Re: [Gnuplot-info] rotation center

[Zhong R. <re...@uc...>]

---- Original message ----
>Date: Sun, 13 Dec 2009 11:51:41 +0100
>From: Hans-Bernhard Bröker <HBB...@t-...>  
>Subject: Re: [Gnuplot-info] rotation center  
>To: re...@uc...
>Cc: gnu...@li...
>
>Zhong Ren wrote:
>> Please consider the following script that draws a sphere.  Why
>> the rotation center is not at the center of the sphere, when
>> the graphics is spun by the mouse?
>
>Why should it be?  Did you really expect gnuplot to have a
special rule 
>like "whenever there is exactly one spherical surface among
the plotted 
>things, rotation will magically center around that"?

No, you didn't get my question.  Please consider my original
case again:

Please consider the following script that draws a sphere.  Why
the rotation center is not at the center of the sphere, when
the graphics is spun by the mouse?  If I change to "set xrange
[ 6 :  44]", the rotation center becomes the center of the
sphere.  This makes the behavior of one of my large
applications unpredictable.  Is there a way to specify the
rotation center?  If not, how is the center selected by Gnuplot?

set terminal x11 0 size 1024,1024
set parametric
set view equal xyz
set xlabel "x"
set ylabel "y"
set zlabel "z"
set xrange [ 6 :  43]
set yrange [80 : 118]
set zrange [13 :  51]
set pm3d implicit at s
splot 10*cos(u)*cos(v)+25,10*cos(u)*sin(v)+99,10*sin(u)+33

The rotation center is indeed at the spherical center when
xrange is [ 6 :  44], because (6+44)/2 = 25, which is exactly
the spherical center.  When xrange is [ 6 :  43], the rotation
center should be (6+43)/2 = 24.5, but it is not.  It is near
x=33.  What makes Gnuplot to select the rotation center?

>
>> If not, how is the center selected by Gnuplot?
>
>Mouse rotates the entire graph box, so, kind-of obviously, it is 
>centered around the center of the graph box.  So if you want
rotation to 
>be around a particular point, make all three axis ranges
symmetric 
>around it:
>
>	set xrange [mid_x - size_x : mid_x + size_x]
>	# likewise for y and z
>

This is what I thought, but it is not working for me as the
example I gave above.  Gnuplot seems to have a complex rule to
select the rotation center.  I really prefer a user selected
rotation center.