T-Junctions re-visited


Re: T-Junctions re-visited
#3
Hi Guys,

If I may jump on the "T junctions are evil" bandwagon:

- If you have a smooth surface, you probably don't have lines covering the edges. But with a T junction, the GPU may leave a 'crack' in the texturing of the surface. So T junctions are a general modeling no-no for this reason even when smoothing isn't considered.

- In order for the lighting to match at the T point, you have to ensure that the calculated normal of the vertex touching the triangle midpoint is the same interpolated normal from the two edges. This pretty much goes against what a smoothing algorithm is trying to do (calculate a normal based on the incident faces).

Even if we can band-aid around T junctions for smoothing, I do not think we should: there exists a way to author the part to fix this (subdivide the 'long' triangle or quad into two triangles or quads at the T point) that will result in un-cracked, smoothly shaded meshes.

I think it would be more productive for developers to write tools to detect and/or repair parts that have T junctions than to code around it in smoothing code; in the later case we endure the development cost for every LDraw app going forward, the performance cost of the more complex algorithm, and the lighting artifacts of having to pick weird normals at every T junction to avoid discontinuities.

I apologize for sounding like a broken record/big fat jerk, but... :-)

...the rules for mesh construction for 3-d graphics and games are already solved for the rest of the 3-d modeling world...we should match the graphics industry's rules for "water tight" meshes; the result will be better visual results and better performance.

Cheers
Ben
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Messages In This Thread
T-Junctions re-visited - by Travis Cobbs - 2013-03-12, 5:38
Re: T-Junctions re-visited - by Tim Gould - 2013-03-12, 6:31
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-12, 18:17
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-12, 18:32
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-12, 18:55
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-13, 18:27
Re: T-Junctions re-visited - by Allen Smith - 2013-03-12, 21:01
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-13, 17:37
Re: T-Junctions re-visited - by Tim Gould - 2013-03-13, 21:31
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-17, 15:43
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-17, 16:02
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-18, 14:30
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-18, 17:57
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-18, 18:27
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-18, 19:23
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-18, 20:41
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-18, 21:59
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-19, 1:09
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-19, 18:43
Re: T-Junctions re-visited - by Tim Gould - 2013-03-20, 10:56
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-20, 17:13
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-20, 17:23
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-20, 17:29
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-20, 17:26
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-20, 18:18
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-20, 13:42
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-20, 13:58
Re: T-Junctions re-visited - by Allen Smith - 2013-03-20, 16:08
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-20, 18:50
Re: T-Junctions re-visited - by Tim Gould - 2013-03-13, 22:56
Re: T-Junctions re-visited - by Sergio Reano - 2013-03-13, 22:51
Re: T-Junctions re-visited - by Tim Gould - 2013-03-14, 0:10
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-15, 3:17
Re: T-Junctions re-visited - by Tim Gould - 2013-03-15, 3:41
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-15, 17:56
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-14, 1:58
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-14, 2:37
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-14, 3:18
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-14, 22:23
Re: T-Junctions re-visited - by Tim Gould - 2013-03-14, 22:41
Re: T-Junctions re-visited - by Tim Gould - 2013-03-14, 2:43
Re: T-Junctions re-visited - by Tim Gould - 2013-03-14, 3:18
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-15, 3:15
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-15, 19:27
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-16, 0:08
Re: T-Junctions re-visited - by Ben Supnik - 2013-03-17, 15:58
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-18, 19:28
Re: T-Junctions re-visited - by Travis Cobbs - 2013-03-18, 19:32
Re: T-Junctions re-visited - by Sergio Reano - 2013-03-26, 21:46
Re: T-Junctions re-visited - by Sergio Reano - 2013-03-27, 20:48

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