Glossary

Ambient illumination
Light that falls on a surface that does not come directly from a light source. For example, if you cast a shadow on the ground on a sunny day, ambient illumination from the blue sky will still illuminate the area inside the cast shadow, even though the direct illumination (the sun) is blocked.

Chromatic aberration
Artifacts in the image caused by different wavelengths of light taking slightly different paths through the lens of the camera. Chromatic aberration artifacts show up as strongly colored pixels on edges, generally away from the center of the image. Chromatic aberration correction algorithms reduce these artifacts and can produce higher quality data.

Direct illumination
Light that falls on a surface that comes directly from a light source. For example, unblocked light from the sun (outdoors) or a lamp (indoors) is direct illumination. Blocking the direct illumination causes shadows.

Gamma compression
A nonlinear transformation of the image intensity of the form Cnew = C 1/g, where g is the gamma value. If g > 1, then gamma compression will brighten darker areas of the image and reduce contrast in brighter areas of the image. If g < 1, then gamma compression will reduce contrast in darker areas of the image and enhance contrast in brighter areas. Commonly used gamma values for brightening dark areas are in the range 1.8-2.2.

Illumination map
An image that shows the color and intensity of light illuminating each surface in the scene. The illumination is modulated by shadows, where a light source is blocked; shading, as the geometry of the surface changes; and interreflection, where light bounces off one object onto another.

Linear image
An image with a linear relationship between the pixel values and the number of photons hitting the camera sensor. If twice as many photons hit a camera sensor, then the pixel value in a linear image will be twice as large. Camera raw images preserve the linearity of the image data. A gamma compression operation on an image breaks the linear relationship.

Lit area
An area of a scene where the direct light source is unblocked. A fully-lit area of a scene is where the light source is unblocked and the surface is facing directly towards the light source.

Penumbra
An area in a scene where the direct light source is partially blocked. It is the transition area between a lit area and a shadowed area.

Product image
The image obtained after multiplying an illumination map with a reflectance map. If the illumination map and reflectance map are unmodified, the product image is identical to the original image.

Reflectance map
An image that shows the effects of the material color and surface properties on a light source, no matter what the light source is. An orange reflects less blue light and more red light, for example, independent of the color of the illumination. The reflectance map shows the image as it would appear assuming a constant, even illumination on all portions of the image.

Scaling
A multiplication of all image intensities by the same value: Cnew = KC. If K = 2, for example, then all of the image intensities will be twice their original value. A value of K > 1 brightens the image and increases the contrast in all areas. A value of K < 1 darkens the image and decreases the contrast in all areas.

Shadow
An area in a scene where the direct light source is completely blocked. The area will still be lit by ambient illumination.

White Balance
An independent scaling of the three color channels that controls what color value appears as white. Often, white balance is set by having a user select a surface in the scene that should be white and computing the necessary scaling factors automatically from the selection.