How to adjust the brightness of an image in YCbCr color space
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You'd do it the same way you'd do it on any other image, only you'd operate on the Y component alone.
That said, "adjust the brightness" is a bit vague. Depending on what you want to accomplish, you should be able to start with imadjust(). There are other tools.
Again, depending on what you want to do, you may wind up pushing pixels out of gamut in the process. For severe cases, you may find parts of the image migrating toward saturated colors. Consider the example:
inpict = imread('peppers.png');
outpict = rgb2ycbcr(inpict); % convert to YCbCr
outpict(:,:,1) = outpict(:,:,1) + 33; % shift Y
outpict = ycbcr2rgb(outpict); % convert back to RGB
This simple operation pushes colors out of gamut; the truncation upon conversion back to RGB ends up oversaturating certain areas (the yellow and orange regions). As a consequence, the luma in those regions is not increased by the specified amount. The same color adjustment would look like this if luma were actually preserved:
Which is appropriate depends on what your goals and your workflow are.