[brlcad-tracker] [ brlcad-Bugs-2561346 ] Tessellation problem for Extrusions mixing Arcs and Beziers

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Bugs item #2561346, was opened at 2009-02-03 11:52
Message generated for change (Comment added) made by johnranderson
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Category: Geometry Conversion
Group: unexpected behavior
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: aegio (aegio)
Assigned to: Nobody/Anonymous (nobody)
Summary: Tessellation problem for Extrusions mixing Arcs and Beziers

Initial Comment:
Tessellation routines seems to present problems for sketches
mixing arcs with Beziers and arcs with arcs.

Attached herein there's an example of correct behavior
(file correct.g) consisting in a sketch composed of closed
beziers and circles that extrudes and converts well to (for
example) STL format:

$ g-stl -o correct.stl correct.g ext
428 triangles written

In addition there's the problematic example mixing arcs
and beziers in a topological correct closed sketch (see incorrect.asc
for details about correctness of vertex linking) that extrudes well on mged
and can be raytraced without problems but makes g-stl complain
about some issue regarding the fact that sketch loop is not closed:

$ g-stl -o incorrect.stl incorrect.g ext
rt_extrude_tess: A loop is not closed in sketch sk
	tessellation failed!!
nmg_booltree_leaf_tess(ext): tessellation failure
db_walk_subtree() FAIL on '/ext'
0 triangles written

alf...@gm...

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>Comment By: John Anderson (johnranderson)
Date: 2009-02-04 21:27

Message:
The problem with the sketch in "incorrect.g" is that the segments are not
consistent in their direction. Every segment (line, circular arc, bezier)
has a start vertex and an end vertex. For a loop to be closed, each end
vertex must be the start vertex of another segment. In "incorrect.g" there
are segments that start at a vertex that is the start vertex of another
segment, and segments with an end vertex that is the end vertex of another
segment. Reversing the direction of two of the segments in "incorrect.g"
will allow g-stl to succeed. This change can be accomplished using the
command:
db adjust sk SL { { bezier D 3 P { 3 2 1 0 } } { bezier D 3 P { 4 5 6 7 }
} { carc S 0 E 4 R 0.8669825810741810245829697 L 1 O 0 } { carc S 7 E 3 R
0.8617633860033965254032751 L 1 O 0 } }

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