A Grayscale Image Posteriser
A simplistic tool to help design value steps and shapes when drawing or painting, whether as a snapshot of a subject or to check your drawing to assist in maintaining a clear lighting/value hierarchy.
This tool uses 5 value steps, which is probably more than is required depending on the image being uploaded. There are heaps of image editing apps out there that can do this and much more with a posterise and related functions. I just needed a simple and easy to access online tool for demonstrating value compression.
The basic idea is to design value shapes of distinct values, resisting any temptation to blend or model to retain full control of the steps in value before working on transitions or edges, and to resist putting too much detail into the shadows until a clear value hierarchy is established.
This just uses a simple javascript algorithm that works in your browser, so it functions completely locally.