STEPS_AUTO

[BryBo617]

Posterizer.STEPS_AUTO = -1;

It appears that the default value should be 1 and not -1. The validation in setParameters is looking for a number between 1 and 255.

if (params.steps && !Array.isArray(params.steps) && (!utils.isNumber(params.steps) || !utils.between(params.steps, 1, 255))) { throw new Error('Bad \'steps\' value'); }