Simulates a pencil sketch. We convolve the image with a Gaussian operator
   of the given radius and standard deviation (sigma). For reasonable
   results, radius should be larger than sigma.  Use a radius of 0 and
   Imagick::sketchImage() selects a suitable radius for you. Angle gives the
   angle of the blurring motion. 
   This method is available if Imagick has been compiled against ImageMagick version 6.2.9 or newer.