title: Reduce bit-depth of PNG files from the command line
body: <p>What command or series of commands could I execute from the CLI to recursively traverse a directory tree and reduce the bit-depth of all PNG files within that tree from 24bpp to 16bpp? Commands should preserve the alpha layer and should not increase the file size  of the PNGs - in fact a decrease would be preferable.</p>
  
  <p>I have an OSX based system at my disposal and am familiar with the <code>find</code> command so am really more keen to to locate a suitable PNG utility command.</p>