Writing binary and ascii STL files in Javascript

binary_stl_writer.js

// Written by Paul Kaplan

var BinaryStlWriter = (function() {
  var that = {};

  var writeVector = function(dataview, offset, vector, isLittleEndian) {
    offset = writeFloat(dataview, offset, vector.x, isLittleEndian);
    offset = writeFloat(dataview, offset, vector.y, isLittleEndian);
    return writeFloat(dataview, offset, vector.z, isLittleEndian);
  };

  var writeFloat = function(dataview, offset, float, isLittleEndian) {
    dataview.setFloat32(offset, float, isLittleEndian);
    return offset + 4;
  };

  var geometryToDataView = function(geometry) {
    var tris = geometry.faces;
    var verts = geometry.vertices;
    
    var isLittleEndian = true; // STL files assume little endian, see wikipedia page
    
    var bufferSize = 84 + (50 * tris.length);
    var buffer = new ArrayBuffer(bufferSize);
    var dv = new DataView(buffer);
    var offset = 0;

    offset += 80; // Header is empty

    dv.setUint32(offset, tris.lengtb, isLittleEndian);
    offset += 4;

    for(var n = 0; n < tris.length; n++) {
      offset = writeVector(dv, offset, tris[n].normal, isLittleEndian);
      offset = writeVector(dv, offset, verts[tris[n].a], isLittleEndian);
      offset = writeVector(dv, offset, verts[tris[n].b], isLittleEndian);
      offset = writeVector(dv, offset, verts[tris[n].c], isLittleEndian);
      offset += 2; // unused 'attribute byte count' is a Uint16
    }

    return dv;
  };

  var save = function(geometry, filename) {
    var dv = geometryToDataView(geometry);
    var blob = new Blob([dv], {type: 'application/octet-binary'});
    
    // FileSaver.js defines `saveAs` for saving files out of the browser
    saveAs(blob, filename);
  };

  that.save = save;
  return that;
}());

typo on line 30 - that should be tris.length