DNQT#2 - Fake Triangles

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DNQT#2 - Fake Triangles

Postby DredNicolson » Wed May 10, 2017 7:51 pm

DredNicolson's Quick Tips #2

A fake triangle is a quad face with a double vertex.

dnqt2-1.jpg
One of these triangles is not like the other.


A fake triangle has a different origin/pivot point and behaves differently under subsurfing. A real triangle subsurfs into a rounded egg shape; a fake triangle retains a sharp point at the double vertex for a teardrop shape.

dnqt2-2.jpg
Real (L) and Fake (R) triangles after 4 subsurf levels.*


Fake triangles make it easier to accurately model objects with sharp points, like horns or the spikey tips of anime hair. Four fake triangles converging to a point form a 0-area quad face, or quadrupel vert.

dnqt2-3.jpg
3-side pyramidal horn with single vert tip (L) -vs- 4-side pyramidal horn and quadrupel vert tip (R)


When working with fake triangles, be careful about using the Remove Doubles tool, lest you turn them all into real ones. One precaution is to put intentional double verts into a vertex group, and deselect that group before removing accidental doubles. (Hit Z for wireframe mode and C for brush select to ensure you get all verts at the same spot.)

*Blender subsurfing. Depending on how they handle smooth subdivision, other 3D packages may give different results.
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Re: DNQT#2 - Fake Triangles

Postby jujube » Wed May 10, 2017 11:01 pm

This is actually a really, really good tip that I had no idea existed. I'm already thinking of all the stuff I could do with this...
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fake triangle pyramid.png
subsurf on fake triangle pyramid vs real triangle pyramid
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Re: DNQT#2 - Fake Triangles

Postby blindsaypatten » Thu May 11, 2017 10:48 am

Which raises the question, will makeclothes accept fake triangles, hence loosening that restriction? Or will strange things happen.
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Re: DNQT#2 - Fake Triangles

Postby punkduck » Thu May 11, 2017 9:19 pm

blindsaypatten wrote:Which raises the question, will makeclothes accept fake triangles, hence loosening that restriction? Or will strange things happen.


Makeclothes is working. But the Catmull-Clark algorithm (smoothing) of MakeHuman is not able to work when you put two vertices at the same location ... but you can load it without subdivision surface smoothing switched on in Makehuman. When you press the smoothing button or load it with smooting switched on it will not work. The logfile tells me that step13/20 of the subdivision fails.

The same thing will even happen if you put 3 vertices on one edge of the triangle. Especially the last result was new for me.

I use a similar method of very small faces (not zero size) when I want to avoid smoothing, look at the pyramids in the picture below:

http://www.makehumancommunity.org/forum/download/file.php?id=4132&mode=view

In Blender people often switch on subdivision surfaces and add a few loopcuts to make a sharp edge. More or less the same way.
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Postby DredNicolson » Thu May 11, 2017 10:53 pm

Yeah, if subsurfing tools in other apps won't dance with zero-area edges, use almost-double verts in the fake triangles. (Like scale factor 0.01/0.02 instead of straight 0.)
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