illumwhite

Estimate illuminant using White Patch Retinex algorithm

Syntax

illuminant = illumwhite(A)

illuminant = illumwhite(A,topPercentile)

illuminant = illumwhite(___,Name,Value)

Description

illuminant = illumwhite(A) estimates the scene illumination in RGB image A by assuming that the top 1% brightest red, green, and blue values represent the color white.

illuminant = illumwhite(A,topPercentile) estimates the illumination using the topPercentile percentage brightest red, green, and blue values.

illuminant = illumwhite(___,Name,Value) estimates the illumination using name-value pairs to control additional options.

Examples

Correct White Balance Using White Patch Retinex Algorithm

Open an image and display it. Specify an optional magnification to shrink the size of the displayed image.

A = imread('foosball.jpg');
figure
imshow(A,'InitialMagnification',25)
title('Original Image')

The JPEG file format saves images in the gamma-corrected sRGB color space. Undo the gamma correction by using the rgb2lin function.

A_lin = rgb2lin(A);

Estimate the scene illumination from the top 5% brightest pixels. Because the input image has been linearized, the illumwhite function returns the illuminant in the linear RGB color space.

topPercentile = 5;
illuminant = illumwhite(A,topPercentile)

illuminant = 1×3

    0.7333    0.8314    1.0000

The third coefficient of illuminant is the largest, which is consistent with the blue tint of the image.

Correct colors by providing the estimated illuminant to the chromadapt function.

B_lin = chromadapt(A_lin,illuminant,'ColorSpace','linear-rgb');

To display the white-balanced image correctly on the screen, apply gamma correction by using the lin2rgb function.

B = lin2rgb(B_lin);

Display the corrected image, setting the optional magnification.

figure
imshow(B,'InitialMagnification',25)
title(['White-Balanced Image using White Patch with topPercentile=' ...
    num2str(topPercentile)])

Input Arguments

A — RGB image

m-by-n-by-3 numeric array

RGB image, specified as an m-by-n-by-3 numeric array.

Data Types: single | double | uint8 | uint16

topPercentile — Percentile of brightest colors

1 (default) | numeric scalar

Percentile of brightest colors to use for illuminant estimation, specified as a numeric scalar in the range [0, 100). To return the maximum red, green, and blue values, set topPercentile to 0.

The image indicates the red, green, and blue value that is selected to estimate the illuminant. The selection is separate for each color channel.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: illuminant = illumwhite(I,'Mask',m) estimates the scene illuminant using a subset of pixels in image I, selected according to a binary mask, m.

'Mask' — Image mask

m-by-n logical or numeric array

Image mask, specified as the comma-separated pair consisting of 'Mask' and an m-by-n logical or numeric array. The mask indicates which pixels of the input image A to use when estimating the illuminant. The computation excludes pixels in A that correspond to a mask value of 0. By default, the mask has all 1s, and all pixels in A are included in the estimation.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

Output Arguments

illuminant — Estimate of scene illumination

3-element numeric row vector

Estimate of scene illumination, returned as a 3-element numeric row vector. The three elements correspond to the red, green, and blue values of the illuminant.

Data Types: double

References

[1] Ebner, Marc. "White Patch Retinex." Color Constancy. Chichester, West Sussex: John Wiley & Sons, 2007.