title: Image filtering with scikit-image?
body: <p>I'm moving to python from a Matlab background, and there are a few elementary operations I've yet to conquer in Python/skimage:</p>
  
  <ol>
  <li><p>How can I apply a user-generated linear filter (given as a small 2d array) to an image?  I can do it with scipy.ndimage.convolve, but is there a method in skimage?</p></li>
  <li><p>In Matlab, image filtering always returns a result of the same numeric type as its input, be it uint8 or float.  Does skimage behave the same way?</p></li>
  <li><p>Does skimage include unsharp masking somewhere?  (I've found an unsharp masking filter in PIL but that's a bit of a pain, as PIL uses its own Image class, rather than ndarrays).</p></li>
  <li><p>Is there a method, maybe similar to Matlab's "colfilt" by which a user can apply a non-linear filter to an image?  The idea is that the user supplies a function which produces a single number from a 3x3 array, say; then that function is applied across the image as a spatial filter.</p></li>
  </ol>