Photo-to-Coloring Page Plugin

I need a native library that can be dropped directly into Unity and called via DllImport to convert photos into clean black-on-white coloring pages.
The input will be an image provided as an RGBA byte buffer with width and height, and the output should be a processed RGBA buffer containing black contours on a white background.

The solution must work 100% offline on both Android and iOS, with no intermediate file I/O — everything should happen in memory.

Because this runs inside a game, performance, image quality, and asynchronous execution are all important.
The result should preserve fine contours even in photos with many overlapping objects, while keeping background noise and speckling to a minimum so the image is suitable for coloring.

Technical expectations
C-ABI entry points only (callable via Unity DllImport)
No disk access inside the library
Thread-safe, non-blocking API (async processing)
Black lines on white background, clean and readable on most everyday photos

Deliverables
Pre-built binaries for:
Android
iOS
A single C/C++ header exposing the public API
A small Unity C# example showing how to call the API asynchronously
Brief build notes so binaries can be reproduced if needed

Experience with OpenCV, classical image processing, edge extraction, or similar computer-vision pipelines is highly preferred.