Question

I have 3d mesh and I would like to draw each face a 2d shape.

What I have in mind is this:
for each face
1. access the face normal
2. get a rotation matrix from the normal vector
3. multiply each vertex to the rotation matrix to get the vertices in a ‘2d like ‘ plane
4. get 2 coordinates from the transformed vertices

I don’t know if this is the best way to do this, so any suggestion is welcome.

At the moment I’m trying to get a rotation matrix from the normal vector,
how would I do this ?

UPDATE:

Here is a visual explanation of what I need:

At the moment I have quads, but there’s no problem
converting them into triangles.

I want to rotate the vertices of a face, so that
one of the dimensions gets flattened.

I also need to store the original 3d rotation of the face.
I imagine that would be inverse rotation of the face
normal.

I think I’m a bit lost in space 🙂

Here’s a basic prototype I did using Processing:

void setup(){
  size(400,400,P3D);
  background(255);
  stroke(0,0,120);
  smooth();
  fill(0,120,0);

  PVector x = new PVector(1,0,0);
  PVector y = new PVector(0,1,0);
  PVector z = new PVector(0,0,1);

  PVector n  = new PVector(0.378521084785,0.925412774086,0.0180059205741);//normal
  PVector p0 = new PVector(0.372828125954,-0.178844243288,1.35241031647);
  PVector p1 = new PVector(-1.25476706028,0.505195975304,0.412718296051);
  PVector p2 = new PVector(-0.372828245163,0.178844287992,-1.35241031647);
  PVector p3 = new PVector(1.2547672987,-0.505196034908,-0.412717700005);

  PVector[] face = {p0,p1,p2,p3};
  PVector[] face2d = new PVector[4];
  PVector   nr = PVector.add(n,new PVector());//clone normal

  float rx = degrees(acos(n.dot(x)));//angle between normal and x axis
  float ry = degrees(acos(n.dot(y)));//angle between normal and y axis
  float rz = degrees(acos(n.dot(z)));//angle between normal and z axis

  PMatrix3D r = new PMatrix3D();
  //is this ok, or should I drop the builtin function, and add 
  //the rotations manually
  r.rotateX(rx);
  r.rotateY(ry);
  r.rotateZ(rz);

  print("original: ");println(face);
  for(int i = 0 ; i < 4; i++){
    PVector rv = new PVector();
    PVector rn = new PVector();
    r.mult(face[i],rv);
    r.mult(nr,rn);
    face2d[i] = PVector.add(face[i],rv);
  }
  print("rotated: ");println(face2d);
  //draw
  float scale = 100.0;
  translate(width * .5,height * .5);//move to centre, Processing has 0,0 = Top,Lef
  beginShape(QUADS);
  for(int i = 0 ; i < 4; i++){
   vertex(face2d[i].x * scale,face2d[i].y * scale,face2d[i].z * scale);
  }
  endShape();
  line(0,0,0,nr.x*scale,nr.y*scale,nr.z*scale);

  //what do I do with this ?
  float c = cos(0), s = sin(0);
  float x2 = n.x*n.x,y2 = n.y*n.y,z2 = n.z*n.z; 
  PMatrix3D m = new PMatrix3D(x2+(1-x2)*c,  n.x*n.y*(1-c)-n.z*s,  n.x*n.z*(1-c)+n.y*s,  0,
                              n.x*n.y*(1-c)+n.z*s,y2+(1-y2)*c,n.y*n.z*(1-c)-n.x*s,0,
                              n.x*n.y*(1-c)-n.y*s,n.x*n.z*(1-c)+n.x*s,z2-(1-z2)*c,0,
                              0,0,0,1);
}

Update

Sorry if I’m getting annoying, but I don’t seem to get it.

