Edit: /usr/share/graphviz/gvpr/color (1440B)
/* color edges based on normalized length alpha.
* If we have lim[i-1] <= alpha <= lim[i], the color
* is linearly interpolated between color[i-1] and color[i].
* Edges already having a color attribute are left unchanged.
*/
BEGIN {
double lens [edge_t];
double maxlen, len, minlen=1.7976931348623157e+308;
double alpha, x0, y0, x1, y1;
double h0, s0, v0;
int i;
int ncolors; /* number of colors: ncolors >= 2 */
/* start of color i.
* lim[0]=0 < lim[1] < ... < lim[ncolors-1]=1
*/
double lim[int];
/* HSV values for color i */
double h[int];
double s[int];
double v[int];
/* simple default */
ncolors = 2;
lim[0] = 0;
lim[1] = 1;
h[0] = 0;
h[1] = 0.67;
s[0] = 1;
s[1] = 1;
v[0] = 1;
v[1] = 1;
}
E{
sscanf ($.tail.pos, "%f,%f", &x0, &y0);
sscanf ($.head.pos, "%f,%f", &x1, &y1);
len = sqrt ((x0-x1)*(x0-x1) + (y0-y1)*(y0-y1));
lens[$] = len;
if (len > maxlen) maxlen = len;
if (len < minlen) minlen = len;
}
BEG_G {
if (!isAttr($,"E","color"))
setDflt($,"E","color","");
}
E{
if ($.color != "") return;
alpha = (lens[$]-minlen)/(maxlen-minlen);
for (i = 1; i < ncolors; i++) {
if (alpha < lim[i]) break;
}
alpha = (alpha - lim[i-1])/(lim[i] - lim[i-1]);
h0 = (1-alpha)*h[i-1] + alpha*h[i];
s0 = (1-alpha)*s[i-1] + alpha*s[i];
v0 = (1-alpha)*v[i-1] + alpha*v[i];
$.color = sprintf ("%.02f %.02f %.02f", h0, s0, v0);
}