How can I defin new SimplifyModifier on A-frame ?
Uncaught ReferenceError: SimplifyModifier is not defined
SimplifyModifier is no part of the three.js core.
Similar to other extensions like loaders, controls or post-processing passes, you have to import modifiers from the examples directory. I suggest you study how the official example handles the ES6 import:
https://threejs.org/examples/webgl_modifier_simplifier
Depending on your app structure, you probably want to import the globals script version instead from the examples/js directory.
