Gamma

Gamma is the relationship between the numerical value of a pixel stored in an image file (think JPG or TIFF)  and the brightness of that pixel when viewed on-screen.

  • A computer translates the numerical values in an image file into voltages that are sent to pixels on a monitor. A higher voltage means a brighter pixel.
  • This relationship between stored value and appearance is non-linear, so a change in voltage does not translate directly into an equivalent change in brightness.
  • For many TVs and computer monitors, to double the apparent brightness of a pixel might require 2.5 times more voltage.
  • In the above example the gamma for such a device is said to be 2.5.

Gamut

A colour gamut defines a more specific range of colours from the range of colours identifiable by the human eye.

  • The range of perceived colours (visible to a human observer) is always greater than the range that can be reproduced by any digital process, or display device (screen, monitor, projector).
  • Digital cameras, scanners, monitors, and printers are all limited to the range of colours they can sense, store and reproduce. A colour gamut is established to make these differences clear and to reconcile the colours that can be used in common between devices.
  • A device that can reproduce the entire visible colour space corresponding with human perception is an unrealized goal within the engineering of display devices and printing processes.
  • When colours within a colour space cannot be reproduced using a specific colour space or display device those colours are said to be out of gamut.

Ganglion cell

A retinal ganglion cell is a type of neuron located in the retina of the human eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells.  Retinal ganglion cells transmit image-forming and non-image forming visual information to several regions in the thalamus, hypothalamus and midbrain.

  • Retinal ganglion cells are located near the boundary between the retina and the central chamber containing vitreous humour.  They collect and process all the visual information gathered directly or indirectly from the forty-something types of rod, cone, bipolar, horizontal and amacrine cells and, once finished, transmit it via their axons towards higher visual centres within the brain.
  • The axons of ganglion cells form into the fibres of the optic nerve that synapse at the other end on the lateral geniculate nucleus. Axons are like long tails and typically conduct electrical impulses, often called action potentials, away from a neuron. They take the form of long slender fibre-like projections of the cell body.
  • A single ganglion cell communicates with as few as five photoreceptors in the fovea at the centre of the macula. This produces images containing the maximum possible resolution of detail. At the extreme periphery of the retina, a single ganglion cell receives information from many thousands of photoreceptors.
  • Around twenty distinguishable functional types of ganglion cells resolve the information received from 120 million rods and cones into a million parallel streams of information about the world surveyed by a human observer throughout every day of their lives. Their functions complete the construction of the foundations of visual experience by the retina, ordering the eyes response to light into the fundamental building blocks of vision.  Ganglion cells enable retinal encodings to ultimately converge into a unified representation of the visual world.
  • As described above cone cells are attuned to different bands of wavelengths, with peak biases at 560 nm, 530 nm, and 420 nm and are concerned with trivariance – three discernible differences in the overall composition of visible light entering the eye.
  • Ganglion cells also play a critical role in trichromacy but the way they function might be thought of as being determined by limitations on bandwidth within the optic nerve.
  • Ganglion cells not only deal with colour information streaming in from rod and cone cells in real time but also with the deductions, inferences, anticipatory functions and modifications suggested by bipolar, amacrine and horizontal cells. Their challenge, therefore, is to enable all this data to converge and to assemble it into a high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the data-carrying capacity of the optic nerve.
  • It is not hard to imagine the kind of challenges that have to be dealt with:
  • Information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediately present visual impressions whenever needed.
  • Information must correspond with the outstanding discriminatory capacities that enable the visual system to operate a palette that can include millions of perceivable variations in colour.
  • Information about the outside world must be able to be automatically cross-referenced, highly detailed, specifically relevant, spatial and temporally sequenced and available on demand.
  • Information must be subjectively orientated in a way that it is locked at an impeccable level of accurate detail to even our most insane intentions as we leap from rock to rock across a swollen river or dive from an aircraft wearing only a wingsuit and negotiate the topography of a mountainous landscape speeding past at 260km per hour.
  • It is now known that efficient transmission of colour information is achieved by a transformation of the initial three colour mechanisms of rods and cones into one achromatic and two opponent chromatic channels. Opponent type processing clearly represents the optimal necessary step to dynamically readjust the eye’s earlier trivariant responses to meet criteria of efficient colour information complete with all the necessary contextualising detail ready for transmission. We can assume it is in response to these demand that every stimulus to the eye can be accurately and objectively defined in both space and time in ways relevant to everyday circumstances.

https://en.wikipedia.org/wiki/Retinal_ganglion_cell>

Greyscale

Greyscale images are also known as black-and-white or monochrome images and are composed exclusively of shades of grey, varying from black at the weakest intensity to white at the strongest. A greyscale image shows natural colour luminance with hue and saturation removed and so it carries only intensity information.

  • Greyscale images have many shades of grey in between black and white.
  • Greyscale images are distinct from one-bit bi-tonal black-and-white images which, in the context of computer imaging, are images with only two colours: black and white.
  • Greyscale images can be the result of measuring the intensity of light at each pixel against a selected wavelength or a weighted combination of wavelengths. In this case, the selected wavelengths can be from anywhere within the electromagnetic spectrum (e.g. infrared, visible light, ultraviolet.).
  • A colourimetric (or more specifically photometric) grayscale image is an image that has a defined greyscale colourspace, which maps the stored numeric sample values to the achromatic channel of a standard colourspace, which itself, is based on measured properties of human vision.