Brightness/contrast/gamma only can be adjusted on grayscale or true color (24 BPP) images.
Adjust Brightness / Contrast / Gamma can be shown/hidden by using the Colors > BCG: Brightness/Contrast/Gamma menu command or the button on the button tool bar.
When the Brightness, Contrast, Gamma sliders are dragged the image display will be adjusted accordingly.
The image itself will not be modified until the Apply button is pressed.
The Brightness/Contrast sliders lets you make simple adjustments to the tonal range of an image. The default value both controls is 0 (no change).
The gamma slider determines how light or dark an image appears. Its default value is 1.00 (no change).
When an image has a noisy background, most times, by adjusting the gamma and contrast it can be corrected easily.
The above image has low contrast and the background is gray instead white.
After adjusting the Contrast (10),Brightness (19) and Gamma (2.40) values, the image improves substantially:
As its name indicates, this button applies the adjustments to the image. It will be enabled only after the sliders are moved and after it is clicked the sliders will fall back to their default positions and the button will get disabled until you drawn the sliders again.
Only works before clicking the Apply button. It restarts the sliders to their default positions.