Home > Lossy data compression
A lossy data compression method is one where compressing a file and then decompressing it retrieves a file that may well be different to the original, but is "close enough" to be useful in some way. This type of compression is used a lot on the Internet and especially in streaming media and telephony applications. These methods are typically referred to as codecs in this context.There are two basic lossy compression schemes:
- In lossy transform codecs, samples of picture or sound are taken, chopped into small segments, transformed into a new basis space, and quantized. The resulting quantized values are then entropy coded.
- In lossy predictive codecs, previous and/or subsequent decoded data is used to predict the current sound sample or image frame. The error between the predicted data and the real data, together with any extra information needed to reproduce the prediction, is then quantized and coded.
In some systems the two techniques are combined, with transform codecs being used to compress the error signals generated by the predictive stage.
The advantage of lossy methods over
lossless methods is that in some cases a lossy method can
produce a much smaller compressed file than any known lossless method, while still meeting
the requirements of the application.
Lossy methods are most often used for compressing sound or images.
In these cases, the retrieved file can be quite different to the original at the bit level while being indistinguishable to the human ear or eye for most practical purposes.
Many methods focus on the idiosyncrasies of the human anatomy, taking into account, for example, that the human eye can see only certain frequencies of light. The
psychoacoustic model describes how sound can be highly compressed without degrading the perceived quality of the sound. Flaws caused by lossy compression
that are noticeable to the human eye or ear are known as compression artifacts.
1 Lossy compression methods
1.1 Still image compression
1.2 Moving image compression
- Flash (also supports JPEG sprites)
- H.261
- H.263
- MNGMNG (pronounced ming is a public file format for animated images. The name stands for Multiple-image Network Graphics''. MNG is closely related to the PNG image format. When PNG development started in early 1995, developers decided not to incorporate supp (supports JPEG sprites)
- Motion JPEGMotion JPEG also known as M-JPEG, is a video file format consisting of a sequence of individual JPEG images; unlike other video compression algorithms, each frame is an image unto itself. Often used by Video D-jays and multimedia artists. M-JPEG is normal
- MPEG-1MPEG-1 is the designation for a group of audio and video coding standards agreed upon by MPEG (Moving Pictures Experts Group). MPEG-1 video is used by the Video CD format. The output quality at usual VCD bitrates is less than that of a VCR. MPEG-1 audio l
- MPEG-2MPEG-2 (1994) is the designation for a group of audio and video coding standards agreed upon by MPEG (Moving Pictures Experts Group), and published as ISO standard 13818. MPEG-2 is typically used to encode audio and video for broadcast signals, including
- MPEG-4MPEG-4 introduced in 1998, is the designation for a group of audio and video coding standards agreed upon by MPEG (Moving Picture Experts Group). MPEG-4 is primarily designed to handle low bit rate content, from 4800 bit/s to approximately 4 Mbit/s. The p
- Ogg TheoraTheora is a video codec being developed by the Xiph. org Foundation as part of their Ogg project. Based upon On2 Technologies' VP3 codec, and christened by On2 as the successor in VP3's lineage, Theora is targeted at competing with MPEG-4 video (e. XviD a (noted for its lack of patent restrictions)
- Sorenson video codecThe Sorenson codec (also known as Sorenson Video Codec 3 or SVQ3 is a digital video codec devised by the company Sorenson Media and used by Apple's QuickTime and the newest version of Macromedia Flash, a special version called Sorenson Spark''. The Sorens