Here’s a bit of python using Blender‘s API:

import Blender
from Blender import *
import math
from math import sin,cos,radians,degrees

def getRotMatrix(n):
    c = cos(0)
    s = sin(0)
    x2 = n.x*n.x
    y2 = n.y*n.y
    z2 = n.z*n.z
    l1 = x2+(1-x2)*c, n.x*n.y*(1-c)+n.z*s, n.x*n.y*(1-c)-n.y*s
    l2 = n.x*n.y*(1-c)-n.z*s,y2+(1-y2)*c,n.x*n.z*(1-c)+n.x*s
    l3 = n.x*n.z*(1-c)+n.y*s,n.y*n.z*(1-c)-n.x*s,z2-(1-z2)*c
    m = Mathutils.Matrix(l1,l2,l3)
    return m

scn = Scene.GetCurrent()
ob = scn.objects.active.getData(mesh=True)#access mesh

out = ob.name+'\n'
#face0
f = ob.faces[0]
n = f.v[0].no
out += 'face: ' + str(f)+'\n'
out += 'normal: ' + str(n)+'\n'

m = getRotMatrix(n)
m.invert()

rvs = []
for v in range(0,len(f.v)):
    out += 'original vertex'+str(v)+': ' + str(f.v[v].co) + '\n'
    rvs.append(m*f.v[v].co)

out += '\n'
for v in range(0,len(rvs)):
    out += 'original vertex'+str(v)+': ' + str(rvs[v]) + '\n'

f = open('out.txt','w')
f.write(out)
f.close

All I do is get the current object, access the first face, get the normal, get the vertices, calculate the rotation matrix, invert it, then multiply it by each vertex.
Finally I write a simple output.

Here’s the output for a default plane for which I rotated all the vertices manually by 30 degrees:

Plane.008
face: [MFace (0 3 2 1) 0]
normal: [0.000000, -0.499985, 0.866024](vector)
original vertex0: [1.000000, 0.866025, 0.500000](vector)
original vertex1: [-1.000000, 0.866026, 0.500000](vector)
original vertex2: [-1.000000, -0.866025, -0.500000](vector)
original vertex3: [1.000000, -0.866025, -0.500000](vector)

rotated vertex0: [1.000000, 0.866025, 1.000011](vector)
rotated vertex1: [-1.000000, 0.866026, 1.000012](vector)
rotated vertex2: [-1.000000, -0.866025, -1.000012](vector)
rotated vertex3: [1.000000, -0.866025, -1.000012](vector)

Here’s the first face of the famous Suzanne mesh:

Suzanne.001
face: [MFace (46 0 2 44) 0]
normal: [0.987976, -0.010102, 0.154088](vector)
original vertex0: [0.468750, 0.242188, 0.757813](vector)
original vertex1: [0.437500, 0.164063, 0.765625](vector)
original vertex2: [0.500000, 0.093750, 0.687500](vector)
original vertex3: [0.562500, 0.242188, 0.671875](vector)

rotated vertex0: [0.468750, 0.242188, -0.795592](vector)
rotated vertex1: [0.437500, 0.164063, -0.803794](vector)
rotated vertex2: [0.500000, 0.093750, -0.721774](vector)
rotated vertex3: [0.562500, 0.242188, -0.705370](vector)

The vertices from the Plane.008 mesh are altered, the ones from Suzanne.001’s mesh
aren’t. Shouldn’t they ? Should I expect to get zeroes on one axis ?
Once I got the rotation matrix from the normal vector, what is the rotation on x,y,z ?

Note: 1. Blender’s Matrix supports the * operator 2.In Blender’s coordinate system Z point’s up. It looks like a right handed system, rotated 90 degrees on X.

Thanks

Answer 1

That looks reasonable to me. Here’s how to get a rotation matrix from normal vector. The normal is the vector. The angle is 0. You probably want the inverse rotation.

Is your mesh triangulated? I’m assuming it is. If so, you can do this, without rotation matrices. Let the points of the face be A,B,C. Take any two vertices of the face, say A and B. Define the x axis along vector AB. A is at 0,0. B is at 0,|AB|. C can be determined from trigonometry using the angle between AC and AB (which you get by using the dot product) and the length |AC|.