A human visual system model (HVS model) is used by image processing,

video processing In electronics engineering, video processing is a particular case of signal processing, in particular image processing, which often employs video filters and where the input and output signals are video files or video streams. 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 The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...

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 In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...

, and Cathode-ray tube (CRT) 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 Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance. It is u ...

. Another example is lossy 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 compression, where sound frequencies inaudible to humans are bandstop 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

Assumptions about the HVS

Low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...

characteristic (limited number of rods in human eye): see Mach bands * Lack of colour resolution (fewer cones in human eye than rods) * Motion sensitivity ** More sensitive in

peripheral vision Peripheral vision, or ''indirect vision'', is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in (or out of) the "corner of one's eye". The vast majority of the area in th ...

** Stronger than texture sensitivity, e.g. viewing a camouflaged animal * Texture stronger than disparity - 3D depth resolution does not need to be so accurate * Integral Face recognition (babies smile at faces) ** Depth inverted face looks normal (facial features overrule depth information) *** Upside down face with inverted mouth and eyes looks normalMargaret Thatcher Illusion – Mighty Optical Illusions
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Examples of taking advantage of an HVS model

* Flicker frequency of film and television using persistence of vision to fool viewer into seeing a continuous image * Interlaced television painting half images to give the impression of a higher flicker frequency * Colour 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 quantised) *

Motion estimation Motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent frames in a video sequence. It is an ill-posed problem as the motion is in three dimensions ...

(use luminance and ignore colour) *

Watermark A watermark is an identifying image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations ...

ing and

Steganography Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a computer file, ...

References

{{reflist Computer vision

Block diagram of HVS

Assumptions about the HVS

Examples of taking advantage of an HVS model

See also

References