Equalise

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Equalise evens out the brightness in sub-regions of the picture, increasing visual detail in areas where the detail has been lost in shadow or bright areas.

Varying the size of the sub-region affects the type of the result. As the region reduces in size, the brightness averaging takes place over a smaller and smaller area - this increases the distortion in the picture, and also the detail revealed. This is best illustrated with a black-and-white photo.

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Equalising a colour picture distorts the colour balance of the picture. If this is not required, do the equalisation on an alpha channel which has a monochrome copy of the RGB channels instead. Use this as a selection to brighten parts of the picture, then invert the selection and darken those parts.

OriginalRGB Channels EqualisedAlpha Channel Equalised
OriginalRGB Channels EqualisedAlpha Channel Equalised

Note that with the alpha channel equalisation, colour saturation is reduced.

Note also that the increased detail in shadow areas has revealed JPEG artifacts on the sports shirt. This can be used to interesting effect: