Rastermill is a Node.js image processing library that provides a unified API for probing, transparency inspection, and encoding (resize, convert, byte-budget search). It uses Photon (WASM) in-process for common formats (PNG, JPEG, GIF, WebP, BMP) and falls back to native tools (sips, ImageMagick, GraphicsMagick, ffmpeg, Windows System.Drawing) for formats like HEIC/AVIF or when quality-controlled WebP is needed. The library enforces pixel budgets to prevent decompression bombs, strips metadata by default, handles EXIF/HEIF orientation, and supports cancellation via AbortSignal. It is designed for server-side and agent-facing media pipelines. The codebase is a single well-structured TypeScript module with comprehensive tests and CI coverage.