TexturePacker, a great tool for generating texture atlases, removed support for ATF after TexturePacker version 6.0.1, because data had indicated that ATF was not being widely used by users of TexturePacker.
ATF supports GPU compression, meaning textures are uploaded and stored in the GPU in their compressed state.
The conventional PNG (and other similar conventional formats) on the other hand, while initially compressed, must be decompressed before being uploaded to the GPU and are stored in GPU memory in a completely uncompressed state.
When producing texture atlases, what image format would you still use if you could?
1. ATF
2. PNG