The simplest and most robust free tool for creating texture atlases is [Free Texture Packer](https://github.com/odrick/free-tex-packer).

However, if you are tied to a specifics graphics platform, you may need or find greater efficiency in using the texture packer designed for and included in your platform.  One example would be the [Texture Atlas Tools for NVIDIA](https://www.nvidia.com/en-us/drivers/texture-atlas-tools/), and many platforms have their own.  Each one may be best for its own patform, but may or may not be able to export effeciently or at all to other platforms.  

So for multiple platforms, Free Texture Packer would still be best, as it performs images optimization using the widely-used TinyPNG, and supports the following exports:
- Json
- Xml
- Css
- Pixi.js
- Phaser
- Cocos2d
- Spine
- Unreal Engine
- Starling
- Custom exporters


Can you tell me a specific platform you wish to know more about? Or is there something else I can help you with?
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