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

Image processing

In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...

, video processing

Video processing

In electrical engineering and computer science, video processing is a particular case of signal processing, which often employs video filters and where the input and output signals are video files or video streams. Video processing techniques are used in television sets, VCRs, DVDs, video codecs,...

 and computer vision

Computer vision

Computer vision is a field that includes methods for acquiring, processing, analysing, and understanding images and, in general, high-dimensional data from the real world in order to produce numerical or symbolic information, e.g., in the forms of decisions...

 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

Visual system

The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...

 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

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....

. Another example is image compression, like JPEG

JPEG

In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

. 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 bandpass 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.

Assumptions about the HVS

• Low-pass filter
  Low-pass filter
  A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
  
   characteristic (limited number of rods in human eye): see Mach bands
  Mach bands
  Mach bands is an optical illusion named after the physicist Ernst Mach. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image that have different lightness gradients .-Explanation:The Mach bands effect is due to the spatial...
  
  
• Lack of colour resolution (fewer cones in human eye than rods)
• Motion sensitivity
  • More sensitive in peripheral vision
    Peripheral vision
    Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
    
    
  • Stronger than texture sensitivity, e.g. viewing a camouflaged animal
• Texture stronger that disparity - 3D depth resolution does not need to be so accurate
• Integral Face recognition (babies smile at faces)
  • Depth inverted face looks normal
    Hollow-Face illusion
    The Hollow-Face illusion is an optical illusion in which the perception of a concave mask of a face appears as a normal convex face....
    
     (facial features overrule depth information)
    • Upside down face with inverted mouth and eyes looks normal

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
  Flicker fusion threshold
  The flicker fusion threshold is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer...
  
  
• 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
  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 but the images are a projection of the 3D scene onto a 2D...
  
  (use luminance and ignore colour)