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Polygons: clockwise or counterclockwise


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#1 Aliii   Members   -  Reputation: 1332

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Posted 18 May 2013 - 03:45 PM

In OpenGL the "default direction" for triangles is CCW. Makes sense. However, ...is there a standard direction like that for line-strips/polygons? .....I have polygons that are built up from many triangles. It feels more natural to have the points of the line-strip ordered clockwise but I wanted to know if theres a standard for that or it doesnt matter.



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#2 phil_t   Crossbones+   -  Reputation: 3185

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Posted 18 May 2013 - 04:48 PM

Winding order is used for culling backfaces. So that's why there needs to be an established convention. But you need at least 3 vertices to do that (to calculate a normal that indicates whether something faces in or out of the screen), so it doesn't really make sense for lines.



#3 Aliii   Members   -  Reputation: 1332

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Posted 18 May 2013 - 05:19 PM

Thanks. I was not precise. ....my English. I draw a polygon with the mouse. Which is practically a closed polyline/line-strip. Sometimes I need to reorder the points so the polygon is always "drawn" the same direction. (I dont draw the polygon directly, I build it up from triangles. The triangles are drawn CCW.) Should the points of the polygon ordered CW or CCW? 



#4 phil_t   Crossbones+   -  Reputation: 3185

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Posted 18 May 2013 - 05:26 PM

My point was that it doesn't matter.



#5 Zaoshi Kaba   Crossbones+   -  Reputation: 3605

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Posted 18 May 2013 - 11:46 PM

Hardware can handle both, CW and CCW. If you're drawing triangles with mouse and having problems with backfacing you could render both sides.






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