Editing Lossy compression (section)

====Downsampling/compressed representation scalability====
One may wish to [[downsample]] or otherwise decrease the resolution of the represented source signal and the quantity of data used for its compressed representation without re-encoding, as in [[bitrate peeling]], but this functionality is not supported in all designs, as not all codecs encode data in a form that allows less important detail to simply be dropped. Some well-known designs that have this capability include [[JPEG 2000]] for still images and [[H.264/MPEG-4 AVC]] based [[Scalable Video Coding]] for video.  Such schemes have also been standardized for older designs as well, such as [[JPEG]] images with progressive encoding, and [[MPEG-2]] and [[MPEG-4 Part 2]] video, although those prior schemes had limited success in terms of adoption into real-world common usage. Without this capacity, which is often the case in practice, to produce a representation with lower resolution or lower fidelity than a given one, one needs to start with the original source signal and encode, or start with a compressed representation and then decompress and re-encode it ([[transcoding]]), though the latter tends to cause [[digital generation loss]].

Another approach is to encode the original signal at several different bitrates, and then either choose which to use (as when streaming over the internet – as in [[RealNetworks]]' "[[SureStream]]" – or offering varying downloads, as at Apple's [[iTunes Store]]), or broadcast several, where the best that is successfully received is used, as in various implementations of [[hierarchical modulation]]. Similar techniques are used in [[mipmap]]s, [[pyramid (image processing)|pyramid representation]]s, and more sophisticated [[scale space]] methods. Some audio formats feature a combination of a lossy format and a lossless correction which when combined reproduce the original signal; the correction can be stripped, leaving a smaller, lossily compressed, file. Such formats include [[MPEG-4 SLS]] (Scalable to Lossless), [[WavPack]], [[OptimFROG DualStream]], and [[DTS-HD Master Audio|DTS-HD Master Audio in lossless (XLL) mode]]).