// whisk scan parameters
startAngle =  5.0 * (Math.PI / 180.0);
stopAngle  = -5.0 * (Math.PI / 180.0);
spinFactor = 6;

// compute the current fraction of a full scan
fraction = 0.5 - 
    Math.cos( this.currentScanFraction * spinFactor * 2.0 * Math.PI ) / 2.0;

// compute the current scan angle for this fraction
scanAngle = startAngle + fraction * ( stopAngle - startAngle );

// compute the current spin angle for this fraction
spinAngle = this.currentScanFraction * 2.0 * Math.PI;

// compute the rotation transform
transformResult = 
    this.matrixMultiply( this.makeZRotationMatrix( spinAngle ),
                this.makeXRotationMatrix( scanAngle ) );
