Defold Get_linear_velocity_from_local_point Expects Incorrect Argument Type Bug Report

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Hey guys! Let's dive into a quirky issue reported in Defold, specifically concerning the get_linear_velocity_from_local_point function. It seems there's a bit of a mix-up in the expected argument types, leading to some unexpected errors. Understanding these nuances can save you a lot of headache when you're deep into game development, so let's break it down.

Understanding the Issue

The core problem lies in how Defold's Box2D physics engine interacts with the scripting environment. According to the bug report, the functions .get_linear_velocity_from_local_point and .get_linear_velocity_from_world_point are expecting the first argument to be both a body and a vector3 simultaneously. Now, that's a head-scratcher, right? Let's dig into the details.

The Code Snippet

The user provided a neat little code snippet that triggers this issue. Let's take a look:

function init(self)
    local url = msg.url(nil, "go", "collisionobject")
    local body = b2d.get_body(url)

    b2d.body.get_linear_velocity_from_local_point(body,  vmath.vector3(1, 1, 1))
end

In this code:

  1. We initialize a function init(self). This is a common starting point in Defold scripts.
  2. We create a URL to a collision object named "collisionobject" within the game object "go".
  3. We retrieve the Box2D body associated with the collision object using b2d.get_body(url). This should give us the physics body we're interested in.
  4. Here's the kicker: We then call b2d.body.get_linear_velocity_from_local_point, passing the body and a vector3 representing a local point (1, 1, 1).

The Error

When running this code, the following error pops up:

ERROR:SCRIPT: main/main.script:8: bad argument #1 to 'get_linear_velocity_from_local_point' (vector3 expected, got userdata)
stack traceback:
  [C]:-1: in function get_linear_velocity_from_local_point
  main/main.script:8: in function <main/main.script:1>

This error message is quite telling. It says that the function expected a vector3 as the first argument but received userdata (which is how Defold represents native objects like Box2D bodies). This mismatch indicates a problem in how the function is interpreting its arguments.

Root Cause Analysis

To understand why this is happening, the bug report points to a specific section in the Defold engine's source code on GitHub:

https://github.com/defold/defold/blob/d80ef0c20ad81b332e333f24907e801cd971a17a/engine/gamesys/src/gamesys/scripts/box2d/script_box2d_body.cpp#L450C1-L457C6

This link takes us to the C++ implementation of the get_linear_velocity_from_local_point function within Defold's Box2D scripting interface. Examining the code around this area would likely reveal the logic that's causing the argument type confusion.

In essence, the bug stems from an incorrect expectation of argument types within the C++ code that bridges the Box2D physics engine and the Lua scripting environment in Defold. The function is expecting a vector3 where it should be expecting the Box2D body object.

Impact and Context

This bug can be quite frustrating, especially if you're relying on these functions to calculate velocities from local or world points. It means that the intended way to fetch linear velocities based on a point relative to the body or in the world isn't working as expected.

This issue was reported on Windows 11 using Defold version 1.10.3. This context helps narrow down the scope of the bug and might be crucial for the Defold team to reproduce and fix the issue. Knowing the specific platform and engine version can help determine if the bug is platform-specific or a more general issue.

Diving Deeper into the Technical Details

To really grasp the significance of this bug, let's break down what get_linear_velocity_from_local_point and get_linear_velocity_from_world_point are supposed to do and why they are important in game development.

Understanding Linear Velocity

In physics engines like Box2D, linear velocity refers to the rate of change of an object's position over time. It's essentially how fast an object is moving in a straight line. The linear velocity is represented as a vector, which means it has both magnitude (speed) and direction.

The Importance of Local and World Points

In game development, objects exist within a world coordinate system. However, each object also has its own local coordinate system. This local coordinate system is centered on the object's origin, and its axes rotate with the object.

  • Local Point: A point defined relative to the object's local coordinate system.
  • World Point: A point defined relative to the world coordinate system.

Functions like get_linear_velocity_from_local_point and get_linear_velocity_from_world_point are crucial for tasks such as:

  • Applying forces at specific points: To create realistic physics interactions, you often need to apply forces at points other than the object's center of mass. The velocity at these points can influence how the object rotates and moves.
  • Simulating complex movements: For example, if you have a character with articulated limbs, you might need to calculate the velocity of the hand based on the character's overall motion and the limb's rotation.
  • Debugging physics simulations: By examining the linear velocity at different points, you can gain insights into how forces are being applied and how objects are interacting.

How the Functions Should Work (Ideally)

  1. get_linear_velocity_from_local_point(body, local_point):
    • Takes a Box2D body and a vmath.vector3 representing a point in the body's local coordinates.
    • Calculates the linear velocity at that local point, taking into account the body's linear and angular velocity.
    • Returns a vmath.vector3 representing the linear velocity at the specified point.
  2. get_linear_velocity_from_world_point(body, world_point):
    • Takes a Box2D body and a vmath.vector3 representing a point in world coordinates.
    • Calculates the linear velocity at that world point, considering the body's motion.
    • Returns a vmath.vector3 representing the linear velocity at the given point.

The Bug's Impact on Functionality

Because of the bug, these functions aren't working as intended. The engine is misinterpreting the first argument, leading to incorrect behavior and potentially breaking physics-based gameplay mechanics. This can be a significant obstacle for developers trying to implement realistic and complex physics interactions.

Potential Workarounds (While We Wait for a Fix)

While this bug is present, there might be some workarounds you can explore, although they might not be as efficient or straightforward as using the built-in functions.

  1. Manual Calculation: You could try to manually calculate the linear velocity at a point using the body's linear and angular velocities. This would involve some vector math and might be more complex, but it could provide a temporary solution.
  2. Alternative Approaches: Depending on your specific use case, you might be able to achieve the desired effect using different physics functions or by restructuring your code. For example, you might be able to apply forces or impulses directly to achieve the desired motion.

However, keep in mind that these workarounds might have limitations and could impact performance. It's essential to weigh the costs and benefits before implementing a workaround.

Reporting Bugs: A Community Effort

This bug report is a great example of how the Defold community contributes to the engine's improvement. By reporting issues with clear and concise information, including code snippets and error messages, users help the Defold team identify and fix bugs more effectively.

If you encounter a bug in Defold (or any software), here are some tips for writing a good bug report:

  1. Provide a clear description: Explain the issue in detail, including what you expected to happen and what actually happened.
  2. Include a minimal reproducible example: The code snippet in this bug report is excellent because it's short, self-contained, and clearly demonstrates the issue.
  3. Share error messages and logs: Error messages provide valuable clues about the cause of the bug.
  4. Specify your environment: Include your operating system, Defold version, and any other relevant information.
  5. Be patient and respectful: Bug fixing takes time and effort. Be patient with the developers and provide constructive feedback.

Conclusion

The get_linear_velocity_from_local_point bug highlights the importance of understanding the underlying mechanics of a game engine and how to effectively report issues. While this bug can be a stumbling block, knowing how to work around it and contribute to the community helps everyone in the long run. So, keep coding, keep experimenting, and keep reporting those bugs! And hopefully, a fix for this is on the horizon. Happy developing, guys!