Re: Adding markers on the surface of a Plot3D?

• To: mathgroup at smc.vnet.net
• Subject: [mg89373] Re: Adding markers on the surface of a Plot3D?
• From: Szabolcs <szhorvat at gmail.com>
• Date: Sat, 7 Jun 2008 03:00:11 -0400 (EDT)
• References: <g2b4o8\$nm2\$1@smc.vnet.net>

```On Jun 6, 1:49 pm, AES <sieg... at stanford.edu> wrote:
> I'd like to highlight the two mesh lines that pass through a certain
> central point -- let's call it the point {xc,yc} -- on a Plot3D by
> thickening those two lines relative to the other mesh lines, or giving
> them a distinctive color, or . . .
>
> Or put a dot on the surface at the point {xc, yc, f[xc,yc]} to identify
> this location on the surface . . .
>
> Or run a vertical "post" up through that point using something like
> Line[{xc, yc, 0}, {xc, yc, 2*f[xc,yc]}}] (with the part of the post
> below the surface not being seen if you look at the surface from
> above) . . .
>
> I think I've so far tested and confirmed every possible way in which
> each of these tasks _can't_ be done . . .
>

Is this what you are trying to achieve?

fun[x_, y_] := Exp[-(x^2 + y^2)]

Show[
Plot3D[fun[x, y], {x, -3, 3}, {y, -3, 3}, PlotRange -> All,
Mesh -> {{{0, Thick},
Sequence @@ Table[{x}, {x, -3, 3, .5}]}, {{0, Thick},
Sequence @@ Table[{x}, {x, -3, 3, .5}]}}],
Graphics3D[{{Red, PointSize[0.03], Point[{0, 0, fun[0, 0]}]}, {Thick,
Line[{{0, 0, fun[0, 0]}, {0, 0, 1.2 fun[0, 0]}}]}}]]

> [And would someone want to help me understand _why_  the
> Show[ - - - - -] command (followed by a semicolon) no longer works, or
> can no longer be used, as one of a sequence of multiple expressions
> within a longer cell?  And why it can't even have a semicolon following
> it as the last line in a cell?  I just can't envision the logic of why
> this was a necessary (or optional?) interface design decision -- isn't
> Show[---] an expression?]

It has been explained many times on this list that in Mathematica 6, the
graphics are not shown as a side effect.  Instead they are simply the
return values of plotting functions.  Graphics[] objects are formatted
in a special way in StandardForm/TraditionalForm (the actual graphics
are displayed instead of the expression representing them).  This
behaviour is much more consistent with the design of Mathematica and
IMO a big step forward.

If you need to show graphics as a side effect, Print[] them.  But I am
sure that you were already aware of this.

```

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