Re: Adding markers on the surface of a Plot3D?

• To: mathgroup at smc.vnet.net
• Subject: [mg89375] Re: Adding markers on the surface of a Plot3D?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sat, 7 Jun 2008 03:00:36 -0400 (EDT)
• References: <g2b4o8\$nm2\$1@smc.vnet.net>

```Hi,

a) yes you are right, it' cant be done :

fun[x_, y_] := Sin[x*y]

DynamicModule[{plt},
plt = Plot3D[fun[x, y], {x, -Pi, Pi}, {y, -Pi, Pi}];
Manipulate[
xline = ParametricPlot3D[{x, yc, fun[x, yc]}, {x, -Pi, Pi}];
yline = ParametricPlot3D[{xc, y, fun[xc, y]}, {y, -Pi, Pi}];
Graphics3D[{plt[[1]],
Sphere[{xc, yc, fun[xc, yc]}, 0.1], {AbsoluteThickness[4],
RGBColor[1, 0, 0], xline[[1]], yline[[1]]}}],
{{xc, 0}, -Pi, Pi}, {{yc, 0}, -Pi, Pi}
]
]

and an other impossible example

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

DynamicModule[{plt},
plt = Plot3D[fun[x, y], {x, -Pi, Pi}, {y, -Pi, Pi},
PlotStyle -> Opacity[0.5], PlotRange -> All];
Manipulate[
pnt = Append[xy, fun @@ xy];
Graphics3D[{plt[[1]], Line[{Append[xy, 0], pnt}],
Sphere[pnt, 0.1], {AbsoluteThickness[4], RGBColor[1, 0, 0],
Line[{pnt, pnt*{1, 1, 2}}]}}, PlotRange -> All],
{{xy, {0, 0}}, Slider2D}
]
]

and

Show[] evaluates to a Graphics[]/Graphics3D[] and the FrontEnd
show this graphics lik it show you the result of Expand[(x+1)^1000]
but in older Mathematica versions the FrontEnd need an other program
to display the PostScript and Show[] had a side effect that it paste
a rendered image in the Notebook.

Regards
Jens

AES 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 . . .
>
> [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?]
>

```

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