Thank you all for the interesting comments!

goblinbot: I already was aware of the "& + wait" parallelization, nice
contribution anyways!

tp3: I´ve tried some dummy example to generate several views of a rotating
model using either successive command line calls to OpenSCAD or a single
call with the --animate option. The latter resulted about 2-fold faster (23
s vs 40 s).

To test this I used the following command

#> time /home/mon/OpenSCAD-2021.01-RC6-x86_64.AppImage --animate 37
--colorscheme Tomorrow --camera 0,0,0,90,0,0,5 anim.scad

and the following OpenSCAD script:

// Explicit rotation
echo("a=",$t180);
rotate(a=$t180,v=[0,0,1]) mymodel();

module mymodel()
{
difference()
{
cube([1,1,1],center=true);
sphere(d=1.25,$fn=24);
}
}

Now I wonder if there would be a faster approach, for example, to rotate the
camera (using --camera option) or the viewport (using $vpr) instead of the
direct model rotation.

// Viewport rotation
$vpr=[90,0,-$t*180];
echo("vpr=",$vpr,"vpt=",$vpt,"vpd=",$vpd);
mymodel();

However, using the following command (with or without the --camera option)
produces a very small object.

#> time /home/mon/OpenSCAD-2021.01-RC6-x86_64.AppImage --animate 37
--colorscheme Tomorrow --camera 0,0,0,90,0,0,5 anim.scad

ECHO: "vpr=", [90, 0, -34.0541], "vpt=", [0, 0, 0], "vpd=", 500

I only partially fix this behavior by explicitly setting the $vpd variable
to a manually adjusted value, e.g. 5.

#> time /home/mon/OpenSCAD-2021.01-RC6-x86_64.AppImage --animate 37
--colorscheme Tomorrow -D '$vpd=5' anim.scad

Unfortunately, there was not any significant speedup using $vpr rotation.

In any case, this behavior (having to manually set the $vpd value) differs
from GUI and is quite annoying since in GUI there is no need to specify the
$vpd variable at all.

1. I think that the command line tool should behave exactly the same as the
   GUI version, at least for this case, don´t you agree?

2. Any faster approach to generate different views from a model in OpenSCAD?
   Intuitively it seems that computing a thousand views should take just a
   couple of seconds (the 3D navigation is very responsive) but it takes around
   10 times more, why is this so?

Best,

--
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Thanks nophead for the quick response!

What you say makes sense: "Animation recomputes everything as any aspect of
the geometry could depend on $t, not just the camera position."

But, what if the model does not change at all and what just changes is
camera view? Recomputing everything in this case seems a waste of CPU time,
don´t you agree?

Is there a way to speed up the generation of different view snapshots of
exactly the same object?

(Sorry if I say something stupid since I don´t know how OpenSCAD internally
works :-)

--
Sent from: http://forum.openscad.org/

Thanks all for the interesting contributions and fast responses, you´re very
nice!

The render() parameter does speed up the process more than 10 fold (i.e.
from about 20 to less than 2 seconds!) using either the "Explicit rotation"
or the "Viewport rotation" methods.

However, this only works from GUI. When I run it from command-line "Explicit
rotation" works but takes 20 s and the "Viewport rotation" takes the same
time but the view is not updated (i.e. 36 equal "frozen" snapshots are
produced...). Is this the expected OpenSCAD behavior?

I think that tp3 already advised about this on a previous post:

Me: "2) Will the persistent cache feature persist across multiple separate
calls to OpenSCAD command-lined or just the GUI version?"
tp3: "It's supposed to work with both (but command line will need explicit
setup options, not using the config that is available in GUI)."

What are these "explicit setup options"?

For completeness, these were the two methods used in my tests:

// Explicit rotation
echo("a=",$t180);
rotate(a=$t180,v=[0,0,1])
render()
mymodel();

// Viewport rotation
$vpr=[90,0,-$t*180];
$vpd=5;
echo("vpr=",$vpr,"vpt=",$vpt,"vpd=",$vpd);
render()
mymodel();

And this was the command:

#> time OpenSCAD-2021.01-RC6-x86_64.AppImage --animate 36 --camera
0,0,0,90,0,0,5 ViewRender.scad

Note that the --camera option is required to prevent the annoying automatic
zoom in & out issue. I want that the object appears like if it were placed
in a virtual rotation table, the camera has to be at the same distance all
the time.

By the way, is there some way to tell OpenSCAD to use the initial camera
parameters (i.e. those at $t=0) for the whole animation?

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