Photoclinometry, or shape-from-shading, is the process by which a 2-dimensional image of a surface is transformed into a surface map that represents different levels of elevation. It uses the shadows and light direction as reference points. It is used mostly to depict the surface of sculptures, to give an idea of how it would look in 3-dimensions. The techniques depend on very specific conditions, especially light direction.

The process

When light bounces off an object, it reflects with location-specific brightness that depends on the shape of the object and the position of the lightning source. This reflection map can be used to create a

bump map Bump mapping is a texture mapping technique in computer graphics for simulating bumps and wrinkles on the surface of an object. This is achieved by perturbing the surface normals of the object and using the perturbed normal during lighting calcu ...

of a surface, which uses

grayscale In digital photography, computer-generated imagery, and colorimetry, a grayscale image is one in which the value of each pixel is a single sample representing only an ''amount'' of light; that is, it carries only intensity information. Graysca ...

levels to depict the height of a point on a surface. Knowing the direction and angle of the light source, one can calculate the approximate height that a point has. However, the light direction and angle is usually unknown, which leads to poor quality images, or having to create multiple

s, each time trying different settings.

Usage

* Some artists use photoclinometry to digitize a 3-dimensional representation of a sculpture. *

Geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

s and those that study

planetary science Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their f ...

use it to get an idea of how the surface of a planet looks like, and generate

topographic maps In modern mapping, a topographic map or topographic sheet is a type of map characterized by large- scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...

and

digital elevation model A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete gl ...

s (see

photometric stereo Photometric stereo is a technique in computer vision for estimating the surface normals of objects by observing that object under different lighting conditions. It is based on the fact that the amount of light reflected by a surface is dependent ...

). Planetary applications began with investigations of lunar topography as early as 1951 and has since been applied to the Martian surface. * Glaciologists have used photoclinometry to characterize the surfaces of Antarctica and Greenland. Repeat photoclinometry is a method of building surfaces from a series of satellite image to investigate changes in ice sheet topography over time. * Photoclinometry has been used to measure volcanic plume top topography on Earth.

Problems

Light direction is very important to the quality of a photoclinometric image. Light that comes from directly over the surface (behind the camera) makes it hard to distinguish the shadows. Multiple light sources are also a problem, since they destroy important shadows required for the algorithms to work properly. In order to solve these problems, new missions to other planets plan to use a process similar to

stereoscopy Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...

in order to get a more accurate depiction of the surface on another planet. The

Mars Reconnaissance Orbiter ''Mars Reconnaissance Orbiter'' (MRO) is a spacecraft designed to study the geology and climate of Mars, provide reconnaissance of future landing sites, and relay data from surface missions back to Earth. It was launched on August 12, 2005, an ...

is one of the mission that attempts to do this. This process uses two images of one location taken from two separate lens on a camera, much in the same way humans do with their eyes. By using two images, they can get a 3-dimensional perspective of objects on the surface like we do.

References

External links

Shape from Shading - Photoclinometry

Photographic processes {{photography-stub

The process

Usage

Problems

See also

References

Further reading

External links