UV winding order issue

[miguel-winfield]

Hello~ Is there anyway  in API to filter out all the flipped UV from a selected polygon object.
Maya display this kind of UV shelf as red and normal UV shelf as blue. and its manual tells me that this kind of UV has different winding order, but I havent find an aswer to check if it is in reverse winding order or not. 

Anyone any idea? Thanks a lot  

[joojaa]

windong order points come in clockwise or not. Simple stuff really cross product point then next negative is other way positive other.

[miguel-winfield]

You mean cross product for two UV point?  
I quickly try it in MEL first but I guess I must have misunderstood something. 
These must be wrong but I post it here anyway.

global proc float [] cross_product(float $a [], float $b []) 

{

float $c[];

$c[0] = $a[1]$b[2] - $a[2]$b[1];

$c[1] = $a[2]$b[0] - $a[0]$b[2];

$c[2] = $a[0]$b[1] - $a[1]$b[0];

return $c;

}

float $a[]=polyEditUV -q -u pCube1.map[7];

float $b[]=polyEditUV -q -u pCube1.map[14];

print cross\_product $a $b;

float $c[]=polyEditUV -q -u pCube1.map[8];

float $d[]=polyEditUV -q -u pCube1.map[9];

print cross\_product $c $d;

 

[joojaa]

umm no, each face is a list of connections in order, so the winding order is the vector form faces first point to faces second point times vector form second point to third point. This si a bit awkward to do in mel because you can only get stringdumps of the face orders and youd need to parse the data yourself.

you can use polyInfo to retrieve this info but its much easier to just use openMaya in python, Since you would first need the winding order of the face to deduce the vindingorder of the uv.

PS: use the builtin cross function, tip you can cast 3 long float arrays to vectors and vice versa:

vector $pos={1.0,2.0,4.0};
// Result: <>  //
float $flt[]=$pos;
// Result: 1 2 4 //

[miguel-winfield]

Thanks for the reply, Joojaa.

" Since you would first need the winding order of the face to deduce the vindingorder of the uv."

Does this mean first check if the 3D face is actually flipped in normal , then check the uv winding order? 
Am confused already,

So the check below is to check the face normal?

"so the winding order is the vector form faces first point to faces second point times vector form second point to third point. " 

Sorry my math has gone back to my math teacher for many years~~ Am trying to search and pick it up,  Could you give me more hint?  A train of thought in API way would be quite helpful . please

[miguel-winfield]

You mean use these method to get the point data?

 MFnMesh::getFaceVertexNormal ( int  faceIndex, int  vertexIndex, MVector &  normal, MSpace::Space  space = MSpace::kObject   )

Or this one?

 MFnMesh::getPoint ( int  vertexId, MPoint &  pos, MSpace::Space  space = MSpace::kObject   ) 

They are for checking the face normal, aren't they?

How to get the vector you describe before?
thank you very much. 

[joojaa]

No,

see maya and allmost universally all 3d stores indices not points. so each face is a list of point index numbers, winding order is just if theyt are defined clockwise or counterclockwise thats all. (direction is defined as clockwise is towards you counter away from you wich incidenttaly is the cross product)

Please NOTE: its not a property of a point, its a property of a FACE! That is the property comes from how the face is connected to the points.

Draw op paper 3 points and connect them in order, if you connected them in clockwise manner you have a different winding order than if you connected them in counterclockwise order.

  MFnMesh::getFaceVertexNormal ( int  faceIndex, int  vertexIndex, MVector &  normal, MSpace::Space  space =

  MFnMesh::getPoint ( int  vertexId, MPoint &  pos, MSpace::Space  space = MSpace::kObject   )

neither you need to iterate the faces uv indices.

[joojaa]

Ok, drew a image question is if its face 1 or face 2 type cross(AB,BC) has different z direction than corss(AC,CB) (right handed rule and cross product):

[miguel-winfield]

Thanks for your reply and the diagram, Joojaa~ 

I think I need to sit down and learn the base of the maya mesh architecture before I try to develop a model plugin. I will be back if I still have some problems. Thanks man~

[joojaa]

Well this is sortof base 3d stuff. Even opengl works like this. Granted it does not bother with windingorder of uvs but its still the same thing, just uvs not points in sapce form the veoctors form indices.

[miguel-winfield]

So do you mean:

get all face vertex uv  by itering through every face.

get the vector from the point faceuv[0] to faceuv[1] and faceuv[1] to faceuv[2] 

get the cross product of these two vectors

check the z value of the product, negative  means flipped??? 

[joojaa]

Yes.

[miguel-winfield]

Hello~~
I try this in API codes, and find it works, but for some polygons it just goes wrong :
 
If check the crossproduct of vector(1,2) and vector (2,3), it is still possitive.
So I iter through all face UVs, and check all the possible corss products, and see that if the negative z are more than possitive z or not. if negative more, then it is flipped.

But soon finds there still be tons of exceptions(althrough these condition wont be that much in model workflow, but still need to be considered in. ):
 
It seems that there must be better way to get around this. 

Thanks beforehand!!

[joojaa]

Yes because its a concave polygon. The rule actually only works for the trangulated data sorray about that*. If your polygon is concave then you need to ask for the traingulation, cheking the first triangle is enough. Becuase the rest has a the same function.

*ogl works on the triangulation mostly.

[miguel-winfield]

Sorry got busy outside these days.
Am back and find your reply,  I will try it  
thanks Joojaa, u r really wonderful. 

[joojaa]

Yeah mee too. Trying to build a 200-300$ (excluding cost of software) handheld 3d scanner, seems to wrok out pretty well so far Still anoyingly useless tough*.

• im finding ita  bit hard to calibrate properly at the moment.

[sebastien-paviot]

I was working on some unwrapping script and was wondering what would be the more sensible thing to do to deal with flip UVs and how to check for them. 

thanks a lot for the detailed explainations