Human visual system model: Difference between revisions

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{{tone|date=September 2022}}

A '''human visual system model''' (HVS model) is used by [[image processing]], [[video processing]] and [[computer vision]] experts to deal with biological and psychological processes that are not yet fully understood. Such a model is used to simplify the behavioursbehaviors of what is a very [[complex system]]. As our knowledge of the true [[visual system]] improves, the model is updated.

'''Psychovisual''' study is the study of the psychology of vision.

The human visual system model can be used to produce desired effects in perception and vision. Examples of using an HVS model include [[color television]], [[lossy compression]], and [[Cathode-ray tube]] (CRT) television.

Originally, it was thought that colourcolor television required too high a bandwidth for the then available technology. Then it was noticed that the colourcolor resolution of the HVS was much lower than the brightness resolution; this allowed colourcolor to be squeezed into the signal by [[chroma subsampling]].

Another example is lossy image compression, like [[JPEG]]. Our HVS model says that we cannot see high frequency detail, so in JPEG we can quantisequantize these components without a perceptible loss of quality. Similar concepts are applied in [[Data compression#Lossy audio compression|audio compression]], where sound frequencies inaudible to humans are bandstopband-stop filtered.

Several HVS features are derived from evolution, when we needed to defend ourselves or hunt for food. We often see demonstrations of HVS features when we are looking at optical illusions.

==Block diagram of HVS==

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* [[Low-pass filter]] characteristic (limited number of rods in human eye): see [[Mach bands]]

* Lack of colourcolor resolution (fewer cones in human eye than rods)

* Motion sensitivity

** More sensitive in [[peripheral vision]]

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* [[Flicker fusion threshold|Flicker frequency of film and television using persistence of vision to fool viewer into seeing a continuous image]]

* [[Interlaced video|Interlaced television]] painting half images to give the impression of a higher flicker frequency

* ColourColor television (chrominance at half resolution of luminance corresponding to proportions of rods and cones in eye)

* Image compression (difficult to see higher frequencies more harshly quantisedquantized)

* [[Motion estimation]] (use luminance and ignore colourcolor)

* [[Watermark]]ing and [[Steganography]]