visually lossless
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In
data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electro ...
and
psychoacoustics Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. Psychophysics has b ...
, transparency is the result of
lossy data compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size fo ...
accurate enough that the compressed result is
perceptually
perceptually
indistinguishable from the uncompressed input, i.e. perceptually lossless. In other words, transparent compression has no perceptible
compression artifact A compression artifact (or artefact) is a noticeable distortion of media (including Image, images, Sound recording, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the medi ...
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 Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signal An analog signal is any continuous signal for which the time-varying feature of the signal is a representation of some other time-varying quantity ...
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 Lame or LAME may refer to: Music * "Lame" (song) by Unwritten Law * ''Lame'' (album) by Iame People * Ibrahim Lame (born 1953), Nigerian educator and politician * Jennifer Lame (), American film editor * Quintín Lame (1880-1967), Colombian r ...
MP3 encoder). 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 In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
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 In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit Data rate units, bit per second unit (symbol: ''bit/s' ...
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 research, the observer bias is a form of detection bias originating at a study’s stage of observing or recording information. Different observers may assess subjective criteria differently, and cognitive biases (including preconceptions and ass ...
, in which subjective like/dislike of a certain compression methodology emotionally influences their judgment. This bias is commonly referred to as ''
placebo A placebo ( ) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like saline), sham surgery, and other procedures. In general, placebos can af ...

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 In a blind or blinded experiment, information which may influence the participants of the experiment An experiment is a procedure carried out to support or refute a , or determine the or of something previously untried. Experiments provide ...
tests may be useful. The ABX method is normally used, with a
null hypothesis In inferential statistics, the null hypothesis (often denoted ''H''0) is that there is no difference between two possibilities. The null hypothesis is that the observed difference is due to chance alone. Using statistical tests it is possible to ...
that the samples tested are the same and with an
alternative hypothesis In statistical hypothesis testing A statistical hypothesis test is a method of statistical inference Statistical inference is the process of using data analysis Data analysis is a process of inspecting, Data cleansing, cleansing, Data tran ...
that the samples are in fact different. All
lossless data compression Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipme ...
methods are transparent, by nature.


In image compression

Both the DSC in
DisplayPort DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association VESA (), formally known as Video Electronics Standards Association, is ...

DisplayPort
and the default settings of
JPEG XL JPEG XL is a royalty-free Royalty-free (RF) material subject to copyright Copyright is a type of intellectual property Intellectual property (IP) is a category of property Property is a system of rights that gives people legal con ...
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. There is also a "panning" test that is purportedly more sensitive than the "flicker" test.


Difference from a lack of artifacts

A perceptually lossless compression is always free of
compression artifacts A compression artifact (or artefact) is a noticeable distortion of media (including Image, images, Sound recording, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the medi ...

compression artifacts
, but the inverse is not true: it is possible for a compressor to produce a natural-looking image with altered contents. Such a confusion is widely present in the field of
radiology Radiology is the medical discipline that uses medical imaging Medical imaging is the technique and process of imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the form ...

radiology
(specifically for the study of diagnostically acceptable irreversible compression), where "visually lossless" is taken to mean anywhere from artifact-free to being indistinguishable on a side-to-side view.


See also

*
Codec listening test A codec listening test is a scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world ...
*


References

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


External links


"Transparency"
Hydrogen Audio Wiki Data compression Audio codecs