Brightness, contrast and gamma
This dialog renders the image's brightness, contrast and gamma. The brightness
and contrast are obvious. The gamma changes the intesity of the lighter colors,
for example the color yellow, will (after moving the knot to the right) be
convert to white, while the "darker" colors stay relative dark. This effect is
different to brightness (though it brightens up the picture) that brightness
applies the same amount of brightness to all colors.