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NAME

       im_sharpen - simple coring edge enhancement

SYNOPSIS

       #include <vips/vips.h>

       int
       im_sharpen( in, out, mask_radius, x1, y2, y3, m1, m2 )
       IMAGE *in, *out;
       int mask_radius;
       double x1, y2, y3;
       double m1, m2;

DESCRIPTION

       This  function  selectively  sharpens the L* channel of a LABPACK coded
       image. It is suitable for preparing an image for printing, where  edges
       need  to  be boosted to compensate for blurring introduced by the half-
       tone process, and also for giving any additional ‘crispening’ required.

       The function operates as:

         in    gaussian                                   out
         --+-> blur with ----> subtract --> LUT --> add ---->
           |   mask_radius        ^                  ^
           |                      |                  |
           +----------------------+                  |
           |                                         |
           +-----------------------------------------+

       In  other  words,  the  L*  channel is smoothed with a gaussian average
       function of radius mask_radius and this smoothed  image  is  subtracted
       from the original L* to generate a high-frequency signal.

       This  high-frequency signal is passed through a look-up table specified
       by the x1, y2, y3, m1 and m2 parameters and added back to the  original
       L* channel to generate the sharpened image.

       The LUT is shaped as:

                             ^
                          y2 |- - - - - -----------
                             |         /
                             |        / slope m2
                             |    .../
                     -x1     | ...   |
         -------------------...---------------------->
                     |   ... |      x1
                     |... slope m1
                     /       |
                    / m2     |
                   /         |
                  /          |
                 /           |
                /            |
         ______/ _ _ _ _ _ _ | -y3
                             |

       When  trying  to  understand  the  meaning  of  these parameters, it is
       helpful to imagine a cross-section through an image. Sharpening filters
       boost  apparent  sharpness  by performing the following transformation.
       Consider a cross-section through a soft edge:

                             ^
                             |          ..........
                             |      ....
                             |   ...
                             | ..
                             |.
         --------------------+------------------->
                            .|
                          .. |
                       ...   |
                   ....      |
         ..........          |
                             |

       This becomes an enhanced edge, perhaps:

                             ^
                             |   ...    overshoot
                             |  .   .
                             | .     ............
                             | .
                             |.
                              .
                             .
         --------------------+------------------->
                             .
                            .|
                            .|
                           . |
         .............     . |
                      .   .  |
           undershoot  ...   |
                             |

       There are two features: the transition from black to white  has  become
       steeper,   and   there   are  positive  and  negative  undershoots  and
       overshoots.

       As a general guide, some overshoot  is  good  for  printing  (it  helps
       exaggerate the edge), but not too much, as you will start to see strong
       white fringes.  Undershoot  introduces  black  lines,  which  are  less
       intrusive, so you can allow more under- than over-shoot.

       im_sharpen  uses  the  x1 parameter to distinguish between low and high
       frequency areas. Pixels which differ by less  than  x1  L*  units  from
       their  local  average  are  sharpened  by factor 1+m1, and pixels which
       differ by more than x1 are sharpened by factor  1+m2.  y2  and  y3  set
       limits on the amount of positive and negative sharpening we allow.

       For printing, we recommend the following settings:

          mask_radius == 7
          x1 == 1.5
          y2 == 20         (don’t brighten by more than 20 L*)
          y3 == 50         (can darken by up to 50 L*)

          m1 == 1          (some sharpening in flat areas)
          m2 == 2          (more sharpening in jaggy areas)

       If  you want more or less sharpening, we suggest you just change the m1
       and m2 parameters. For an extreme sharpen, you might try:

          m1 == 2
          m2 == 4

       And for a relatively gentle sharpen, perhaps:

          m1 == 0.5
          m2 == 1.5

       If you want to adjust the x1 parameter, it can be helpful visualise its
       effect by setting:

          m1 == -2
          m2 == 4

       In other words, heavily blur flat areas, and heavily sharpen elsewhere.
       This creates a terrible-looking image, but you  will  be  able  to  see
       clearly what parts of your image are being classified as flat.

       The  y2 and y3 parameters need not usually be adjusted, unless you wish
       to reduce the strength of the finges.

       The mask_radius parameter changes the width of the fringe  and  can  be
       adjusted according to the output printing resolution. As an approximate
       guideline, use 3 for 4 pixels/mm (CRT  display  resolution),  5  for  8
       pixels/mm,  7  for  12  pixels/mm and 9 for 16 pixels/mm (300 dpi == 12
       pixels/mm). These figures refer to the image raster, not the  half-tone
       resolution.

RETURN VALUE

       The function returns 0 on success and -1 on error.

SEE ALSO

       im_conv(3), im_compass(3), im_fastcor(3).

COPYRIGHT

       National Gallery and Birkbeck College, 1995

THANKS TO

       Lindsay!

                                  16 May 1995