Pixijs bounding box relative to certain parents coordinate system

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Summary

The problem revolves around calculating the bounding box of a container in relation to its parent container’s coordinate system using Pixijs. The goal is to scale the entire scene by placing it in a single parent container, while keeping the background and UI unscaled. The challenge lies in finding a bounding box that is neither in the local coordinate system (using localBounds) nor in the world coordinate system (using getBounds), but rather in a coordinate system relative to a specific parent container.

Root Cause

The root cause of the issue is the complexity of transformations in computer graphics, particularly when dealing with nested containers and coordinate systems. The attempted solution of inverting the parent’s worldTransform and prepending the container’s worldTransform may not be accurate due to the intricacies of matrix transformations. Key causes include:

  • Incorrect handling of transformations
  • Insufficient understanding of coordinate systems
  • Complexity of nested containers

Why This Happens in Real Systems

This issue occurs in real systems due to the following reasons:

  • Nested transformations: When containers are nested, their transformations are combined, leading to complex calculations.
  • Coordinate system conversions: Converting between local, parent, and world coordinate systems can be error-prone.
  • Scaling and rotation: Applying scales and rotations to containers can affect the bounding box calculations.

Real-World Impact

The real-world impact of this issue includes:

  • Inaccurate collision detection: Incorrect bounding box calculations can lead to false positives or false negatives in collision detection.
  • Visual artifacts: Incorrect transformations can cause visual artifacts, such as misplaced or distorted objects.
  • Performance issues: Inefficient transformation calculations can lead to performance degradation.

Example or Code

// Calculate the bounding box of a container relative to its parent container
let parent = new PIXI.Container();
let container = new PIXI.Container();
parent.addChild(container);

let parentBounds = parent.getBounds();
let containerBounds = container.getBounds();

// Attempt to calculate the bounding box in the parent's coordinate system
let inverseTransform = parent.worldTransform.invert();
let containerTransform = container.worldTransform;
inverseTransform.prepend(containerTransform);

// Calculate the bounding box using the transformed container
let transformedBounds = containerBounds.applyTransform(inverseTransform);

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Understanding the transformation pipeline: They recognize the order of operations for transformations and how they affect the bounding box calculations.
  • Using the correct transformation methods: They apply the correct transformation methods, such as applyTransform or prepend, to calculate the bounding box in the desired coordinate system.
  • Verifying calculations: They thoroughly verify their calculations to ensure accuracy and correctness.

Why Juniors Miss It

Juniors may miss this issue due to:

  • Lack of experience with transformations: They may not fully understand the complexities of transformations and coordinate systems.
  • Insufficient knowledge of Pixijs: They may not be familiar with the Pixijs library and its specific methods for handling transformations and bounding box calculations.
  • Overlooking details: They may overlook critical details, such as the order of operations or the correct transformation methods, leading to incorrect calculations.