Custom generator of previews. Custom code to generate previews. Please check "file_dialog/thumbnail_size" in EditorSettings to find out the right size to do previews at. If this function returns true the generator will call [method generate] or [method generate_from_path] for small previews too. By default it returns false. Generate a preview from a given resource with the specified size. This must always be implemented. Returning an empty texture is an OK way to fail and let another generator take care. Care must be taken because this function is always called from a thread (not the main thread). Generate a preview directly from a path with the specified size. Implementing this is optional, as default code will load and call [method generate]. Returning an empty texture is an OK way to fail and let another generator take care. Care must be taken because this function is always called from a thread (not the main thread). If this function returns true the generator will automatically generate the small previews from the normal preview texture generated by the methods [method generate] or [method generate_from_path]. By default it returns false. Return if your generator supports this resource type.