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I'm not sure what you are trying to do, but here is how to create a Ramp parameter in VOP and then make it usable at the assets UI.
Start Houdini, and drop a Geometry object, then go inside delete the File SOP
Drop a VOP SOP node and go inside.
Drop a Ramp Paramater VOP node.
Go up one level and see the Ramp parameter for the VOP network.
Go up one level to the Geometry node.
Select "Edit Parameter Interface" to edit the parameters for the Geoemetry object.
Click "From Nodes" tab
Expand "geo1" and "vopsop1".
You should see the "Paramater (ramp)" as a folder, this is the Ramp Parameter from VOP. Drag that to your parameter list.
Remember for the an asset you should do this using the Type Properties and not the Edit Parameter tool menu, but the approach is the same.
The Group SOP highlights the included points or primitives as if they are selected. Is there a way to do this for digital assets where you want to highlight some points or primitives in a similar manner?
Maybe if you used a Group SOP inside the asset and made it the active message node for the asset, then used an expression to have the Group SOP group what you want highlighted. The Group SOP doesn't have to be part of the output for the asset.
I never seen compiled VOP assets. What's the advantage of that? I am not sure how the VEX compiler works exactly but I thought there is a virtual machine that runs the code so could potentially make optimizations based on the machine running the asset?
Also how do you compile it just out of curiosity?
Compiling VEX to asset types greatly reduces cooking times for your HIP scene. This can improve both the load times of a HIP file and render times.
If you'd like to see the difference. Create a subnet, put inside it a sphere and a material network. In the material network create a basic Material Shader Builder material, and assign it to the sphere. Turn the subnet into an asset, and then create 10 copies of that asset. Reload the scene and render a few frames. Next, unlock that asset and go to the material. Right click, and Compile VEX code to Shop Type. Drop the new material asset and tell the sphere to use that. Lock the asset and update. It'll load 10 times faster and renders will start up faster.