"When it comes to tweaking photos, Levels is the most important
weapon in digiKam's arsenal. This tool lets you adjust brightness
and contrast by specifying the location of complete black, complete
white, and midtones in a histogram, which makes it a perfect tool
for fixing under- and overexposed photos as well as improving the
overall tonal range of a photo.
"You can access the Levels tool in the image editor by choosing
Color | Levels Adjust. Select Luminosity from the Channel drop-down
list, and press the Linear button.
"The key element in the Levels Adjust pane is the histogram with
the black point and white point sliders. Simply put, the black
point slider controls shadows, while the white point slider
controls highlights. So to darken shadow areas in the photo, move
the black point slider to the right. Need to boost highlights? Move
the white point slider to the left."
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