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FILTERS(9.1)									     FILTERS(9.1)

       adapt,  ahe,  crispen,  laplace,  edge,	edge2,	edge3, extremum, median, nonoise, smooth,
       shadepic - image neighborhood operators

       fb/adapt [ input ]

       fb/ahe [ input ]

       fb/crispen [ input ]

       fb/laplace [ input ]

       fb/edge [ input ]

       fb/edge2 [ input ]

       fb/edge3 [ input ]

       fb/extremum [ input ]

       fb/median [ input ]

       fb/nonoise [ input ]

       fb/smooth [ input ]

       fb/shadepic [ -lx y z ] [ input ]

       Gathered here are descriptions of programs that compute the pixels of an output	image  by
       performing  some  operation  on a neighborhood of each pixel of their input image (default
       standard input).  Each program writes the output image on standard output.   The  programs
       process multi-channel inputs by treating each channel independently.

       Adapt  performs adaptive contrast enhancement by examining the 7x7 region centered on each
       input pixel, remapping the center pixel linearly in a way that would  send  the	neighbor-
       hood's  maximum	value to 255 and its minimum to 0.  To avoid divide checks, no mapping is
       done if all pixels in the region have the same value.

       Ahe performs adaptive histogram equalization by examining the  17x17  region  centered  on
       each input pixel, counting the number of pixels whose value is less than the center pixel.
       (It counts 1/2 for each pixel equal to the center value.)  Output  pixel  values  are  255
       times the count divided by the window size.

       Crispen examines the 3x3 region surrounding each input pixel, computing 9 times the center
       pixel minus the sum of its eight neighbors.  This is a fairly extreme high-pass filter and
       sharpens edges substantially.

       Laplace	computes 5 times the center pixel minus the sum of its four vertical and horizon-
       tal neighbors.  This adds a 3x3 discrete Laplacian to the original image, and  is  a  less
       extreme high-pass filter than crispen.

       Edge,  edge2,  and  edge3 detect edges in various ways.	Edge examines the 3x3 region sur-
       rounding each input pixel, outputting 8 times the center value minus the sum of its  eight

       Edge2  applies  a Sobel operator to the input image.  It approximates the image's gradient
       by finite differences on a 3x3 neighborhood, outputting the vector length of the  gradient

       Edge3  likewise	approximates  the gradient of the input image.	The output is roughly the
       phase angle of the gradient approximation, scaled between 0 and 255.

       Extremum examines the 3x3 region surrounding each input pixel, outputting the  value  that
       differs most from the center value.  In case of a tie, the larger candidate is chosen.

       Median  does  noise  reduction by replacing each pixel of the input image by the median of
       the 3x3 region surrounding it.

       Nonoise implements the Bayer-Powell noise reduction filter.  It computes the average value
       of  the eight neighbors of each pixel of the input image, and substitutes it for the pixel
       value if the two differ by more than 64.

       Smooth low-pass filters its input image by convolution with a Bartlett window.

       Shadepic treats its input image as an array of elevations.  At each pixel it  approximates
       the normal vector to the height-field by finite differences on a 3x3 neighborhood and out-
       puts 255 times its dot product with the unit vector in the light-source	direction  speci-
       fied  by  option  -l  (default  1,-1,1).  If the dot product is negative, it is clamped at
       zero.  (This computation is just Lambertian diffuse reflection.)



       There are too many weird wired-in sizes.

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