lin2rgb

Apply gamma correction to linear RGB values

Syntax

B = lin2rgb(A)

B = lin2rgb(A,Name,Value)

Description

B = lin2rgb(A) applies a gamma correction to the linear RGB values in image A so that B is in the sRGB color space, which is suitable for display.

B = lin2rgb(A,Name,Value) applies gamma correction using name-value arguments to control additional options.

Examples

Plot Gamma Curve of sRGB and Adobe RGB

Define a range of linear values. This vector defines 257 equally spaced points between 0 and 1.

lin = linspace(0,1,257);

Apply gamma correction to the linear values based on the sRGB standard. Then apply gamma correction to the linear values based on the Adobe RGB (1998) standard.

sRGB = lin2rgb(lin);
adobeRGB = lin2rgb(lin,'ColorSpace','adobe-rgb-1998');

Plot the gamma-corrected curves.

figure
plot(lin,sRGB,'b',lin,adobeRGB,'r')
title('Gamma-Corrected vs. Linear Values')
legend('sRGB','Adobe RGB (1998)','Location','southeast')

For an alternative visualization, plot color bars representing each color space.

cb_lin = ones(30,257) .* lin;
cb_sRGB = ones(30,257) .* sRGB;
cb_adobeRGB = ones(30,257) .* adobeRGB;

figure
subplot(3,1,1); imshow(cb_lin); title('Linear RGB')
subplot(3,1,2); imshow(cb_sRGB); title('sRGB');
subplot(3,1,3); imshow(cb_adobeRGB); title('Adobe RGB (1998)');

The gamma-corrected color spaces get brighter more quickly than the linear color space, as expected.

Apply sRGB Gamma Correction to Linear RGB Image

Open an image file containing minimally processed linear RGB intensities.

A = imread('foosballraw.tiff');

The image data is the raw sensor data after correcting the black level and scaling to 16 bits per pixel. Interpolate the intensities to reconstruct color by using the demosaic function. The color filter array pattern is RGGB.

A_demosaiced = demosaic(A,'rggb');

Display the image. To shrink the image so that it appears fully on the screen, set the optional initial magnification to a value less than 100.

figure
imshow(A_demosaiced,'InitialMagnification',25)
title('Sensor Data Without sRGB Gamma Correction')

The image appears dark because it is in the linear RGB color space. Apply gamma correction to the image according to the sRGB standard, storing the values in double precision.

A_sRGB = lin2rgb(A_demosaiced,'OutputType','double');

Display the gamma-corrected image, setting the optional magnification.

figure
imshow(A_sRGB,'InitialMagnification',25)
title('Sensor Data With sRGB Gamma Correction');

The gamma-corrected image looks brighter than the linear image, as expected.

Input Arguments

A — Linear RGB color values

numeric array

Linear 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") applies gamma correction to an 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") applies gamma correction to an image, I, according to the Adobe RGB (1998) standard.

ColorSpace — Color space of output image

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

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

Data Types: string | char

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: string | char

Output Arguments

B — Gamma-corrected RGB image

numeric array

Gamma-corrected RGB image, returned as a numeric array of the same size as the input A.

Algorithms

Gamma Correction Using the sRGB Standard

The gamma correction to transform linear RGB tristimulus values into sRGB tristimulus values is defined by the following parametric curve:

    f(u) = -f(-u),             u < 0

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

    f(u) = a ⋅ u^ɣ + b,      u ≥ d,

where u represents a color value with these parameters:

    a = 1.055

    b = –0.055

    c = 12.92

    d = 0.0031308

    ɣ = 1/2.4

Gamma Correction Using the Adobe RGB (1998) Standard

The gamma correction to transform linear RGB tristimulus values into Adobe RGB (1998) tristimulus values uses a simple power function [2]:

    v = u^ɣ,           u ≥ 0

    v = -(-u)^ɣ,      u < 0,

with

    ɣ = 1/2.19921875

Gamma Correction Using the ProPhoto (ROMM RGB) Standard

The gamma correction to transform linear RGB tristimulus values into ProPhoto tristimulus values is defined by the following parametric curve [3]:

    f(u) = 0,             u < 0

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

    f(u) = u^1/1.8 + 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

lin2rgb 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".