osg::NodeCallback migration to vsg

[hearga]

Hello,

I'm trying to migrate osg code to vsg. In this code I'm using osg::NodeCallback extensively.
Is there a way to migrate osg::NodeCallback (or osg::Callback) to vsg?

Thank you very much

[VasanthBalguri]

Hi,

Vsg is very different from Osg, so callbacks are not present instead we have to use event handlers and node visitors.
Basically, Callback is abstraction of doing something when draw, cull or update pass happens.

Vsg doesnt seem have concept of draw,cull and update passes, it depends on the Vulkan pipeline that you setup in the scene instead, so a CallBack doesn't make much sense here.

Bottom line is you will have to do it yourself somehow based on your use case.

[robertosfield]

Could you explain what you are using the callbacks for?

The VSG by design doesn't have callbacks like OSG does, neither scene graph strictly needs them, and having them adds memory and branching penalty that all nodes have to pay for even if the nodes that have any callbacks assigned to them. Not having them in the VSG is one of the reasons traversals can be an order of magnitude faster than the OSG.

Like the OSG the VSG has virtual void accept(..) and void travers(..) methods which you can override to customize behaviour. The node callbacks in the OSG are just in effect an alternative to subclassing a node to override the traverse(..) method, everything you can do with a node callback you can do by subclassing from a node and override the traverse method,

Rather than have two ways of doing the same thing, the VSG just aims to make it easy to subclass nodes and override the accept()/traverse(). You have to subclass and override the callback anyway so it's not actually any more code.

Another approach to override nodes is to use a viewer level vsg::EventHandler or an update vsg::Operation. The vsg::EventHandler base class is similar in role to the osgGA::GUIEventHandler. The OSG also a osg::Operation base class. Doing all the update operations together is more efficient trawling a scene graph for update callbacks, so again this is performance and complexity issue that I've decide avoid.

My plan is to have a special node in the VSG that you can attach event handlers and update operations to, these will just provide a mechanism for the event handler and operations to be assigned to/removed from the Viewer rather than get invoked themselves.

As a general note, it may be possible to move the work done in callbacks into a combination of compute, mesh, vertex or fragment shaders.

[hearga]

Thank you for your quick response and ideas.
Callbacks were used mainly to update some custom transformations applied to the geometry.

[robertosfield]

Are you updating transform nodes or the vertex arrays/index arrays?

[hearga]

transform nodes

[robertosfield]

One approach you could take is to subclass from vsg::Transform base class and override the dmat4 Transform::transform(const dmat4& mv) method to provide the transform behaviour you want.

You may well find that the approach of subclassing from transform takes less code than what you did with the OSG node callback and is more efficient as there is no need for casting data types or indirection. You can put all the variables you need into the Transform subclass and provide the public interface in a way that works exactly best for your application. You can also override the Transform::read()/write() methods to serialize out all your variables - for this you'll also need to register your node with the ObjectFactory so that loaders know how to create your node.

[hearga]

Thank you very much, helps me a lot.