There are special variables for the camera which you can set
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Other_Language_Features#.24vpr.2C_.24vpt_and_.24vpd

On Sun, Nov 12, 2017 at 11:01 AM, Vaclav Peroutka vaclavpe@seznam.cz wrote:

On 11/12/2017 9:01 AM, Vaclav Peroutka wrote:

I'm not sure, but I don't think you can.

I'm not really good with the math, but after experimenting for a while
it seems like you can move the camera around, but you can't directly
zoom it.

There are three variables that control the camera:

$vpr controls rotation.  (Note that it controls rotation of the model,
not the camera.  Changing it turns the model rather than turning the
camera.  You can also look at it as causing the camera to orbit around
the model.)

$vpt controls where the center of the viewport is.  Normally this is
used for panning left and right, but it can also pan in and out.

$vpd controls the distance from the center of the viewport to the
camera.  For "zoom" purposes, I believe this is the same as using $vpt
to move the center of the viewport in and out.

What you want is to zoom the image, not to move the camera closer to
it.  The difference is in the angles; zoom magnifies the image but does
not change the angles.  Visibility of an object does not change as you
zoom in and out, while moving the camera out may make more objects visible.

You can zoom, by changing the window size.  However, that's severely
constrained; you can't just make the window ten times bigger than your
screen.

It might be possible to do what you need by applying the right matrix
multiply to your object, to sort of do your viewport translation in the
construction of the object.  But I would I have to think for a long time
about exactly how to do that.

Here's an animation that lets you play with moving the viewport center
and/or the camera distance.  Note that the two cubes on the side are
more visible when the camera is further away; when the camera is close
they are hidden behind the big cube.  Pick one of the $vpt lines and one
of the $vpd lines to enable (remember Ctrl+D and Ctrl+Shift+D are your
friends).  Note that if you enable both calculated values, they exactly
cancel each other out.  If one of them controlled camera position and
the other controlled zoom, that wouldn't be true.

// t will vary from 0 to 1 and back to 0.
t = 1-abs($t-0.5)*2;

#cube(10,center=true);
translate([5.5,0,0]) cube(1,center=true);
translate([-5.5,0,0]) cube(1,center=true);

//$vpt=[0,60t-30,0];  // Varies from -30 to +30
$vpt=[0,0,0];
$vpr=[90,0,0];
$vpd=(60t)+30;    // Varies from 30 to 60
//$vpd=60;

translate($vpt) sphere(0.1);

If you haven't used an animation before:  View/Animate, then in the
fields at the bottom of the window enter values for FPS and Steps.  10
and 100 are working well for me.

On 11/13/2017 3:14 AM, Ronaldo Persiano wrote:

straightforward uniform scaling transformation after applying the
perspective transformation, either before or after discarding the depth
coordinate to project into 2D.  But I'm not sure.

He's trying to match a photograph, so switching to orthogonal view won't
do the right thing either.

You're right - I was speaking informally.  I wasn't drawing a
distinction between rotating the coordinate system (and thus rotating
the objects defined in that coordinate system) and rotating the objects
themselves.  The objects don't move.

Having the camera orbit the center of the viewport is probably still
more correct - if I'm doing my right-hand-rule right, that's what the
numbers say - but I just can't convince my brain that that's what's
happening, at least for most models.  (Maybe for architectural models.)

It's tempting to think that you could achieve something a lot like a
zoom effect by scaling the model, but my experiments suggest not - you
need the scaling to happen after the perspective, and matrix multiply
isn't commutative.  (Or not commutative enough?  Or I haven't figured
out how.)  It seems like maybe you could switch to orthogonal mode and
transform the model to apply both perspective and zoom, but some
experiments suggest that you can't do perspective transforms.  Or maybe
in perspective mode use a transform that's the equivalent of scaling
after perspective, but I suspect that such a matrix would resemble a
perspective matrix.

Od: Jordan Brown
"

On 11/13/2017 3:14 AM, Ronaldo Persiano wrote:
"
There is no way to get zoom effect with OpenSCAD in perspective mode. In
camera lenses, zoom results from changing the camera lens focus distance. In
OpenSCAD, the "camera lens focus distance" seems to be fixed to the
equivalent of a 60mm focus lens for 35mm films. However, you may simulate
zoom in parallel mode by changing the camera distance. To get zoom we would
need to control either the viewport size (equivalent to change the camera
film format) or the focus distance of the camera lens.
"

uniform scaling transformation after applying the perspective
transformation, either before or after discarding the depth coordinate to
project into 2D.  But I'm not sure.

He's trying to match a photograph, so switching to orthogonal view won't do
the right thing either.

"

Exactly. I tried orthogonal view but it seemed to me that it was even worse.
I need to somehow change the "lens focus".