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 ½ for each pixel equal to the center value.)
Output pixel values are 255 times the count divided by the
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FILTERS(9.1) FILTERS(9.1)
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, out-
putting 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, outputting the vector length of the gradi-
ent 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 cen-
ter value. In case of a tie, the larger candidate is cho-
sen.
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
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FILTERS(9.1) FILTERS(9.1)
/sys/src/fb
SEE ALSO
picfile(9.6)
BUGS
There are too many weird wired-in sizes.
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