Vertex-Winding-Order

[Acki]

no, no, not another discussion on what order to use/chose !!!

I have a problem to determine how 3 vertices are connected...

the 3 vertices are placed on a xy plane...
and I have a winding order (v1-v2-v3)...

Code: Select all

// get the vertex positions
position2d v1 = getRandomXYPosition();
position2d v2 = getRandomXYPosition();
position2d v3 = getRandomXYPosition();

//  what is the winding order ???
bool cw = isOrderClockwise(v1, v2, v3);

but I have no clue how to calculate the order ???
(maybe I'm missing something obvious )

[Brainsaw]

Hmm ... not quite sure, but maybe this article could help: http://www.cgafaq.info/wiki/Simple_Polygon_Orientation

[Lonesome Ducky]

The widely accepted method (requiring a known normal), is to compare the dot product of the known normal, and the normal calculated by normalize((V2 - V1) x (V3 - V1)). If it is close to 0, it is counter clockwise, otherwise it is clockwise.

[Acki]

ahhh, normals and dot product !?!?!
but how do you define "close to 0" ???

well, I'll try it...

thank you both !!!

[slavik262]

Irrlicht doesn't employ the use of normals to determine if the triangle is front or back facing though. You can have really messed up normals or no normals at all and it will still properly determine if the triangle is front or back facing.

[Acki]

while my researches I also found that there are solutions with cross products...
but after the keyword "normals" I came up with this "easy" solution:

Code: Select all

bool isOrderClockwise(vector3df v0, vector3df v1, vector3df v2){
  v0.Z = v1.Z = v2.Z = 0.0; // I want the triangle only to be on the xy-plane
  vector3df normal = plane3df(v0, v1, v2).Normal;
  return (normal.Z < 0.0);
}

thanks again, for the hints !!!
cu, Acki

[Lonesome Ducky]

Irrlicht doesn't employ the use of normals to determine if the triangle is front or back facing though. You can have really messed up normals or no normals at all and it will still properly determine if the triangle is front or back facing.

Of course, but the normal calculated by normalize((V2 - V1) x (V3 - V1)) will return a normal that is the opposite of the known normal if the winding is not counter clockwise.

[BlindSide]

You can try getting the middle point and then calculating the angle around that point for a certain axis?

[terier]

If you want only the 2D solution then here's my solution:
- take a point (P3)
- make a line out of other points (P1 --> P2)
- determine if the point is BELOW or ABOVE the line
- below: negative orientation of the tri.
- above: positive orientation of the tri.