rgb2lin

Linearize gamma-corrected RGB values

Syntax

B = rgb2lin(A)

B = rgb2lin(A,Name,Value)

Description

B = rgb2lin(A) undoes the gamma correction of the sRGB values in image A so that B contains linear RGB values.

B = rgb2lin(A,Name,Value) undoes gamma correction using name-value arguments to control additional options.

Examples

Linearize an sRGB Image

Open an image. The JPEG file format saves images in the gamma-corrected sRGB color space.

A = imread('foosball.jpg');

Display the image.

imshow(A)
title('Scene With sRGB Gamma Correction')

Undo the gamma correction and linearize the image by using the rgb2lin function. Optionally, specify the data type of the linearized values.

B = rgb2lin(A,'OutputType','double');

Display the linearized image. Shadows in the linearized image are darker than in the original image, as expected.

imshow(B)
title('Scene Without sRGB Gamma Correction')

Input Arguments

A — Gamma-corrected RGB color values

numeric array

Gamma-corrected RGB color values, specified as a numeric array in one of the following formats.

• c-by-3 colormap. Each row specifies one RGB color value.

• m-by-n-by-3 image

• m-by-n-by-3-by-p stack of images

Data Types: single | double | uint8 | uint16

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: B = lin2rgb(I,ColorSpace="adobe-rgb-1998") linearizes the gamma-corrected image, I, according to the Adobe RGB (1998) standard.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: B = lin2rgb(I,"ColorSpace","adobe-rgb-1998") linearizes the gamma-corrected image, I, according to the Adobe RGB (1998) standard.

ColorSpace — Color space of input image

"srgb" (default) | "adobe-rgb-1998" | "prophoto-rgb"

Color space of the input image, specified as "srgb", "adobe-rgb-1998", or "prophoto-rgb".

Data Types: char | string

OutputType — Data type of output RGB values

"double" | "single" | "uint8" | "uint16"

Data type of the output RGB values, specified as "double", "single", "uint8", or "uint16". By default, the output data type is the same as the data type of A.

Data Types: char | string

Output Arguments

B — Linearized RGB color values

numeric array

Linearized RGB color values, returned as a numeric array of the same size as the input A.

Algorithms

Linearization Using the sRGB Standard

sRGB tristimulus values are linearized using this parametric curve:

    f(u) = -f(-u),                u < 0

    f(u) = c ⋅ u,                0 ≤ u < d

    f(u) = (a ⋅ u + b)^ɣ,      u ≥ d,

where u represents a color value with these parameters:

    a = 1/1.055

    b = 0.055/1.055

    c = 1/12.92

    d = 0.04045

    ɣ = 2.4

Linearization Using the Adobe RGB (1998) Standard

Adobe RGB (1998) tristimulus values are linearized using a simple power function [2]:

    v = u^ɣ,

with

    ɣ = 2.19921875

Linearization Using the ProPhoto (ROMM RGB) Standard

ProPhoto (ROMM RGB) tristimulus values are linearized using this parametric curve [3]:

    f(u) = 0,             u < 0

    f(u) = u/16,             0 ≤ u < 16*E_t

    f(u) = u^1.8 + b,      16*E_t < u < 1,

    f(u) = 1,             u ≥ 1

with

    E_t = 1/512

References

[1] Ebner, Marc. "Gamma Correction." Color Constancy. Chichester, West Sussex: John Wiley & Sons, 2007.

[2] Adobe Systems Incorporated. "Inverting the color component transfer function." Adobe RGB (1998) Color Image Encoding. Section 4.3.5.2, May 2005, p.12.

[3] ISO 22028-2:2013 Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 2: Reference output medium metric RGB colour image encoding (ROMM RGB). https://www.iso.org/standard/56591.html

Version History

Introduced in R2017b

R2022b: Support for ProPhoto (ROMM RGB) color space

rgb2lin now supports the ProPhoto (ROMM RGB) color space, which has a wider gamut than the sRGB and Adobe RGB 1998 color spaces. To use the ProPhoto color space, specify the ColorSpace name-value argument as "prophoto-rgb".