index.js

module.exports = geoArc;

function geoArc(options) {

  var geo = {
    positions: [],
    cells: [],
    uvs: []
  };

  options = options || {};
  options.cellSize = options.cellSize || 3;
  options.x = options.x || 0;
  options.y = options.y || 0;
  options.z = options.z || 0;
  options.startRadian = options.startRadian || 0;
  options.endRadian = options.endRadian || Math.PI * 1.5;
  options.innerRadius = typeof options.innerRadius == 'number' ? options.innerRadius : 40;
  options.outerRadius = options.outerRadius || 200;
  options.numBands = options.numBands || 2;
  options.numSlices = options.numSlices || 40;
  options.drawOutline = options.drawOutline !== undefined ? options.drawOutline : true;

  createGeometry(options, geo.positions, geo.cells, geo.uvs);

  return geo;
}

function createGeometry(options, positions, cells, uvs) {

    var o = options;
    var idxSize = o.cellSize;
    var radDist = o.endRadian - o.startRadian;
    var numSlices = Math.floor(Math.abs(radDist) / (Math.PI * 2) * o.numSlices);
    var radInc = radDist / numSlices;
    var numBandIncs = (o.numBands == 1) ? 1 : o.numBands - 1;
    var bandInc = (o.outerRadius - o.innerRadius) / numBandIncs;
    var cRad, x, y, z, cRadius, curSlideIdx, prevSlideIdx;

  for(var i = 0, len = numSlices; i <= len; i++) {

    cRad = i * radInc + o.startRadian;
    prevSlideIdx = (i - 1) * o.numBands;
    curSlideIdx = i * o.numBands;

    for(var j = 0, lenJ = o.numBands; j < lenJ; j++) {

      cRadius = o.innerRadius + bandInc * j;

      x = Math.cos(cRad) * cRadius + o.x;
      y = o.y;
      z = Math.sin(cRad) * cRadius + o.z;

      positions.push([ x, y, z ]);
      uvs.push([i/numSlices, j/numBandIncs])

      //if we've added in positions then we'll add cells
      if(idxSize == 1) {

        cells.push([ curSlideIdx + j ]);
      } else if(idxSize == 2) {

        if(i > 0 && j + 1 < lenJ) {

          cells.push( [ 
                        prevSlideIdx + j, 
                        curSlideIdx + j 
                      ]);

          cells.push( [ 
                        curSlideIdx + j + 1, 
                        prevSlideIdx + j + 1 
                      ]);

          if( !o.drawOutline ) {
            
            cells.push( [ 
                          curSlideIdx + j, 
                          curSlideIdx + j + 1 
                        ]);
          }
        }
      } else if(idxSize == 3) {

        if(i > 0 && j + 1 < lenJ) {

          cells.push( [ 
                        curSlideIdx + j,
                        prevSlideIdx + j + 1, 
                        prevSlideIdx + j
                      ]);

          cells.push( [ 
                        curSlideIdx + j, 
                        curSlideIdx + j + 1, 
                        prevSlideIdx + j + 1 
                      ]);
        }
      }
    }
  }

  //cap it off
  if(idxSize == 2) {
    
    // if it's going all the way around then we wont put the connecting line
    if( radDist % Math.PI * 2 != 0 ) {

      for(var j = 0, lenJ = o.numBands - 1; j < lenJ; j++) {

        cells.push([ 
                      curSlideIdx + j, 
                      curSlideIdx + j + 1 ]);
      }

      curSlideIdx = 0;

      for(var j = 0, lenJ = o.numBands - 1; j < lenJ; j++) {

        cells.push([ 
                      curSlideIdx + j, 
                      curSlideIdx + j + 1 ]);
      }
    }
  }
}