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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
It won't need a processor unless you're using an input photograph that matches the composition you want. For the mlsd input, maybe try draw two horizontal white lines on a black canvas, for where you want the floor and ceiling meet the walls, and play with the strength. There's also tools like https://zhuyu1997.github.io/open-pose-editor/ and https://app.posemy.art/ and various extensions. I know you said "without resorting to creating a background"... just google some gym photographs?