Unreal Tournament: Fixing Vector Division By Zero Error

Hey guys! Let's dive into a classic head-scratcher from the good old days of Unreal Tournament. Specifically, we're talking about that pesky vector division by zero error that popped up in v469 RC10. If you've ever scratched your head at a cryptic error message, or worse, experienced unexpected player deaths, you're in the right place. Let's break down what happened, why it happened, and how it all ties together.

The Error: Function Engine.Decoration.Bump:0093

So, what exactly went wrong? The error message points to a specific line of code within the Decoration.Bump class. Here’s the problematic snippet:

speed = VSize(Other.Velocity);
Velocity = Other.Velocity * FMin(120.0, 20 + speed)/speed;

The core issue lies in this line: Velocity = Other.Velocity * FMin(120.0, 20 + speed)/speed;. Notice the division by speed? Well, what happens when speed is zero? Boom! Division by zero error. It’s like trying to split a pizza into zero slices – the universe just doesn’t allow it.

The variable speed is calculated using VSize(Other.Velocity). The VSize function calculates the magnitude (or length) of the Other.Velocity vector. So, if Other.Velocity is a zero vector (i.e., (X=0, Y=0, Z=0)), then speed becomes zero, leading to our infamous error.

Why Does Other.Velocity Become Zero?

This is the million-dollar question. In the context of the Decoration.Bump class, Other.Velocity likely refers to the velocity of another object (like a player) that interacts with a decoration in the game world. If a player is standing perfectly still on top of a decoration, or if the game engine momentarily calculates the player's velocity as zero, then Other.Velocity could indeed be (0, 0, 0).

The Jump Start Scenario

The original post also raises a very insightful point: could this be the reason for player deaths when a jump starts on top of a decoration? The answer is a resounding yes. Here's why:

1. Instantaneous Velocity: When a player initiates a jump, there might be a fleeting moment where their velocity is calculated as zero, especially if the jump starts from a standstill.
2. Decoration Interaction: If the player is standing on a decoration that uses the Decoration.Bump code during this moment, the VSize(Other.Velocity) calculation could return zero.
3. Division by Zero: The division by zero error occurs, potentially leading to unpredictable behavior, including game crashes or, in some cases, player deaths due to corrupted game state.

This scenario perfectly illustrates how a seemingly minor coding oversight can have significant consequences in a real-time gaming environment.

Understanding the Code: A Deeper Dive

Let's break down the code snippet line by line to fully grasp what's happening:

speed = VSize(Other.Velocity);
Velocity = Other.Velocity * FMin(120.0, 20 + speed)/speed;

1. speed = VSize(Other.Velocity);

   • This line calculates the magnitude of the Other.Velocity vector. In simpler terms, it figures out how fast the other object (usually a player) is moving. The result is stored in the speed variable.
   • VSize is a function that takes a vector as input and returns its length as a floating-point number. For example, if Other.Velocity is (3, 4, 0), then speed would be 5 (because the square root of (3^2 + 4^2 + 0^2) is 5).

2. Velocity = Other.Velocity * FMin(120.0, 20 + speed)/speed;

   • This line is where the actual velocity adjustment happens. It takes the original velocity of the other object (Other.Velocity), modifies it, and assigns the result to the Velocity variable.
   • FMin(120.0, 20 + speed): This part calculates the smaller value between 120.0 and 20 + speed. It's essentially capping the influence of the speed on the velocity adjustment. If speed is very high, the value will be limited to 120.0. If speed is low, it will be 20 + speed.
   • /speed: This is the critical part where the division by zero error occurs. The result of FMin(120.0, 20 + speed) is divided by speed. If speed is zero, this operation is undefined and causes the error.
   • Other.Velocity * ...: Finally, the original velocity is multiplied by the result of the division. This scales the velocity vector, effectively changing its magnitude while preserving its direction.

In essence, this code snippet attempts to adjust the velocity of an object based on its speed. However, the division by speed without proper handling of the zero case introduces a fatal flaw.

Potential Solutions and Workarounds

So, how can we fix this? There are a few ways to tackle this issue:

1. Adding a Check for Zero Speed: The most straightforward solution is to add a conditional check to ensure that speed is not zero before performing the division.

speed = VSize(Other.Velocity); if (speed > 0.001) // Adding a small epsilon value to avoid floating-point inaccuracies { Velocity = Other.Velocity * FMin(120.0, 20 + speed)/speed; } else { Velocity = vect(0,0,0); // Or some other default value }

    This code snippet checks if `speed` is greater than a small value (0.001) to avoid potential floating-point inaccuracies. If `speed` is indeed zero (or very close to it), the code assigns a default value (like a zero vector) to `Velocity` to prevent the division by zero error.

2.  **Using a Smooth Minimum Function:** Another approach is to use a smooth minimum function that avoids the abrupt transition of `FMin`. This can provide a more stable result, especially when `speed` is close to zero.

3.  **Re-evaluating the Logic:** Sometimes, the best solution is to rethink the entire logic behind the code. Is the velocity adjustment really necessary? Could there be a better way to achieve the desired effect without relying on division?

## Implications for Game Development

This seemingly small error highlights several important lessons for game developers:

*   **Defensive Programming:** Always anticipate potential edge cases and handle them gracefully. In this case, the division by zero error could have been easily avoided with a simple check.
*   **Floating-Point Precision:** Be aware of the limitations of floating-point numbers and potential inaccuracies. Using small epsilon values in comparisons can help prevent unexpected behavior.
*   **Testing:** Thoroughly test your code with a variety of scenarios, including edge cases and extreme values. This can help uncover hidden bugs and prevent them from making their way into the final product.

## Conclusion

The vector division by zero error in Unreal Tournament v469 RC10's `Decoration.Bump` class is a classic example of how a small coding oversight can lead to significant problems. By understanding the root cause of the error and implementing appropriate solutions, we can prevent game crashes and ensure a smoother gaming experience. Remember, defensive programming, awareness of floating-point precision, and thorough testing are essential tools for any game developer. Happy coding, and may your vectors always be non-zero!