Thread Subject: path along surf plot

Hi everybody,

starting from many (x,y,z) points, I've built a 3d surface
plot with the following code:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
xlin = linspace(min(x),max(x),35);
ylin = linspace(min(y),max(y),35);
[X,Y] = meshgrid(xlin,ylin);

Z = griddata(x,y,z,X,Y,'cubic');

mesh(X,Y,Z) %interpolated
axis tight; hold on
plot3(x,y,z,'.','MarkerSize',15)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

What I would like to do now is to plot a certain line laying
on this surface, connecting some points, in order to
highligh a certain path on the surface itself.
The surface is not regular, so if I just fit the points I
wanna connect the line intersect the surface, it doesn't
follow its shape.
Any ideas?
If I need to change the code for plotting the surface, no
problem, I'm more interested in the "line" than in the surface.

Many thanks in advance

> What I would like to do now is to plot a certain line
laying
> on this surface, connecting some points, in order to
> highligh a certain path on the surface itself.
> The surface is not regular, so if I just fit the points I
> wanna connect the line intersect the surface, it doesn't
> follow its shape.
> Any ideas?
> If I need to change the code for plotting the surface, no
> problem, I'm more interested in the "line" than in the
surface.
>
> Many thanks in advance

It sounds like you want to plot a line that follows along a
plotted surface without intersecting it. There is one
problem in doing this that I believe you may have run into.

When you plot a surface using either the "surf"
or "surface" command, the surface is graphically rendered
as a bunch of triangular sections. Therefore, the rendered
surface will graphically appear to be faceted. When you
use, for example, "interp2" to find the surface height at
any given point(s) of the data-defined surface, these height
(s) may lie above or below a facet of the graphically-
defined surface, since the interpolation will generally
represent a smoother contour than the graphical surface
would.

So you see, if you want to find a line that exactly tracks
along a rendered surface, you will have to use a different
algorithm than interp. You will likely have to write your
own ray-triangle intersection algorithm. I just finished
working on something like this for a newer version of a
game I have on the FEX: Staker. In the first version, I
used "interp2" to determine if a bomb had hit the surface
of a terrain. Because the interpolated surface doesn't
exactly match the graphical surface, the bomb sometimes
appeared to explode above or below the surface.

If you would like any help, or a sample of my surface
intersection code, let me know. I think it could help you,
and I was considering posting on the FEX anyway.

Ken