Werner's Miscellanea

Werner's Miscellanea Commit Details

Date:	2011-07-23 20:23:30 (8 years 3 months ago)
Author:	Werner Almesberger
Commit:	3f820dc41d82c78d972efe3c69f139f209a742f9
Message:	cad/test2/README: added more results and cleaned up the text
Files:	cad/test2/README (5 diffs)

Change Details

cad/test2/README
17	17	New findings and hypotheses
18	18	===========================
19	19
	20	The first test revealed several problems in both OpenSCAD and
	21	Cadmium. It turned out that most of them could be resolved by making
	22	small changes to the models.
	23
20	24
21	25	The road to hell is paved with good intentions
22	26	----------------------------------------------
...	...
34	38	Before: 4901.0 4904.4 1.5 8362 3183
35	39	After: 316.3 315.9 0.3 2942 1473
36	40
37		This makes Cadmium only about 3.7 times slower than OpenSCAD's CGAL
	41	This makes Cadmium only about 4-5 times slower than OpenSCAD's CGAL
38	42	rendering, a marked improvement from the previous factor of 57. The
39	43	resulting mesh is now less than half the size of the mesh generated
40	44	by OpenSCAD.
...	...
43	47	understandable without these extra tolerances.
44	48
45	49
46		Context: preventing artefacts
	50	Background: preventing artefacts
	51	--------------------------------
47	52
48	53	When subtracting volumes from each other, surfaces that coincide in
49		both the original volume and the one subtracted from it, can remain
	54	both the original volume and the one subtracted from it can remain
50	55	as arbitrarily thin artefacts.
51	56
52	57	This 2D example illustrates the effect:
53	58
54		- given an original shape A like this
	59	- given an original shape like this
55	60
56	61	+---------------+
57	62	\| \|
...	...
99	103	+---------+
100	104
101	105
102		Artefacts in OpenSCAD's mesh may also be rounding errors
	106	Artefacts in OpenSCAD's mesh were rounding errors
	107	-------------------------------------------------
	108
	109	The artefacts found in the mesh produced by OpenSCAD were also caused
	110	by rounding errors. In this case, the problem were parts that were
	111	fused along a common surface. As in the above example, this could be
	112	solved by making the parts overlap a little.
103	113
104		The artefacts found in the mesh produced by OpenSCAD may also be
105		caused from rounding errors. In this case, the problem would be parts
106		that are fused along a common surface. As in the above example, this
107		could be solved by making the parts overlap a little.
	114	This change affected the rendering performance as follows:
	115
	116	real user system faces vertices
	117	Before: 85.5 85.0 0.4 7798 3351
	118	After: 68.8 65.4 0.5 6508 3258
108	119
109		Whether this is truly the source of the artefacts still needs to be
110		verified.
	120	The resulting mesh looks as good as the one generated by Cadmium.
111	121
112	122
113	123	OpenCSG problems may be an OpenGL compatibility issue
...	...
118	127	different color without actually removing them is caused by using
119	128	OpenGL emulation in Mesa. This needs further investigation.
120	129
	130
	131	Conclusion
	132	==========
	133
	134	The model used in the first test contained constructs that were meant
	135	to help avoid ambiguous situations. It turned out that Cadmium did
	136	not need this "help" and removing the constructs substantially
	137	increased the rendering speed.
	138
	139	OpenSCAD is the exact opposite: it produces clearly visible
	140	artefacts in ambiguous cases and rendering speed also benefits from
	141	manual resolution of these issues. The cost of doing so is an
	142	increase of the complexity of the model.
	143
	144	Performance after the model changes (times are in seconds):
	145
	146	real user system faces vertices
	147	OpenSCAD: 68.8 65.4 0.5 6508 3258
	148	Cadmium: 316.3 315.9 0.3 2942 1473
	149
	150	The sources of the models used are in
	151	http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/cad/test2/
	152
	153	The meshes can be downloaded from
	154	http://downloads.qi-hardware.com/people/werner/cad/test2/
	155
121	156	---------------------------------------------------------------------

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