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GRADIENT

Section: C Library Functions (3)
Updated: 16 Mar 1998
Index

NAME

gradient, grad, prewitt, roberts, sobel - gradientoperator for edge detection

SYNTAX

int gradient( IBAND input, ISS_BAND convx,
    ISS_BAND convy, double xscale,
    double yscale, IBAND magnitude, IBAND angle,
    IBAND pseudo, int verbose );

int sobel( IBAND input, IBAND magnitude,
IBAND angle, IBAND pseudo, int verbose );

int prewitt( IBAND input, IBAND magnitude,
IBAND angle, IBAND pseudo, int verbose );

int roberts( IBAND input, IBAND magnitude,
IBAND angle, IBAND pseudo, int verbose );

int grad( IBAND input, IBAND magnitude,
IBAND angle, IBAND pseudo, int verbose );

DESCRIPTION

gradientperforms a convolution of inputand convx,scaled by xscale(or 0.0 for autoscaling), giving gx,and a convolution of inputand convy,scaled by yscale((or 0.0 for autoscaling), giving gy.gxand gyarecombined into magnitude, angle and a pseudo image.If verboseis TRUE some messages are witten with Info().

magnitude = |gx| + |gy|
angle = atan(gy/gx);
pseudo = combination of magnitude and angle.

sobelcalculates gx and gy as

        -1 0 1             -1 -2 -1
gx =   -2 0 2       gy =   0  0  0
        -1 0 1              1  2  1

prewittcalculates gx and gy as

        -1 0 1             -1 -1 -1
gx =   -1 0 1       gy =   0  0  0
        -1 0 1              1  1  1

robertscalculates gx and gy as

        1  0              0  1
gx =               gy =
        0 -1             -1  0

gradcalculates gx and gy as

       -1  1             -1  0
gx =               gy =
        0  0              1  0

PSEUDO

The pseudo comination of magnitude and pseudo:
mag level = magnitude / 32;
  Level  First Length
      0      0      4
      1      4     12
      2     16     20
      3     36     28
      4     64     36
      5    100     44
      6    144     52
      7    196     60
pseudo = first[mag_level] + angle/256* length[mag_level];

The pseudo-image may be displayed with a "gradient"color-table.

AUTHOR

Otto Milvang

ID

$Id: gradient_op.c,v 1.34 1997/06/11 09:44:52 svein Exp $

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