OpenCV, T-Block & PhysTwin: Compatibility & Simulation

Hey guys! I've been diving into this awesome project using PhysTwin for policy training and evaluation, and it's been a blast. The code is super easy to get up and running, thanks to the uv environment setup. I did hit a snag, though, and I've also got some questions about the T-block simulation. Let's break it down.

OpenCV Compatibility: The imshow Headache

So, the first thing I ran into was an OpenCV compatibility issue when trying to run keyboard_teleop.py. Basically, the code uses cv2.imshow to display the interactive window, but it threw an error. Specifically, the error message: cv2.error: OpenCV(4.11.0) /io/opencv/modules/highgui/src/window.cpp:1301: error: (-2:Unspecified error) The function is not implemented. This essentially means that the OpenCV library wasn't built with the necessary support for displaying windows, like GTK+ 2.x or Cocoa. If you're on Ubuntu or Debian, you might need to install libgtk2.0-dev and pkg-config and then rebuild OpenCV.

The root of the problem lies in the pyproject.toml dependencies. The real2sim-eval part of the project needs a full opencv-python or opencv-contrib-python for the display window to work. However, inside the policy submodules, the pyproject.toml file specifically requires opencv-python-headless. The headless version is designed for environments without a graphical user interface, so it doesn't include the display features. Resolving this is usually pretty straightforward: make sure that the main environment uses the full opencv-python or opencv-contrib-python. It's a common issue when different parts of a project have conflicting dependencies, so it's good to be aware of how to debug and fix it.

It's important to make sure that the correct OpenCV libraries are installed in your environment before running the code that uses cv2.imshow. This ensures that the interactive playground, which is a great tool for understanding and interacting with the simulations, functions correctly. The interactive window is essential because it allows users to visually observe the simulation and control the actions of the agent, and without the proper OpenCV setup, you'll be stuck without a visual representation of the simulation. This visual feedback is crucial for testing the trained policies and seeing how they interact with the environment, so making sure it's working is the first step toward getting the most out of this project.

Clarification on Interactive Window Controls

Another small thing I noticed was a bit of a discrepancy in the instructions for the interactive window. The terminal output tells you to use WASD for XY movement and QE for Z movement. But, in reality, to control the Z (up/down) scale, you actually need to use f and r. This isn't a huge deal, but updating the terminal output would make things a little less confusing for anyone trying to navigate the interactive playground. Minor tweaks like this can greatly improve the overall user experience, especially for newcomers to the project who are still getting the hang of things. It's the small details that make a big difference in how easily people can jump in and start playing around with the code.

Decoding the T-Block: Rigidity, Spring-Mass, and Gaussian Points

Now, let's get into the more interesting stuff: the T-block and its simulation. I've been pondering how the T-block, being a rigid object, is represented in the simulation. This is where it gets a bit more complex. The core of the question is, how can a simple spring-mass system effectively simulate a rigid object like the T-block?

I dug into the T-block data on Hugging Face and found that the optimized spring-stiffness is set to 0. Yet, when you run the demo, the T-block is using the default spring stiffness value of 3e4 in PhysTwin. This seemed a little odd, and it led to some interesting observations while running the demo. Using the default spring stiffness causes the T-block to behave in ways that aren't entirely consistent with how a rigid object should behave in a physical simulation. Specifically, I noticed the T-block sometimes appeared deformable or was being pulled by the pusher. This sort of behavior is not exactly what you'd expect from a solid block in a realistic simulation.

The simulation of the T-block involves a spring-mass system, which, by its nature, introduces some level of flexibility. When the pusher gets too close and digs into the T-block, this can create pulling actions and cause the block to deform, which is not compatible with the expected behavior of a rigid object interaction. This occurs because the spring-mass model, even with a high spring stiffness, still allows for some deformation or give. It's like the block is made of a very stiff, but slightly squishy, material rather than being perfectly solid.

Also, the contact radius parameter plays a significant role in how the interaction between the pusher and the T-block is modeled. If the contact radius is set too large, the pusher can get