I'm half surprised that this isn't utilizing canvas + wasm. I know it wouldn't work for all web clients, but would really be a good way forward to shift the processing to the browser/client for many devices.
As to variations in rendering, for some things like this, there isn't that much different. A project I work on at work does similar overlays with scanned images for marking up manual verifications and creates a final composite document in a similar way (react fe, .net core be).
As to variations in rendering, for some things like this, there isn't that much different. A project I work on at work does similar overlays with scanned images for marking up manual verifications and creates a final composite document in a similar way (react fe, .net core be).