False color
False colors and pseudo colors respectively refers to a group of color rendering methods used to display images in colors which were recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an image that depicts an object in colors that differ from those a photograph would show. In this image, colors have been assigned to three different wavelengths that human eyes cannot normally see.
In addition, variants of false colors such as pseudocolors, density slicing, and choropleths are used for information visualization of either data gathered by a single grayscale channel or data not depicting parts of the electromagnetic spectrum.
Types of color renderings
True color
The concept behind true color can help in understanding false color. An image is called a true-color image when it offers a natural color rendition, or when it comes close to it. This means that the colors of an object in an image appear to a human observer the same way as if this same observer were to directly view the object: A green tree appears green in the image, a red apple red, a blue sky blue, and so on.File:Burns cliff.jpg|thumb|Burns Cliff inside of Endurance crater on Mars. The color is approximate true color because, instead of the red spectral band, infrared was used. The result is a metameric failure in the color of the sky, which is slightly green in the image – had a human observer been present, then that person would have perceived the actual sky color to have a bit more orange in it. The Opportunity rover which captured this image does have a red filter, but it is often not used, due to the higher scientific value of images captured using the infrared band and the constraints of data transmission.
Absolute true-color rendering is impossible. There are three major sources of color error :
- Different spectral sensitivities of the human eye and of an image capture device.
- Different spectral emissions / reflections of the object and of the image render process.
- Differences in spectral irradiance in the case of reflective images or reflective objects – see color rendering index for details.
Approximate true-color images gathered by spacecraft are an example where images have a certain amount of metameric failure, as the spectral bands of a spacecraft's camera are chosen to gather information on the physical properties of the object under investigation, and are not chosen to capture true-color images.
False color
In contrast to a true-color image, a false-color image sacrifices natural color rendition in order to ease the detection of features that are not readily discernible otherwise – for example the use of near infrared for the detection of vegetation in satellite images. While a false-color image can be created using solely the visual spectrum, typically some or all data used is from electromagnetic radiation outside the visual spectrum. The choice of spectral bands is governed by the physical properties of the object under investigation.As the human eye uses three spectral bands, three spectral bands are commonly combined into a false-color image. At least two spectral bands are needed for a false-color encoding, and it is possible to combine more bands into the three visual RGB bands – with the eye's ability to discern three channels being the limiting factor. In contrast, a "color" image made from one spectral band, or an image made from data consisting of non-EM data is a pseudocolor image.
For true color, the RGB channels from the camera are mapped to the corresponding RGB channels of the image, yielding a "RGB→RGB" mapping. For false color this relationship is changed. The simplest false-color encoding is to take an RGB image in the visible spectrum, but map it differently, e.g. "GBR→RGB". For traditional false-color satellite images of Earth a "NRG→RGB" mapping is used, with "N" being the near-infrared spectral band – this yields the typical "vegetation in red" false-color images.
False color is used for satellite and space images: Examples are remote sensing satellites, space telescopes or space probes. Some spacecraft, with rovers being the most prominent examples, have the ability to capture approximate true-color images as well. Weather satellites produce, in contrast to the spacecraft mentioned previously, grayscale images from the visible or infrared spectrum.
False color has a range of scientific applications. Spacecraft often employ false-color methods to help understand the composition of structures in the universe such as nebula and galaxies. The frequency of light emitted by different ions in space are assigned contrasting colors, allowing the chemical composition of complex structures to be better separated and visualised. The image of the Eagle Nebula above is a typical example of this; the Hydrogen and Oxygen ions have been assigned green and blue respectively. The large amounts of green and blue in the image show that there is a large amount of Hydrogen and Oxygen in the nebula.
On 26 October 2004, the NASA/ESA Cassini-Huygens spacecraft captured a false-color image of Titan, Saturn's largest moon. The image was captured in Ultraviolet and Infrared wavelengths, both invisible to the human eye. In order to provide a visual representation, false color techniques were used. The infrared data was mapped to red and green colors, and ultraviolet mapped to blue.
Pseudocolor
A pseudocolor image is derived from a grayscale image by mapping each intensity value to a color according to a table or function. Pseudo color is typically used when a single channel of data is available, in contrast to false color which is commonly used to display three channels of data.Pseudocoloring can make some details more visible, as the perceived difference in color space is bigger than between successive gray levels alone. On the other hand, the color mapping function should be chosen to make sure the lightness of the color is still monotonic, or the uneven change would make it hard to interpret levels, for both normal and colorblind viewers. One offender is the commonly used "rainbow" palette, with a back-and-forth change in lightness.
A typical example for the use of pseudo color is thermography, where infrared cameras feature only one spectral band and show their grayscale images in pseudo color.
Another familiar example of pseudo color is the encoding of elevation using hypsometric tints in physical relief maps, where negative values are usually represented by shades of blue, and positive values by greens and browns.
Depending on the table or function used and the choice of data sources, pseudocoloring may increase the information contents of the original image, for example adding geographic information, combining information obtained from infrared or ultra-violet light, or other sources like MRI scans.
A further application of pseudocoloring is to store the results of image elaboration; that is, changing the colors in order to ease understanding an image.