code: function handleMouseDown(event) {
          mouseDown = true;
          lastMouseX = event.clientX;
          lastMouseY = event.clientY;
          startX = event.clientX;
          startY = event.clientY;
  
          //get coordinates within canvas (with the right orientation)
          var r = viewer._canvas.getBoundingClientRect();
          var viewX = startX - r.left;
          var viewY = viewer._height - (startY - r.top);
  
          //this is for picking
          id = viewer._getID(viewX, viewY);
  
          /**
          * Occurs when mousedown event happens on underlying canvas.
          *
          * @event xViewer#mouseDown
          * @type {object}
          * @param {Number} id - product ID of the element or null if there wasn't any product under mouse
          */
          viewer._fire('mouseDown', {id: id});
  
  
          //keep information about the mouse button
          switch (event.button) {
              case 0:
                  button = 'left';
                  break;
  
              case 1:
                  button = 'middle';
                  break;
  
              case 2:
                  button = 'right';
                  break;
  
              default:
                  button = 'left';
                  break;
          }
  
          viewer._disableTextSelection();
      }
docs: set initial conditions so that different gestures can be identified