imflatfield

Load a grayscale image. This image has severe shading distortion on the left side and in the upper-right corner.

I = imread('printedtext.png');
imshow(I)
title('Distorted Image')

Perform the flat-field correction.

sigma = 30;
Iflatfield = imflatfield(I,sigma);

Display the result. The corrected image has more uniform brightness.

imshow(Iflatfield)
title(['Flat-Field Corrected Image, \sigma = ',num2str(sigma)])

Load a color image that has vignetting, or darkening of the corners.

I = imread('fabric.png');
imshow(I)
title('Image with Vignetting')

Perform the flat-field correction.

sigma = 20;
Iflatfield = imflatfield(I,sigma);

Display the result. The corrected image has more uniform brightness.

imshow(Iflatfield)
title(['Flat-Field Corrected Image, \sigma = ',num2str(sigma)])

Load a color image. This image has a shading defect in the lower right corner.

I = imread('hands1.jpg');
imshow(I)
title('Image with Dark Corner')

Try applying flat-field correction to the entire image.

sigma = 25;
Iflatfield = imflatfield(I,25);
imshow(Iflatfield)
title(['Flat-Field Corrected Image, \sigma = ',num2str(sigma)])

The shading defect in the corner is corrected, but the center of the image is too bright and the hand has changed color. To avoid this brightening artifact, apply flat-field correction just to the background of the image.

Load the mask of this image. In the original mask, maskHand, the segmented hand is the region of interest (ROI). Invert the mask so that the background is the ROI. Display the mask, which shows the ROI in white.

maskHand = imread('hands1-mask.png');
maskBackground = ~maskHand;
imshow(maskBackground)
title('Background Mask')

Perform the flat-field correction on the background of the image using the mask maskBackground. The hand is not a region of interest in the mask, therefore flat-field correction is not applied to pixels in the hand.

Iflatfield2 = imflatfield(I,sigma,maskBackground);

Display the corrected image. The shading defect in the corner is corrected, and the hand retains its original color.

imshow(Iflatfield2)
title(['Flat-Field Corrected Background, \sigma = ',num2str(sigma)])