Lottie4J, a JavaFX-based Lottie animation renderer, has been significantly improved to match the output of the official Lottie web player. The key change is a new frame-by-frame comparison test harness that renders animations via both a headless Chrome instance running the official dotlottie-wc player and the JavaFX renderer, then diffs each frame against a tolerance threshold. Rendering fixes were made across easing (ported BezierEaser from lottie-web), mattes (inverted-alpha type now handled), gradients (alpha/colour stop merging and densification), blur/blend modes, and text. The work was done using Theia IDE with the Claude API in a disciplined AI-assisted workflow: an @Architect agent wrote scoped plans, a @Coder agent executed them, and SonarQube reviewed the output. The worst-performing test file now achieves 95.2% similarity, with most files above 98%.
