% SPDX-FileCopyrightText: 2023 Felix Thaler
% SPDX-License-Identifier: GPL-3.0-or-later

# Animation

## Introduction

By default, the add-on simulates the growth process of the snowflake and keeps the end result as a static mesh.
However, you might also want to visualize the full process instead of only the final state and that’s exactly where the animation feature steps in.
An example of simple a animation of snowflake growth is shown in {numref}`anim-example`.

```{figure} ../_static/growing.gif
:width: 512px
:name: anim-example

The full growth process of a snowflake.
```

Technically, the add-on saves the simulated frames to a Universal Scene Description (USD) file.
This can then be reimported into Blender for further processing and rendering.
Note however that the whole process is fully automated, so apart from clicking a button to enable the feature, no manual user interaction is required.

## Export a Snowflake Growth Animation

Enabling the animation feature is easy: just click on `Export Animation` in the sidebar.
If you click that button, three new settings appear: `Steps per frame`, `Apply animation`, and `Animation file`.

The setting `Steps per frame` essentially controls the animation speed, that is, how many simulation steps happen during one animation frame.
Therefore, more simulation steps per frame means faster animation.

`Apply animation` defines, if the animation is directly applied to the given snowflake as a Blender mesh cache modifier.
The animation data is always exported to an external Universal Scene Description (USD) file.
If this setting is enabled, the resulting USD file is automatically reimported, and you can directly scroll through the timeline to see your animated simulation result.

Finally, `Animation file` controls, to which file the animation data is saved.
This has to be some path with *.usd*, *.usda*, or *.usdc* extension.

```{caution}
Make sure to specify unique file names if you animate multiple snowflake in one scene.
If not, one snowflake’s output can easily be overwritten by another’s!
```

If you now grow your snowflake – no matter if some fixed number of steps or interactively –, the growth process will be saved to the given USD file.
You can even simulate some steps, change parameters, simulate some more steps, etc., and everything will be exported to USD.

If `Apply animation` is set, you can also directly see the updated animation when scrolling through the timeline;
If not, you can later re-import the USD file manually using Blender’s standard USD import functionality.
Or import it into any other software which supports USD!