Does VSG support state inheritance/override via the vsg::StateGroup class, similar to override in osg?

[tmonk22]

I read a vsgt file and want to change its display state to wireframe mode, but I don't want to use an intrusive method to change its graphics pipeline. Is it possible to define a StateGroup at the top and use the graphics pipeline of the StateGroup to override the graphics pipeline of all its child nodes? It's similar to override in osg. But this doesn't work.

auto node = vsg::read_cast<vsg::Node>("models/teapot.vsgt", options);
auto group = vsg::StateGroup::create();
group->stateCommands = wireframeStateCommands;
group->addChild(node);

[AnyOldName3]

If you've got state commands that alter the polygon mode (i.e. whether you get triangles, their outlines, or their points), you need to have set up the pipeline with the polygon mode configured as dynamic state. If state isn't dynamic, it's baked into the pipeline and can only be altered by replacing the whole pipeline.

[tmonk22]

Yes, but the whole replacement pipeline will pollute the original data. Is it possible to achieve it through the clone node?

[robertosfield]

State inheritance in the OSG works best with fixed function pipeline, with shader based scene graphs it is a bit more limited. As the VSG/Vulkan is entirely shader based what you can control is more limited, simply because the granularity that state is controllable is coarser grained.

In the case of polygon mode by default this state is fixed by the vkPipeline (vsg::GraphicsPipeline) unless the pipeline is setup explicitly to make a particular pipeline state component dynamic state. The vsg::DynamicState pipeline state needs to be configured with the appropriate VkDynamciState values and passed in during creation of the pipelines.

The vsgdynamicstate example illustrates how this is done with adjusting the line width.

[tmonk22]

I'm just giving an example. There are many ways to switch the wireframe mode. My idea is whether it is possible to achieve state coverage similar to that in osg. Even if the granularity is coarse, I don't want to change the original data?

[robertosfield]

The original pipeline needs to be setup to support dynamic state, so you either have to tell the scene graph creation code/loader to setup use vsg::DynamicState set you want, or you load the data and modify it.

The vsg::ShaderSet::defaultGraphicsPipelineStates can be used to pass in custom pipeline states like vsg::DynamicState, and then pass in the custom shaderset to the loader via the vsg::Options::shaderSets map. The vsgcustomshaderset example illustrates how a custom shader set is passed in to the loaders.

You can clone a whole subgraph and cherry pick the bits you want to duplicate and the parts you want to share, but this is more complex than just setting up the scene graph to be flexible in the ways you want to use it.

This is more complex than OpenGL/OSG fixed function pipeline that you might be used to, but unfortunately the VSG has to work with the constraints of Vulkan, which is designed specifically for performance and getting the most out of modern hardware, and has to grapple with complexity of just how flexible a shader based rendering can be.