Transparency (data compression)

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In data compression and psychoacoustics, transparency is the result of lossy data compression accurate enough that the compressed result is perceptually indistinguishable from the uncompressed input, i.e. perceptually lossless.

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A transparency threshold is a given value at which transparency is reached. It is commonly used to describe compressed data bitrates. For example, the transparency threshold for MP3 to linear PCM audio is said to be between 175 and 245 kbit/s, at 44.1 kHz, when encoded as VBR MP3 (corresponding to the -V3 and -V0 settings of the highly popular LAME MP3 encoder). [1] This means that when an MP3 that was encoded at those bitrates is being played back, it is indistinguishable from the original PCM, and the compression is transparent to the listener.

The term transparent compression can also refer to a filesystem feature that allows compressed files to be read and written just like regular ones. In this case, the compressor is typically a general-purpose lossless compressor.

Determination

Transparency, like sound or video quality, is subjective. It depends most on the listener's familiarity with digital artifacts, their awareness that artifacts may in fact be present, and to a lesser extent, the compression method, bit rate used, input characteristics, and the listening/viewing conditions and equipment. Despite this, sometimes general consensus is formed for what compression options "should" provide transparent results for most people on most equipment. Due to the subjectivity and the changing nature of compression, recording, and playback technology, such opinions should be considered only as rough estimates rather than established fact.

Judging transparency can be difficult, due to observer bias, in which subjective like/dislike of a certain compression methodology emotionally influences their judgment. This bias is commonly referred to as placebo , although this use is slightly different from the medical use of the term.

To scientifically prove that a compression method is not transparent, double-blind tests may be useful. The ABX method is normally used, with a null hypothesis that the samples tested are the same and with an alternative hypothesis that the samples are in fact different.

All lossless data compression methods are transparent, by nature.

In image compression

Both the DSC in DisplayPort and the default settings of JPEG XL [2] are regarded as visually lossless. The losslessness is usually determined by a flicker test: the display initially shows the compressed and the original side-by-side, switches them around for a tiny fraction of a second and then goes back to the original. This test is more sensitive than a side-by-side comparison ("visually almost lossless"), as the human eye is highly sensitive to temporal changes in light. [3] There is also a panning test that is purportedly more representative of sensitivity in the case of moving images than the flicker test. [4]

Difference from a lack of artifacts

A perceptually lossless compression is always free of compression artifacts, but the inverse is not true: it is possible for a compressor to produce a signal that appears natural but with altered contents. Such a confusion is widely present in the field of radiology (specifically for the study of diagnostically acceptable irreversible compression), where visually lossless is taken to mean anywhere from artifact-free [5] to being indistinguishable on a side-to-side view, [6] neither being as stringent as the flicker test.

See also

Related Research Articles

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In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression reduces bits by identifying and eliminating statistical redundancy. No information is lost in lossless compression. Lossy compression reduces bits by removing unnecessary or less important information. Typically, a device that performs data compression is referred to as an encoder, and one that performs the reversal of the process (decompression) as a decoder.

<span class="mw-page-title-main">Lossy compression</span> Data compression approach that reduces data size while discarding or changing some of it

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Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistical redundancy. By contrast, lossy compression permits reconstruction only of an approximation of the original data, though usually with greatly improved compression rates.

<span class="mw-page-title-main">MP3</span> Digital audio format

MP3 is a coding format for digital audio developed largely by the Fraunhofer Society in Germany under the lead of Karlheinz Brandenburg, with support from other digital scientists in other countries. Originally defined as the third audio format of the MPEG-1 standard, it was retained and further extended—defining additional bit rates and support for more audio channels—as the third audio format of the subsequent MPEG-2 standard. A third version, known as MPEG-2.5—extended to better support lower bit rates—is commonly implemented but is not a recognized standard.

MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to about 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting (DAB) practical.

<span class="mw-page-title-main">PNG</span> Family of lossless-compression image file formats

Portable Network Graphics is a raster-graphics file format that supports lossless data compression. PNG was developed as an improved, non-patented replacement for Graphics Interchange Format (GIF)—unofficially, the initials PNG stood for the recursive acronym "PNG's not GIF".

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<span class="mw-page-title-main">Compression artifact</span> Distortion of media caused by lossy data compression

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Generation loss is the loss of quality between subsequent copies or transcodes of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be considered a form of generation loss. File size increases are a common result of generation loss, as the introduction of artifacts may actually increase the entropy of the data through each generation.

A codec listening test is a scientific study designed to compare two or more lossy audio codecs, usually with respect to perceived fidelity or compression efficiency.

JPEG XR is an image compression standard for continuous tone photographic images, based on the HD Photo specifications that Microsoft originally developed and patented. It supports both lossy and lossless compression, and is the preferred image format for Ecma-388 Open XML Paper Specification documents.

<span class="mw-page-title-main">Audio coding format</span> Digitally coded format for audio signals

An audio coding format is a content representation format for storage or transmission of digital audio. Examples of audio coding formats include MP3, AAC, Vorbis, FLAC, and Opus. A specific software or hardware implementation capable of audio compression and decompression to/from a specific audio coding format is called an audio codec; an example of an audio codec is LAME, which is one of several different codecs which implements encoding and decoding audio in the MP3 audio coding format in software.

Display Stream Compression (DSC) is a VESA-developed video compression algorithm designed to enable increased display resolutions and frame rates over existing physical interfaces, and make devices smaller and lighter, with longer battery life. It is a low-latency algorithm based on delta PCM coding and YCGCO-R color space.

JPEG XS is an interoperable, visually lossless, low-latency and lightweight image and video coding system used in professional applications. Applications of the standard include streaming high quality content for virtual reality, drones, autonomous vehicles using cameras, gaming, and broadcasting. It was the first ISO codec ever designed for this specific purpose. JPEG XS, built on core technology from both intoPIX and Fraunhofer IIS, is formally standardized as ISO/IEC 21122 by the Joint Photographic Experts Group with the first edition published in 2019. Although not official, the XS acronym was chosen to highlight the eXtra Small and eXtra Speed characteristics of the codec. Today, the JPEG committee is still actively working on further improvements to XS, with the second edition scheduled for publication and initial efforts being launched towards a third edition.

References

  1. ↑ cjxl(1)  –  Linux General Commands Manual
  2. ↑ ISO/IEC JTC 1/SC 29. "Annex B. Forced choice paradigm with interleaved images test protocol". ISO/IEC 29170-2:2015 Information technology — Advanced image coding and evaluation — Part 2: Evaluation procedure for nearly lossless coding. ISO.
  3. ↑ Allison, Robert; Wilcox, Laurie; Wang, Wei; Hoffman, David; Hou, Yuqian; Goel, James; Deas, Lesley; Stolitzka, Dale. Large Scale Subjective Evaluation of Display Stream Compression. The Society for Information Display's annual Display Week 2017.
  4. ↑ European Society of Radiology (April 2011). "Usability of irreversible image compression in radiological imaging. A position paper by the European Society of Radiology (ESR)". Insights into Imaging. 2 (2): 103–115. doi: 10.1007/s13244-011-0071-x . PMC   3259360 . PMID   22347940.
  5. ↑ Kim, Kil Joong; Kim, Bohyoung; Lee, Kyoung Ho; Mantiuk, Rafal; Richter, Thomas; Kang, Heung Sik (September 2013). "Use of Image Features in Predicting Visually Lossless Thresholds of JPEG2000 Compressed Body CT Images: Initial Trial". Radiology. 268 (3): 710–718. doi: 10.1148/radiol.13122015 . PMID   23630311.
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