Dynamic command graph for multipass.

[trinhtuanlong]

I am passing an image through multiple filter for processing. Now I want to have the option to enable and disable any part of the pipeline during runtime. How do I do this? Say I am setting up the viewer like this:

auto vkViewer = createViewer(window, instrumentation, vkCamera, {computeCommandGraph1, computeCommandGraph2, graphicsCommandGraph1});

vsg::ref_ptr<vsg::Viewer> createViewer(vsg::ref_ptr<vsg::Window> window, vsg::ref_ptr<vsg::Instrumentation> instrumentation, vsg::ref_ptr<vsg::Camera> camera, std::vector<vsg::ref_ptr<vsg::CommandGraph>> commandGraph)
{
	auto viewer = vsg::Viewer::create();
	viewer->addWindow(window);
	viewer->assignRecordAndSubmitTaskAndPresentation(commandGraph);
	viewer->assignInstrumentation(instrumentation);
	viewer->compile();
	viewer->addEventHandler(vsg::Trackball::create(camera));
	viewer->addEventHandlers({vsg::CloseHandler::create(viewer)});
	return viewer;
}

Now during the main loop I want to remove the computeCommandGraph2 and switch the graphicsCommandGraph1 to a different one. How do I do this?

[robertosfield]

To change the contents of a Viewer/RecordAndSubmitTask you'd need to use a viewer.deviceWaitIdle() call to make sure all the rendering is complete and then change things.

Or if you can just stick a vsg::Switch above the subgraphs you want to toggle. It may be possible to do this individually, so leave the CommandGraph's in place at the top RecordAndSubmitTask level and then just let the VSG decide whether to submit work. I haven't ever tried this though.

I'm just looking through CommandGraph and RenderGraph classes, there isn't presently any controls for switch it on/off, perhaps there needs to be. There might be considerations w.r.t threading that might need to take this into account, so it might be more awkward that just adding a bool/mask that is checked each traversal.

With tasks like this that try pushing the outer boundaries of the current VSG implementation it can be useful to create a new vsgExamples that attempts to implement the required functionality using existing features, and then each have other members of the community help tweak it to make sure it's robust or cleaner, or for modifications to the core VSG to tried out to simply and clean up the task being carried out by the example. Adding a bool/mask to CommandGraph might be such a modification.

One really needs this concrete test cast that illustrates the feature required with a provisional implementation before ones starts changing the core VSG, as it might be that existing functionality works perfectly well, or that the problem being explained by the user really isn't understood correctly by various parties so modifications just don't fix the problem in hand,

[trinhtuanlong]

Is there an example of using vsg::Switch during main loop? I mean using a bool imported from outside to toggle the switch before recordandsubmit?

[robertosfield]

A vsg::Switch node can be a child of CommandGraph, but you can't make a Switch node a child of a RecordAndSubmitTask as it requires it's children to be CommandGraph.

Essentially the scene graph and starts below the CommandGraph, so all the flexibility in how you compose things starts there. RecordAndSubmitTask is a very specific class for managing the recording and submission of command buffers to Vulkan and handling all the required synchronization primitives.

However, it is possible to nest CommandGraph within CommandGraph as part of the scene graph hierarchy, and you can decorate those nested CommandGraph with Switch nodes or other scene graph nodes. The vsg::View utilizes this capability with CommandGraph's internal to the ViewDependentState object that the View can have for handling the lighting and shadow map rendering. These internal CommandGraph are traversed dependent on the needs of the View/ViewDependentState/Lighting.

So.. if you could do your compute and rendering management within a top level CommandGraph then you'd be able use Switch or your own custom node.

Alternatively we'd need top level support built into RecordAndSubmitTask and CommandGraph for toggling on/off. Or you just stop the rendering, wait for device idle, reconfigure the viewer and then restart.