  The Cherenkov Telescope Array (CTA) is a ground-based $\gamma$-ray
observatory that will observe the full sky in the energy range from 20 GeV to
100 TeV from facilities in both hemispheres. It is proposed to consist of more
than 100 telescopes and the large amount of data produced will exceed the
volume of current VHE Imaging Atmospheric Cherenkov Telescopes by $\sim$two
orders of magnitude. This volume of data represents a new challenge to the
community, which is looking for new data formats to transfer and store the CTA
data. One of the prototypes currently under study is the ROI (Regions Of
Interest) file format for camera images. It can store only those pixels of a
camera image that are close to the shower, thus removing the major part of the
night sky background (NSB) while keeping all pixels that might belong to the
shower. Simple on-the-fly compression is used to reduce the file size even
further. Here, we explain the ROI prototype in detail and present preliminary
results when applied to simulations.
