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Geometry modifiers allow an object to be modified at the beginning of the render process. For example, a coarse geometry object could be refined just before starting the render, depending on some scene setup.
Procedural geometry extensions enable maxwell to render mathematically defined primitives. It is geometry created on-the-flyat render time. It can also generate its own texture coordinates.
Procedural textures generate texture colors on-the-fly are calculated while rendering.