# FreeBSD Manual Pages

FILTERS(9.1) FILTERS(9.1)NAMEadapt, ahe, crispen, laplace, edge, edge2, edge3, extremum, median, nonoise, smooth, shadepic - image neighborhood operatorsSYNOPSISfb/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[-lxyz] [input]DESCRIPTIONGathered 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.Adaptperforms adaptive contrast enhancement by examining the 7x7 region centered on each input pixel, remapping the center pixel lin- early 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.Aheperforms 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 countsA1/2 for each pixel equal to the center value.) Output pixel values are 255 times the count divided by the window size.Crispenexamines 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.Laplacecomputes 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 thancrispen.Edge,edge2, andedge3detect edges in various ways.Edgeexamines the 3x3 region surrounding each input pixel, outputting 8 times the center value minus the sum of its eight neighbors.Edge2applies 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.Edge3likewise approximates the gradient of the input image. The out- put is roughly the phase angle of the gradient approximation, scaled between 0 and 255.Extremumexamines the 3x3 region surrounding each input pixel, out- putting the value that differs most from the center value. In case of a tie, the larger candidate is chosen.Mediandoes noise reduction by replacing each pixel of the input image by the median of the 3x3 region surrounding it.Nonoiseimplements the Bayer-Powell noise reduction filter. It com- putes 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.Smoothlow-pass filters its input image by convolution with a Bartlett window.Shadepictreats 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 reflection.)SOURCE/sys/src/fbSEE ALSOpicfile(9.6)BUGSThere are too many weird wired-in sizes. FILTERS(9.1)

NAME | SYNOPSIS | DESCRIPTION | SOURCE | SEE ALSO | BUGS

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