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# Order of triangles's vertices

Posted on 2006-11-06
Medium Priority
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I plot some 3D functions and extract triangles, but some draw clockwise and other counter-clockwise so they are falsely colored (but all normal vector are directed to same direction)

For example, I use v=sign(x0*y1-x1*y0 + x1*y2-x2*y1 + x2*y0-x0*y2) to determine how triangle is shown:

(-0.422 -0.100 0.900)
(-0.400 -0.167 0.900)
(-0.400 -0.100 0.911)
0.001481 - clockwise

(-0.422 0.100 0.900)
(-0.400 0.167 0.900)
(-0.400 0.100 0.911)
-0.001481 - counterclockwise

I am looking for the method that will allow sorting of triangle vertices that they are all shown in right direction (counterclockwise for instance).

Thank you.
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Question by:Neoliten
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LVL 84

Expert Comment

ID: 17887449
Is that clockwise when looking in the z direction?
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Author Comment

ID: 17887473
Actually it not directly visible. I see on the display that some triangles colored on front, but other colored on the back, however normals are pointer to the same direction, so all triangles should be colored on front.
I made calculations and I see that v=sign(... gives negative and positive results, which means that triangles drawn in clockwise/counterclockwise order. I am trying to sort vertices so they are all drawn in clockwise direction.
In this way coloring will depend on direction of normal vector.
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LVL 84

Accepted Solution

ozo earned 500 total points
ID: 17887489
Can you just swap a pair of vertices when v is negative?
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Author Comment

ID: 17887500
I am trying...
However, which pair? Or swap until v become positive?
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LVL 84

Expert Comment

ID: 17887549
It doesn't matter, swaping any pair will change the sign
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Author Comment

ID: 17887570
Yes, works. Thank you.

However seems that I need to organize vertices according normal direction. For example, if I plot sphere, then one half colored on front and other half also on front (which is wrong, second half should be colored on back - inside the sphere).

I need sort of check, if normal points back, then color front (v < 0); if normal points front, then color back (v > 0). (just different checks for v, greater then 0 or less than 0)
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