A false-color image is an image that depicts a subject in colors that differ from those a full-color photograph would show.
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A true-color image of a subject is an image that appears to the human eye just like the original subject would: a green tree appears green in the image, a red apple red, a blue sky blue, etc. When applied to black-and-white images, true-color means that the perceived lightness of a subject is preserved in its depiction. Absolute true-color is impossible to achieve due to the differences between the chemistry of the display medium and the materials in the original scene.
In a false-color set, human visual perception is limited to three independent coordinates (combination of red, green, and blue); at most, three measurements can be depicted in such a way.
A pseudo-color image is derived from a greyscale image by mapping each pixel value to a color according to a table or function. A familiar example is the encoding of altitude using hypsometric tints in physical relief maps, where negative values (below sea level) are usually represented by shades of blue, and positive values by greens and browns. Pseudo-coloring can make some details more visible, by increasing the distance in color space between successive gray levels. Pseudo-coloring can be used to store the results of image elaboration; that is, changing the colors in order to ease understanding the image.[1] Alternatively, depending on the table or function used, pseudo-coloring may increase the information contents of the original image, for example adding geographic information, combining information obtained from infra-red or ultra-violet light, or MRI scans.[2]
Pseudo-color images differ from false-color images in that they are made from only one original gray-scale image, rather than two or three.
False-color and pseudo-color images are frequently used for viewing satellite images, such as from weather satellites, the Hubble Space Telescope, and the Cassini-Huygens space probe's images of the rings of Saturn. Infrared cameras used for thermal imaging often show their image in false colors. In the notes for a toolkit called GIPSY that might be used for this application area (written in 2001) it is said:
The used term PseudoColor refers to a pixel encoding category for X11 systems that represents pseudo color. This category is still present in the current implementation provided by Xorg. Even if hardware based 8 bit palletized modes might be a rather common bitness variant for the semiconductor based implementation (the most prominent ancestor is the IBM VGA graphics adapter) of pseudo-color modes still the principle is not at all limited to that depth. E.g. announcements for medical devices from 1999 report RAMDAC designs with 10 bit being fed with image data of 12 bit width using a 12-to-10-bit conversion table plus several other modes.[4]
A choropleth is an image or map in which areas are colored or patterned proportionally to the category or value of one or more variables being represented, with little regard for details of the form of the underlying object. A choropleth map of a geographic area is thus an extreme form of false-color.