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# How Look Files Work

To gain a better understanding of what Look Files are and how they can be used we are going to look in more detail at how they actually work.

The geek-eye view of a Look File is that it's a 'scene graph diff'. In other words it's a list of all the changes that need to be made to a sub-branch of the scene graph hierarchy to take from an initial state (typically a model without any materials assigned) to a new transformed state (typically the model with all its materials assigned to correct pieces of geometry, and any additional overrides such as shader values or to change visibility flags). When you do a LookFileResolve all those changes are re-played back onto the relevant scene graph locations.

To make material assignments work all the materials assigned within the hierarchy are also written out into the Look File. Similarly, any renderer procedurals required are also written out into the Look File.

For each render pass a separate scene graph diff is supplied. There are two caveats we should mention about Look Files:

• The data in Look Files is non-animating, so you can't use them to hand off animating data such as flashing disco lights or lightning strikes. Animating data like this can be handled in a number of ways, including making the animating data part of the 'naked' asset, or by using Katana Macros and Live Groups to hand off actual Katana node that can have animating parameters.

• Currently you can't delete scene graph locations using Look Files, you can only add new locations or modify existing ones. For instance, to hide an object you should set its visibility options rather than pruning it from the scene graph.