jsketcher/web/app/3d/triangulation.js

import libtess from 'libtess'

function initTesselator() {
  // function called for each vertex of tesselator output
  function vertexCallback(data, polyVertArray) {
    polyVertArray.push(data);
  }
  function begincallback(type) {
    if (type !== libtess.primitiveType.GL_TRIANGLES) {
      console.log('expected TRIANGLES but got type: ' + type);
    }
  }
  function errorcallback(errno) {
    console.log('error callback');
    console.log('error number: ' + errno);
  }
  // callback for when segments intersect and must be split
  function combinecallback(coords, data, weight) {
    // console.log('combine callback');
    return [coords[0], coords[1], coords[2]];
  }
  function edgeCallback(flag) {
    // don't really care about the flag, but need no-strip/no-fan behavior
    // console.log('edge flag: ' + flag);
  }

  var tessy = new libtess.GluTesselator();
  // tessy.gluTessProperty(libtess.gluEnum.GLU_TESS_WINDING_RULE, libtess.windingRule.GLU_TESS_WINDING_POSITIVE);
  tessy.gluTessCallback(libtess.gluEnum.GLU_TESS_VERTEX_DATA, vertexCallback);
  tessy.gluTessCallback(libtess.gluEnum.GLU_TESS_BEGIN, begincallback);
  tessy.gluTessCallback(libtess.gluEnum.GLU_TESS_ERROR, errorcallback);
  tessy.gluTessCallback(libtess.gluEnum.GLU_TESS_COMBINE, combinecallback);
  tessy.gluTessCallback(libtess.gluEnum.GLU_TESS_EDGE_FLAG, edgeCallback);

  return tessy;
}

export function Triangulate(contours, normal) {
  const tessy = initTesselator();
  // libtess will take 3d verts and flatten to a plane for tesselation
  // since only doing 2d tesselation here, provide z=1 normal to skip
  // iterating over verts only to get the same answer.
  // comment out to test normal-generation code
  //tessy.gluTessNormal(0, 0, 1);
  tessy.gluTessNormal(normal[0], normal[1], normal[2]);
  
  const triangleVerts = [];
  tessy.gluTessBeginPolygon(triangleVerts);

  for (let contour of contours) {
    tessy.gluTessBeginContour();
    for (let coords of contour) {
      tessy.gluTessVertex(coords, coords);
    }
    tessy.gluTessEndContour();
  }

  // finish polygon (and time triangulation process)
  tessy.gluTessEndPolygon();
  return triangleVerts;
}