Human visual system model: Difference between revisions

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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 behaviours of what is a very complex system. As our knowledge of the true [[visual system]] improves, the model is updated.

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.

It is common to think of "taking advantage" of the HVS model to produce desired effects. Examples of taking advantage of an HVS model include colour television. Originally it was thought that colour television required too high a bandwidth for the then available technology. Then it was noticed that the colour resolution of the HVS was much lower than the brightness resolution; this allowed colour to be squeezed into the signal by [[chroma subsampling]]. Another example is image compression, like [[JPEG]]. Our HVS model says that we cannot see high frequency detail so in JPEG we can quantise these components without a perceptible loss of quality. Similar concepts are applied in [[Audio_data_compression#Lossy_audio_compression| audio compression]], where sound frequencies inaudible to humans are bandpass filtered.

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

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.

The human visual system model can 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 color television required too high a bandwidth for the then available technology. Then it was noticed that the color resolution of the HVS was much lower than the brightness resolution; this allowed color to be squeezed into the signal by [[chroma subsampling]].

Another example is lossy image compression, like [[JPEG]]. Our HVS model says we cannot see high frequency detail, so in JPEG we can quantize 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 band-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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==Assumptions about the HVS==

* [[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]]

** Stronger than texture sensitivity, e.g. viewing a camouflaged animal

* Texture stronger thatthan disparity -{{En dash}} 3D depth resolution does not need to be so accurate

* Integral Face recognition (babies smile at faces)

** [[Hollow-face illusion| Depth inverted face looks normal]] (facial features overrule depth information)

*** Upside down face with inverted mouth and eyes looks normal<ref>[http://www.moillusions.com/2006/09/margaret-thatcher-illusion.html Margaret Thatcher Illusion -– Mighty Optical Illusions<!-- Bot generated title -->]</ref>

==Examples of taking advantage of an HVS model==

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

* [[Interlacing|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]]

==See also==

* [[Psychoacoustics]]

* [[Visual system]]

* [[Visual perception]]

* [[Depth perception]]

==References==