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Chapter 2 ■ Edge-Detection Techniques 67

/* Follow the gradient direction, as indicated by the direction of
the vector (xc, yc); retain pixels that are a local maximum. */

if (fabs(yc) > fabs(xc))
{

/* The Y component is biggest, so gradient direction is
basically UP/DOWN */
xx = fabs(xc)/fabs(yc);
yy = 1.0;

g2 = norm (dx[i-1][j], dy[i-1][j]);
g4 = norm (dx[i+1][j], dy[i+1][j]);
if (xc*yc > 0.0)
{
g3 = norm (dx[i+1][j+1], dy[i+1][j+1]);
g1 = norm (dx[i-1][j-1], dy[i-1][j-1]);
} else
{
g3 = norm (dx[i+1][j-1], dy[i+1][j-1]);
g1 = norm (dx[i-1][j+1], dy[i-1][j+1]);
}

} else
{

/* The X component is biggest, so gradient direction is
basically LEFT/RIGHT */
xx = fabs(yc)/fabs(xc);
yy = 1.0;

g2 = norm (dx[i][j+1], dy[i][j+1]);
g4 = norm (dx[i][j-1], dy[i][j-1]);
if (xc*yc > 0.0)
{
g3 = norm (dx[i-1][j-1], dy[i-1][j-1]);
g1 = norm (dx[i+1][j+1], dy[i+1][j+1]);
}
else
{
g1 = norm (dx[i-1][j+1], dy[i-1][j+1]);
g3 = norm (dx[i+1][j-1], dy[i+1][j-1]);
}
}

/* Compute the interpolated value of the gradient magnitude */
if ( (g > (xx*g1 + (yy-xx)*g2)) &&
(g > (xx*g3 + (yy-xx)*g4)) )