FILTERS(9.1)															      FILTERS(9.1)

NAME

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

SYNOPSIS

       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 ]

DESCRIPTION

       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 neighborhood'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 horizontal 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 surrounding each input pixel, outputting 8 times the
       center value minus the sum of its eight neighbors.

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

       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 outputs 255 times its dot product with the unit vector in the light-source direction specified by
       option -l (default 1,-1,1).  If the dot product is negative, it is clamped at zero.  (This computation is just Lambertian  diffuse  reflec-
       tion.)

SOURCE

       /sys/src/fb

SEE ALSO

       picfile(9.6)

BUGS

       There are too many weird wired-in sizes.

																      FILTERS(9.1)