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'''. In other words, transparent compression has no perceptible [[compression artifact]]s.
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|44.1 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
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=== 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 sensitiverepresentative 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-lookingimagebut 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.
