c4dbc3758f
This patch improves the straight skeleton implementation used by GrTessellator to collapse overlap regions in the alpha gradient border. The resulting quality improvement can be seen in the "thinconcavepaths" GM, for example, where the coverage values of the "thin right angle", "thin rect and triangle" and "skinny snake" are now lower and match the raster backend more closely. It also improves correctness, such as on the linked Chromium bugs below. Previously, the straight skeleton was performed using the same Vertex and Edge classes used for tessellation, but this led to fragility in maintaining the connectivity and ordering required by those classes. Instead, that functionality has been moved to new SSEdge and SSVertex classes. Their construction results in alternating SSVertex and SSEdges around the boundary of the each overlap region, shrunk by one SSEdge as the boundary collapses. Applying events may now also create further events (chained events), as intersections between newly-adjacent bisectors change the structure of the skeleton. This is always calculated from the bisectors of original boundary, not the shrunk boundary of the straight skeleton, which is why each SSEdge points at its originating Edge. If the edges are parallel or near-parallel, the bisector may be infinite. This is handled by a new flavour of create_event(), which does a Line/Line intersection (rather than an Edge/Edge intersection) to find the intersection between the infinite bisector and an adjacent bisector. Several ancillary bugs were fixed: the priority queue used to represent edge events was sorting the inner edges incorrectly. These need to be sorted in descending not ascending order of coverage. Its implementation was moved from Skia's TDPQueue to std::priority_queue(), which is more flexible in specifying a comparator. check_for_intersection()'s partner synthesis code was moved into a new function, compute_bisector(), also used by the chained skeleton events code. Degenerate edges are now removed during the simplify_boundary() pass. They were previously detected but ignored, causing incorrect inner and outer tangents to be computed in stroke_boundary(). An fSynthetic flag was added to Vertex, in order to detect vertices which cannot be moved by an edge collapse event (e.g., intersections with non-boundary edges, merged vertices, straight skeleton vertices). More raw implementation notes: Connect straight skeleton vertices as we find them, so we don't have to use partnering (ss_connect()). Only disconnect edges which are still alive after event application. Add a check for near-parallel lines in compute_bisector(). Don't use edge type to determine direction to offset for bisectors. The winding should already include this information. Move Event ownership to SSEdge. If we're down to the last two edges in a skeleton, don't check for events. Add concave_arc_and_circle GM. Add a collapsepaths GM. Bug: 941429, 913349, 929915, 945853 Change-Id: Ib89e231d0e8611f8735fd3592db6391da096369d Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215094 Commit-Queue: Stephen White <senorblanco@chromium.org> Reviewed-by: Robert Phillips <robertphillips@google.com> |
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Skia is a complete 2D graphic library for drawing Text, Geometries, and Images. See full details, and build instructions, at https://skia.org.