ProPhoto RGB color space explained

ProPhoto RGB
Long Name:	Reference Output Medium Metric RGB
Native Name Lang:	en
Status:	Published
Version:	2013
Committee:	ISO/TC 42
Authors:	Kodak
Base Standards:	RGB
Abbreviation:	ROMM RGB
Domain:	Color space, color model

The ProPhoto RGB color space, also known as ROMM RGB (Reference Output Medium Metric), is an output referred RGB color space developed by Kodak. It offers an especially large gamut designed for use with photographic output in mind. The ProPhoto RGB color space encompasses over 90% of possible surface colors in the CIE L*a*b* color space, and 100% of likely occurring real-world surface colors documented by Michael Pointer in 1980,^{[1] [2]} 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.

When working in color spaces with such a large gamut, it is recommended to work in 16-bit color depth to avoid posterization 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 (Reference Input Medium Metric) intended to encode standard dynamic range scene space images, and ERIMM RGB intended to encode extended dynamic-range scene space images.^{[3] [4]}

Development

The development of ProPhoto RGB and other color spaces is documented in an article^[5] summarizing a presentation by one of its developers Geoff Wolfe at Kodak, now senior research manager at Canon Information Systems Research Australia, at the IS&T/SPIE Color Imaging Conference in 2011.

Encoding primaries

Color	CIE x	CIE y	CIE X	CIE Y	CIE Z
red	0.734699	0.265301	0.79767	0.28804	0.00000
green	0.159597	0.840403	0.13519	0.71188	0.00000
blue	0.036598	0.000105	0.03134	0.00009	0.82491
white	0.345704	0.358540	0.96420	1.00000	0.82489

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 point is specified by the chromaticity values for CIE Standard illuminant D50 (

x = 0 . 345704 , y = 0 . 358540 ).

• 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

X' _{ROMM} = I _{MAX} ⋅ \begin{cases} 0 ; & X _{ROMM} < 0 . 0 \ 16 ⋅ X _{ROMM;} & 0 . 0 \le X _{ROMM} < E _{t} \ X ^{1 / 1 . 8} _{ROMM} ; & E _{t} \le X _{ROMM<1.0} \ 1 ; & X _{ROMM} \ge 1 . 0 \end{cases}

where

X = R, G, or B

and

I _{MAX} is the maximum integer value used in the encoding function (e.g. 255 for 8-bit configuration)

and

E _{t} = 16 ^{1.8/(1-1.8)} = 2 ^{-9} = 1 / 512 = 0 . 001953125

External links

• Specification of ROMM RGB
• Information page about ROMM RGB including a downloadable ICC format profile.
• Understanding ProPhoto RGB
• Color Spaces: Beyond Adobe RGB
• Why Use the ProPhoto RGB Color Space?

Notes and References

1. The Gamut of Real Surface Colours. Color Res. Appl., 5. Pointer, M. R.. Color Research & Application. 145–155. 10.1002/col.5080050308. 1980. 5. 3. 2007-03-15. 2021-04-19.
2. Web site: The Pointer's Gamut - The coverage of real surface colors by RGB color spaces and wide gamut. TFT Central. Kid. Jansen. 2014-02-19. 2017-10-29.
3. Web site: ISO - ISO/TS 22028-3:2012 - Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 3: Reference input medium metric RGB colour image encoding (RIMM RGB). ISO. ISO. International Electrotechnical Commission. 2012. August 2012. 2021-04-19.
4. Web site: ANSI/I3A IT10.7466-2002 - Photography - Electronic still picture imaging - Reference Input Medium Metric RGB Color encoding (RIMM-RGB). 2002. 2021-04-19. International Imaging Industry Association. International Imaging Industry Association. ANSI Webstore.
5. Web site: Real-Time Rendering · 2011 Color and Imaging Conference, Part VI: Special Session. www.realtimerendering.com. en-US. 2011-12-21. 2017-10-29. Naty.