extend a line - improved!

[Hul Tytus]

comp.graphics.algorithms
extending a line in xyz notation

I'm in need of a routine for extending a line as shown by the beginnings
below. As can be seen, it is the merest start. Anyone have some code that does
the job or maybe a suggestion or two? Would appreciate it.

Hul

/*
extend a line dst distance from pt1 perpendicular to line to pt2 and in
the plane and direction defined by pt3. res[] is for the point at the
end of the extended line.
*/
whatev(pt1, pt2, pt3, dst, res)
double pt1[], pt2[], pt3[];
double dst, res[];
{

[Hans-Bernhard Bröker]

Post by Hul Tytus
/*
extend a line dst distance from pt1 perpendicular to line to pt2 and in
the plane and direction defined by pt3. res[] is for the point at the
end of the extended line.
*/
whatev(pt1, pt2, pt3, dst, res)
double pt1[], pt2[], pt3[];
double dst, res[];
{

While it's an improvement that this time round you at least found the
right newsgroup, the problem statement is still lacking clarity to a
degree that makes it almost incomprehensible.

For starters, there is no "plane defined by a point". And "line to pt2"
is equally vague: line from where to pt2?

In other words: please try again, and try to be less cryptic about it.