rgb2lab

Convert RGB to CIE 1976 L*a*b*

Syntax

lab = rgb2lab(RGB)

lab = rgb2lab(RGB,Name,Value)

Description

lab = rgb2lab(RGB) converts sRGB values to CIE 1976 L*a*b* values.

lab = rgb2lab(RGB,Name,Value) specifies additional conversion options, such as the color space of the RGB image, using one or more name-value pair arguments.

Examples

Convert RGB White to L*a*b*

Use rgb2lab to convert the RGB white value to L*a*b.

rgb2lab([1 1 1])

ans = 1×3

  100.0000         0    0.0000

Convert Color Value to L*a*b* Specifying Color Space

Convert an Adobe RGB (1998) color value to L*a*b* using the ColorSpace parameter.

rgb2lab([.2 .3 .4],'ColorSpace','adobe-rgb-1998')

ans = 1×3

   30.1783   -5.6902  -20.8223

Convert RGB color to L*a*b* Specifying Reference White

Use rgb2lab to convert an RGB color to L*a*b using the D50 reference white.

rgb2lab([.2 .3 .4],'WhitePoint','d50')

ans = 1×3

   31.3294   -4.0732  -18.1750

Convert RGB Image to L*a*b* and Display L* Component

Read RGB image into the workspace.

rgb = imread('peppers.png');

Convert the RGB image to the L*a*b* color space.

lab = rgb2lab(rgb);

Display the L* component of the L*a*b* image.

imshow(lab(:,:,1),[0 100])

Input Arguments

RGB — RGB color values

numeric array

RGB color values to convert, specified as a numeric array in one of these 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: lab = rgb2lab([0.25 0.40 0.10],WhitePoint="d50")

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

Example: lab = rgb2lab([0.25 0.40 0.10],"WhitePoint","d50")

ColorSpace — Color space of input RGB values

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

Color space of the input RGB values, specified as "srgb", "adobe-rgb-1998", "prophoto-rgb", or "linear-rgb". If you specify "linear-rgb", then rgb2lab assumes the input RGB values are linearized sRGB values.

Data Types: string | char

WhitePoint — Reference white point

"d65" (default) | "a" | "c" | "e" | "d50" | "d55" | "icc" | 1-by-3 vector

Reference white point, specified as a 1-by-3 vector or one of the CIE standard illuminants listed in the table.

Value	White Point
"a"	CIE standard illuminant A, [1.0985, 1.0000, 0.3558]. Simulates typical, domestic, tungsten-filament lighting with correlated color temperature of 2856 K.
"c"	CIE standard illuminant C, [0.9807, 1.0000, 1.1822]. Simulates average or north sky daylight with correlated color temperature of 6774 K. Deprecated by CIE.
"e"	Equal-energy radiator, [1.000, 1.000, 1.000]. Useful as a theoretical reference.
"d50"	CIE standard illuminant D50, [0.9642, 1.0000, 0.8251]. Simulates warm daylight at sunrise or sunset with correlated color temperature of 5003 K. Also known as horizon light.
"d55"	CIE standard illuminant D55, [0.9568, 1.0000, 0.9214]. Simulates mid-morning or mid-afternoon daylight with correlated color temperature of 5500 K.
"d65"	CIE standard illuminant D65, [0.9504, 1.0000, 1.0888]. Simulates noon daylight with correlated color temperature of 6504 K.
"icc"	Profile Connection Space (PCS) illuminant used in ICC profiles. Approximation of [0.9642, 1.000, 0.8249] using fixed-point, signed, 32-bit numbers with 16 fractional bits. Actual value: [31595,32768, 27030]/32768.

Data Types: single | double | string | char

Output Arguments

lab — Converted L*a*b* color values

numeric array

Converted L*a*b* color values, returned as a numeric array of the same size as the input. The output type is double unless the input type is single, in which case the output type is also single.

Attribute	Description
L*	Luminance or brightness of the image. Values are in the range [0, 100], where 0 specifies black and 100 specifies white. As L* increases, colors become brighter.
a*	Amount of red or green tones in the image. A large positive a* value corresponds to red/magenta. A large negative a* value corresponds to green. Although there is no single range for a*, values commonly fall in the range [-100, 100] or [-128, 127).
b*	Amount of yellow or blue tones in the image. A large positive b* value corresponds to yellow. A large negative b* value corresponds to blue. Although there is no single range for b*, values commonly fall in the range [-100, 100] or [-128, 127).

Data Types: double | single

Tips

• If you specify the input RGB color space as "linear-rgb", then rgb2lab assumes the input values are linearized sRGB values. If instead you want the input color space to be linearized Adobe RGB (1998), then you can use the lin2rgb function.

  For example, to convert linearized Adobe RGB (1998) image RGBlinadobe to the CIE 1976 L*a*b* color space, perform the conversion in two steps:

  RGBadobe = lin2rgb(RGBlinadobe,"ColorSpace","adobe-rgb-1998");
  LAB = rgb2lab(RGBadobe,"ColorSpace","adobe-rgb-1998");
  

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• rgb2lab supports the generation of C code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

• When generating code, all character vector input arguments must be compile-time constants.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• The ColorSpace name-value argument only supports the values "srgb", "adobe-rgb-1998", and "linear-rgb".

• When generating code, all character vector input arguments must be compile-time constants.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced in R2014b

R2022b: Support for ProPhoto (ROMM RGB) color space

rgb2lab 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".

R2022b: Support for thread-based environments

rgb2lab now supports thread-based environments.