Aerial perspective
Encyclopedia
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
between the object and its background decreases, and the contrast of any markings or details within the object also decreases. The colours of the object also become less saturated
Saturation (color theory)
In colorimetry and color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color. Colorfulness is the degree of difference between a color and gray. Chroma is the colorfulness relative to the brightness of another color...
and shift towards the background color, which is usually blue, but under some conditions may be some other color (for example, at sunrise or sunset distant colors may shift towards red).
History
Aerial perspective was used in paintings from the Netherlands in the 15th Century, and explanations of its effects were with varying degrees of accuracy written by polymaths such as Leon Battista Alberti and Leonardo da VinciLeonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...
. The latter used aerial perspective in many of his paintings such as the Mona Lisa
Mona Lisa
Mona Lisa is a portrait by the Italian artist Leonardo da Vinci. It is a painting in oil on a poplar panel, completed circa 1503–1519...
and The Last Supper
The Last Supper (Leonardo)
The Last Supper is a 15th century mural painting in Milan created by Leonardo da Vinci for his patron Duke Ludovico Sforza and his duchess Beatrice d'Este...
.
Optics
The major component affecting the appearance of objects during daylightDaylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
is scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of light, called skylight
Diffuse sky radiation
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
, into the line of sight
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
of the viewer. Scattering occurs from molecules of the air and also from larger particles in the atmosphere such as water vapor and smoke (see haze
Haze
Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...
). Scattering adds the sky light as a veiling luminance onto the light from the object, reducing its contrast with the background sky light. Skylight usually contains more short wavelength light than other wavelengths (this is why the sky usually appears blue), which is why distant objects appear bluish (see Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
for detailed explanation). A minor component is scattering of light out of the line of sight of the viewer. Under daylight, this either augments the contrast loss (e.g., for white objects) or opposes it (for dark objects). At night there is effectively no skylight (unless the moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
is very bright), so scattering out of the line of sight becomes the major component affecting the appearance of self-luminous objects. Such objects have their contrasts reduced with the dark background, and their colours are shifted towards red.
Why reducing contrast reduces clarity
The ability of a person with normal visual acuityVisual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
to see fine details is determined by his or her contrast sensitivity. Contrast sensitivity is the reciprocal
Multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
of the smallest contrast for which a person can see
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
a sine-wave grating
Grating
A grating is any regularly spaced collection of essentially identical, parallel, elongated elements. Gratings usually consist of a single set of elongated elements, but can consist of two sets, in which case the second set is usually perpendicular to the first...
. A person's contrast sensitivity function is contrast sensitivity as a function of spatial frequency
Spatial frequency
In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance. The SI unit of spatial frequency is...
. Normally, peak contrast sensitivity is at about 4 cycles per degree of visual angle
Visual angle
The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size....
. At higher spatial frequencies, comprising finer and finer lines, contrast sensitivity decreases, until at about 40 cycles per degree even the brightest of bright lines and the darkest of dark lines cannot be seen.
The high spatial frequencies in an image give it its fine details. Reducing the contrast of an image reduces the visibility of these high spatial frequencies because contrast sensitivity for them is already poor. This is how a reduction of contrast can reduce the clarity of an image—by removing its fine details.
It is important to emphasize that reducing the contrast is not the same as blurring
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
an image. Blurring is accomplished by reducing the contrast only of the high spatial frequencies. Aerial perspective reduces the contrast of all spatial frequencies.
In art
In art, especially painting, aerial perspective refers to the technique of creating an illusion of depth by depicting distant objects as paler, less detailed, and usually bluer than near objects.(One caution: in common speech, the words perspective and viewpoint tend to be used interchangeably; however, in art, aerial perspective does not imply an aerial viewpoint, such as that forming the basis of the aerial landscape genre. The example by Frans Koppelaar
Frans Koppelaar
Frans Thomas Koppelaar , is a Dutch painter, who was born in The Hague, Netherlands.From 1963 - 1969 he attended the Royal Academy of Visual Arts at The Hague...
pictured here shows the difference. This landscape is a good example of aerial perspective; however, it is not an aerial landscape, since the observer is apparently standing on the ground.)
See also
- Aerial landscape artAerial landscape artAerial landscape art includes paintings and other visual arts which depict or evoke the appearance of a landscape from a perspective above it—usually from a considerable distance—as it might be viewed from an aircraft or spacecraft. Sometimes the art is based not on direct observation but on aerial...
- Aerial perspective (disambiguation)Aerial perspective (disambiguation)Aerial perspective can refer to:*Aerial perspective or atmospheric perspective is the effect on the appearance of an object by the atmosphere between it and a viewer....
- Aerial shotAerial shotAerial shots are usually done with a crane or with a camera attached to a special helicopter to view large landscapes. This sort of shot would be restricted to exterior locations. A good area to do this shot would be a scene that takes place on a building. If the aerial shot is of a character it...
- HazeHazeHaze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...
- Landscape artLandscape artLandscape art is a term that covers the depiction of natural scenery such as mountains, valleys, trees, rivers, and forests, and especially art where the main subject is a wide view, with its elements arranged into a coherent composition. In other works landscape backgrounds for figures can still...
- Rayleigh scatteringRayleigh scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
- Tyndall effectTyndall effectThe Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth...