# Preprocessing System

Preprocessing system is responsible for setting up or modifying blocks to be used throughout the preview.

# preprocess Callback

System loops through all entities in level sequentially based on their preprocess callback order. For each entity preprocess callback is executed.

# Remarks

preprocess callback is the only callback where writing to normally immutable blocks is allowed. Therefore it is great for preparing data that are unknown at compile time, yet required at runtime.

Due to the nature of preprocessing system running only once before the start of a level, it is recommended to use preprocess callback to do all immediately needed calculations that are required at runtime instead of doing them at runtime, to improve performance.

However, because of potentially large amount of entities a level may contain, it is recommended to defer calculations that are not immediately needed to render callback to avoid overly long preprocessing time.