. the caps on a stroke)
      are estimated based on the length of the segment.
   
   3. Flattened curves are estimated by tiling a line whose length is
      derived based on a conservative estimate of the curve's arc length.
      The segment count is forced to be at least as large as the Wang's
      estimate used for the LineSoup count.
   
   This implementation currently has two major shortcomings:
   
   I. Clipping is not taken into account, so the count is overstimated when
      layers are present or when geometry lies outside the viewport. There
      are ways to address this but they will require some changes:
   
      a) Most of the count accummulation happens at encode time but the
         bounds of the viewport are only known at render time. This could
         be changed so that most of the accummulation happens at render
         time and additional data (like the bounding box of each shape) get
         tracked during encoding.
   
      b) It might make sense to track the clip stack to test each shape's
         bounding box against the clip geometry while resolving the counts
         and use that as input to a heuristic. It is also possible to
         discard shapes that lie completely outside the bounds of the clip
         geometry. All of this would require additional tracking that
         impacts CPU time and memory usage.
   
   II. A rotation that is present in the transform has an impact on tile
       crossings. We can precisely account for this for explicit lines with
       known endpoints but we have to use a heuristic for curves. The
       estimator doesn't track detailed shape data, so a heuristic must be
       used when appending a scene fragment. We currently inflate all
       segment counts as if they have a 90 degree rotation whenever a
       transform should apply.
   
   Overall the segment count tends to be overestimated 3x-10x. There is one
   known failure mode when the count is underestimated with _very_ small
   scale factors ("conflation artifacts" test scene).

   

   armansito committed Mar 14, 2024
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