ProPhoto RGB color space
Encyclopedia
RGB color space
An RGB color space is any additive color space based on the RGB color model. A particular RGB color space is defined by the three chromaticities of the red, green, and blue additive primaries, and can produce any chromaticity that is the triangle defined by those primary colors...
developed by Kodak. It offers an especially large gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...
designed for use with photographic output in mind. The ProPhoto RGB color space encompasses over 90% of possible surface colors in the CIE
International Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
L*a*b* color space
Lab color space
A Lab color space is a color-opponent space with dimension L for lightness and a and b for the color-opponent dimensions, based on nonlinearly compressed CIE XYZ color space coordinates....
, and 100% of likely occurring real world surface colors making ProPhoto even larger than the Wide Gamut RGB color space. The ProPhoto RGB primaries were also chosen in order to minimize hue rotations associated with non-linear tone scale operations. One of the downsides to this color space is that approximately 13% of the representable colors are imaginary colors that do not exist and are not visible colors. This means that potential color accuracy is wasted for reserving these unnecessary colors.
When working in color spaces with such a large gamut, it is recommended to work in 16-bit color depth to avoid posterization
Posterization
Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This was originally done with photographic processes to create posters...
effects. This will occur more frequently in 8-bit modes as the gradient steps are much larger.
There are two corresponding scene space color encodings known as RIMM RGB intended to encode standard dynamic range scene space images, and ERIMM RGB intended to encode extended dynamic range scene space images.
ProPhoto RGB (ROMM RGB) Encoding Primaries
| Color | CIE x | CIE y |
|---|---|---|
| red | 0.7347 | 0.2653 |
| green | 0.1596 | 0.8404 |
| blue | 0.0366 | 0.0001 |
| white | 0.3457 | 0.3585 |
Viewing Environment
- Luminance level is in the range of 160-640 cd/m2.
- Viewing surround is average.
- There is 0.5-1.0% viewing flare.
- The adaptive white pointWhite pointA white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...
is specified by the chromaticityChromaticityChromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
values for CIE Standard Illuminant D50 (x = 0.3457, y = 0.3585). - The image color values are assumed to be encoded using flareless (or flare corrected) colorimetric measurements based on the CIE 1931 Standard Colorimetric Observer.
Encoding Function
where
and
is the maximum integer value used in the encoding function (e.g. 255 for 8-bit configuration)and
