Editing Transparency (data compression)

{{Short description|Perceptually indistinguishable 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 [[perception|perceptually]] indistinguishable from the uncompressed input, i.e. '''perceptually lossless'''.

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&nbsp;kbit/s, at [[44.1 kHz|44.1&nbsp;kHz]], when encoded as [[Variable_bitrate|VBR]] MP3 (corresponding to the -V3 and -V0 settings of the highly popular [[LAME]] MP3 encoder).<ref name="LAME Recommended Encoder Settings">{{citation
  | title = LAME Recommended Encoder Settings 
  | publisher = hydrogenaudio 
  | url = http://wiki.hydrogenaud.io/index.php?title=LAME }}</ref> 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-expectancy effect|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 test|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 compression|lossless data compression]] methods are transparent, by nature.

=== In image compression ===
Both the DSC in [[DisplayPort]] and the default settings of [[JPEG XL]]<ref>{{man|1|cjxl|ManKier}}</ref> 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.<ref>{{cite book |title=ISO/IEC 29170-2:2015 Information technology — Advanced image coding and evaluation — Part 2: Evaluation procedure for nearly lossless coding|url=https://www.iso.org/standard/66094.html |url-access=subscription |chapter=Annex B. Forced choice paradigm with interleaved images test protocol|publisher=International Organization for Standardization |language=en}}</ref> There is also a ''panning'' test that is purportedly more representative of sensitivity in the case of moving images than the ''flicker'' test.<ref>{{cite conference |author1=Allison, Robert |author2=Wilcox, Laurie |author3=Wang, Wei |author4=Hoffman, David |author5=Hou, Yuqian |author6=Goel, James |author7=Deas, Lesley |author8=Stolitzka, Dale |conference=The Society for Information Display's annual Display Week 2017 |title=Large Scale Subjective Evaluation of Display Stream Compression |url=https://www.researchgate.net/publication/317425815}}</ref>

== 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<ref>{{cite journal |author=European Society of Radiology |title=Usability of irreversible image compression in radiological imaging. A position paper by the European Society of Radiology (ESR) |journal=Insights into Imaging |date=April 2011 |volume=2 |issue=2 |pages=103–115 |doi=10.1007/s13244-011-0071-x |doi-access=free |pmid=22347940 |pmc=3259360}}</ref> to being indistinguishable on a side-to-side view,<ref>{{cite journal |last1=Kim |first1=Kil Joong |last2=Kim |first2=Bohyoung |last3=Lee |first3=Kyoung Ho |last4=Mantiuk |first4=Rafal |last5=Richter |first5=Thomas |last6=Kang |first6=Heung Sik |title=Use of Image Features in Predicting Visually Lossless Thresholds of JPEG2000 Compressed Body CT Images: Initial Trial |journal=Radiology |date=September 2013 |volume=268 |issue=3 |pages=710–718 |doi=10.1148/radiol.13122015|pmid=23630311 |doi-access=free }}</ref> neither being as stringent as the ''flicker'' test.

==See also==
*[[Codec listening test]]
*{{Section link|High fidelity|Listening tests}}

==References==
{{reflist}}
{{refbegin}} 
* Bosi, Marina; Richard E. Goldberg. ''Introduction to digital audio coding and standards''. Springer, 2003. {{ISBN|1-4020-7357-7}}
* Cvejic, Nedeljko; Tapio Seppänen. ''Digital audio watermarking techniques and technologies: applications and benchmarks''. Idea Group Inc (IGI), 2007. {{ISBN|1-59904-513-3}}
* Pohlmann, Ken C. ''Principles of digital audio''. McGraw-Hill Professional, 2005. {{ISBN|0-07-144156-5}}
* Spanias, Andreas; Ted Painter; Venkatraman Atti. ''Audio signal processing and coding''. Wiley-Interscience, 2007. {{ISBN|0-471-79147-4}}
* Syed, Mahbubur Rahman. ''Multimedia technologies: concepts, methodologies, tools, and applications, Volume 3''. Idea Group Inc (IGI), 2008. {{ISBN|1-59904-953-8}}
{{refend}} 

==External links==
*[http://wiki.hydrogenaud.io/index.php?title=Transparent "Transparency"], Hydrogen Audio Wiki

[[Category:Data compression]]
[[Category:Audio codecs]]