New feature request: add ability to create "phantom" objects in the VRE To my knowledge, all objects in the simulation must have >0 mass and >0 density. While this makes sense for creating objects with which you want to interact in a physically realistic environment, it creates a shortfall for testing our motor decode algorithms within the VRE with human subjects. Ideally, we would like the ability to create translucent "target spheres" that stay in place (i.e. unaffected by gravity) and have no density (i.e. any object can move through it uninterrupted). These spheres would be used as targets for our subjects to try to reach with a specific decoded finger movement with MoCap deactivated. In the same test, other phantom spheres would be placed over the other finger tips at their rest positions. The subject would have to reach the target sphere with a specific finger while keeping the others at their resting position. These spheres could then change color based on whether the finger tips are in their appropriate positions, either in their resting positions or successfully moving to the target. We have used this experiment previously in another VRE to demonstrate the number of hand DOFs we can achieve with our motor decodes. This experiment is fundamentally doable with MuJoCo in its current state by taking advantage of the hx_read_sensors() function. Whether or not a finger is positioned properly can be determined by returning the amount of motor actuation from an hx_read_sensors() call. However, this is not very helpful for the the subject who would have very little visual feedback as to whether their fingers are oriented properly. Currently, the only cues we can give the subject within the VRE are by updating the message via an mjhx_message() call. In a nutshell, we would like the ability to create zero-mass, zero-density phantom objects in the simulations. Combined with my previously-requested collision detection ability, the aforementioned DOF-determining experiment could be carried out in MuJoCo. Even better, we would also like to use the API to change the position and color of these phantom spheres.