Commits on Mar 12, 2024

1. [encoding] BumpEstimator utility …

   Several vello stages dynamically bump allocate intermediate data
   structures. Due to graphics API limitations the backing memory
   for these data structures must have been allocated at the time of
   command submission even though the precise memory requirements
   are unknown.
   
   Vello currently works around this issue in two ways (see #366):
   1. Vello currently prescribes a mechanism in which allocation failures
      get detected by fencing back to the CPU. The client responds to
      this event by creating larger GPU buffers using the bump
      allocator state obtained via read-back. The client has the
      choice of dropping skipping a frame or submitting the fine
      stage only after any allocations failures get resolved.
   
   2. The encoding crate hard-codes the buffers to be large enough to be
      able to render paris-30k, making it unlikely for simple scenes to
      under-allocate. This comes at the cost of a fixed memory watermark
      of >50MB.
   
   There may be situations when neither of these solutions are desirable
   while the cost of additional CPU-side pre-processing is not considered
   prohibitive for performance. It may also be acceptable to pay the cost
   of generally allocating more than what's required in order to make the
   this problem go away entirely (except perhaps for OOM situations).
   
   In that spirit, this change introduces the beginnings of a
   heuristic-based conservative memory estimation utility. It currently
   estimates only the LineSoup buffer (which contains the curve flattening
   output) within a factor of 1.1x-3.3x on the Vello test scenes (paris-30k
   is estimated at 1.5x the actual requirement).
   
   - Curves are estimated using Wang's formula which is fast to evaluate
     but produces a less optimal result than Vello's analytic approach.
     The overestimation is more pronounced with increased curvature
     variation.
   
   - Explicit lines (such as line-tos) get estimated precisely
   
   - Only the LineSoup buffer is supported.
   
   - A BumpEstimator is integrated with the Scene API (gated by a feature
     flag) but the results are currently unused. Glyph runs are not
     supported as the estimator is not yet aware of the path data stored
     in glyph cache.

   

   armansito committed Mar 12, 2024
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