# illumwhite

Estimate illuminant using White Patch Retinex algorithm

## 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.

example

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

illuminant = illumwhite(___,Mask=mask) estimates the illumination using only the pixels within the ROI defined by a binary mask.

## Examples

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Open and display an image with poor white balance.

imshow(A)
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.

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.

imshow(B)
title("White-Balanced Image using White Patch with topPercentile="+ ...
num2str(topPercentile))

## Input Arguments

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

Data Types: single | double | uint8 | uint16

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

Image mask, specified as an m-by-n logical or numeric matrix. 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

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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.

## Version History

Introduced in R2017b