[osg-users] Recommended way to render a scene from multiple independent viewpoints

[Hannes Naude]

Thanks Riccardo and Robert for your inputs.

Robert, yes you are correct that the only issue I had with the
CompositeViewer was that the same Node's callback would get called as many
times as views that it appeared in. This means that for example if I have a
simple update that would translate a node a fixed amount, then nodes that
appear in mulitple views would move faster than those that appear in a
single view only. Also, as I add more cameras nodes end up moving faster.

Obviously I can fix this in the update callback itself, by checking
something like simulationTime (and I would ultimately have to do this
anyway to make my motion happen at the same speed, irrespective of frame
rate), but I would prefer to not have the callbacks called at all when not
required.

Incidentally, I found that the (non-composite) viewer did not immediately
solve this. It would only go away if all my cameras shared the exact same
root node. Now I have some symbology that I wish to display on one camera,
but not the others, but I managed to achieve this by setting the nodemask
appropriately.

I am not really doing anything fancy with the callbacks. I created a class
which extends osg::Callback and overrode the run method to update a
MatrixTransform node (via getMatrix and setMatrix). I then created another
class which extends MatrixTransform and in the constructor I call

this->setUpdateCallback

providing an instance of my callback class as the argument. Now whenever I
add an instance of my MatrixTransform class to the scenegraph, it
implements the motion I want.

This seems to work, except for the multiple update problem.

Hannes


On Tue, Apr 11, 2017 at 3:09 PM, Robert Osfield <robert.osfield at gmail.com>
wrote:

> HI Hannes,
>
> The CompositeViewer was written specifically for your usage case -
> i.e. multiple Views.
>
> I wouldn't recommend using slave Camera's for doing multiple views,
> while possible it's just a mess in terms of management.  slave
> Camera's are tools for helping rendering a single view, but with a
> view that is composed of several components - either spread across
> multiple windows, or a view that requires multiple passes such as
> distortion correction, field of view etc.
>
> The only reason you drawback you state about using CompositeViewer is
> multiple update traversals. Is this correct?  If so then the
> discussion should be about what problems you are having with
> callbacks, as the solution will likely related to how you are doing
> callbacks rather high level viewer configuration.
>
> Robert.
>
> On 11 April 2017 at 12:08, Hannes Naude <naude.jj at gmail.com> wrote:
> > Hi all
> >
> > I am trying to render a single scene from multiple viewpoints. I
> initially
> > implemented this with a compositeviewer as per the osgthirdpersonview
> > example. This worked fine except that my update callbacks appeared to be
> > getting called more than once per render cycle. I assumed that the update
> > traversal was being done for each view separately and therefore nodes
> that
> > are present in multiple views will have their update callbacks called
> > multiple times. So, at this point I tried to do the same thing but with a
> > single View, somewhat similar to the osgCamera example. But, I do not
> want
> > to add my cameras with viewer.addSlave as I want them to move
> independently
> > of one another. So I tried adding them into the scene graph and giving
> each
> > their own GraphicsContext, but even though the windows corresponding to
> > these GraphicsContexts get created, it appears as if all rendering is
> done
> > in a single window with multiple viewpoints being rendered over one
> another.
> >
> > Obviously there are many ways to skin this cat, but I would appreciate
> some
> > guidance on the recommended approach. To recap my requirements are :
> >  - Multiple cameras viewing the same scene.
> >  - Camera positions and orientations must be independently controlled.
> >  - Node update callbacks should be called only once per Node per render
> > cycle.
> >
> > Any help will be appreciated
> >
> > Regards
> > Hannes Naude
> >
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> >
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