Re: Scripting : Align along normal

[Raffaele Fragapane <jaco(at)thejaco.com>]

to create a 3x3 matrix you need 9 numbers, and that's it :D

however, what you want to do (orienting an object without shearing or 
non-uniform scaling) requires an orthogonal 3x3 matrix.
that is what you need to create a set of 3 axis that define an orientation.
to do that you need 3 normalized vectors orthogonal to each other.

a cross product performed on any 2 non aligned vectors, returns a 3rd 
that is orthogonal to both, which is at the base of the double cross 
product system I pimped in previous emails and posts on other sites.

given one vector (the poly normal, which is what you want to align the Y 
axis to), another vector to up vector this one(safest upvector to use is 
a unit vector aligned along the smallest absolute value of your initial 
vector), and 2 cross products, the result is a set that is guaranteed to 
create an orthogonal matrix.

as for your script, afaik the polygon node collection is meant to 
represent edge junctions in your polygon, therefore the normal of the 
first node you retrieve won't be the same of the polygon's, unless you 
have a completely flat subsection there.

so either use the old geometry2D (which I don't like nor reccomend, but 
everytime I neglected it I got told off :P) or use facets or the 
normalsArray if you want a polygon's normal without the overhead of 
extrapolating it yourself from the points' normals.

and after you get a normal from your own operations always normalize it 
in place, even if it shouldn't be a problem when you operate on already 
normalized vectors and a set number of operands, precision errors 
creeping in at any point in transformations are always pesky to track if 
they cascade down.

******************************
|     Raffaele Fragapane       |
|     Rising Sun Pictures      |
| "Remember, TD is for TopDog" |
******************************



guillaume laforge wrote:

> Thanks for the help Kim. Raf function gives me a clearer vision of 
> matrix use with XSI :-). So two vector cross products are needed to 
> create the 3X3 matrix ? Not so complicated :-)
>  
> However, I found my problem. It came from a wrong normal vector. Maybe 
> some users on this list learn 3d math like me so here is the answer : 
>  
>  In my previous script the polygon normal is :
> //---------------------------------------------------------
> var polySel = Selection.item(0).subcomponent.componentcollection;
> var oPolygonFace = polySel(0);
> //Get the Polygon Normal
> var oPolygonNode = oPolygonFace.Nodes(0);
> var oNormal = oPolygonNode.Normal;
> //-------------------------------------------------------
>  
> In fact it returns a point normal.
> So I use this snipet to find the polygon normal :
>
> //---------------------------------------------------------
> //Get the selected polygon
> var polySel = Selection.item(0).subcomponent.componentcollection;
> var oPolygonFace = polySel(0);
> nbPoints = oPolygonFace.NeighborVertices (1);
> var oNormal = XSIMath.CreateVector3();
> for(i=0;i<nbPoints.count;i++)
> {
>    oNormal.AddInPlace(nbPoints.item(i).normal);
> }
> oNormal.ScaleInPlace(1/nbPoints.count);
> //---------------------------------------------------------
>
> Thanks to Heldge Mathee for his scripting tutorials (Getting Started 
> with Scripting in SOFTIMAGE|XSI ). It helps a lot !
>
> Cheers,
>
> Guillaume Laforge
> Cg Artist | Jr Td
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