Brightness

This transformation lets you adjust the brightness of an image.

Amount

The amount control lets you control how much and in which direction the brightness of the image is changed. Positive values make the image brighter; negative values make the image darker. You can apply the transformation to the entire image, making it brighter or darker, or you can specify an amount mask to brighten or darken different parts of the input image by different amounts.

When you use an amount mask, the input image is left unchanged where the mask image is a mid-level gray; it is brightened where the mask is brighter and darkened where the mask is darker. You can control the degree of brightening and darkening caused by the mask image using the double slider.

Color Space

If the input image is color then this control lets you select whether you want to work in the HSV, HSL or the RGB color space. When you lighten an image using HSL, it becomes whiter and may appear washed out. When you lighten an image using HSV, it retains the relative proportions of its RGB color components. When you modify an image using RGB, each channel is affected independently.

Preserve

There are many ways to lighten or darken an image. This control lets you select what parts of the gray scale of the input image are most affected and what parts of the gray scale remain unchanged:

Black and White -- lightens or darkens the input image by applying a curve that keeps blacks black and whites white and has its strongest effect on the image mid-tones. Lightening increases shadow contrast and decreases highlight contrast. Darkening decreases shadow contrast and increases highlight contrast.

White -- lightens or darkens the input image by applying a curve that keeps blacks black and has its strongest effect on the image highlights. Lightening changes pure black to gray and reduces highlight contrast. Darkening can clip shadows and increases highlight contrast.

Black -- lightens or darkens the input image by applying a curve that keeps whites white and has its strongest effect on the image shadows. Lightening increases shadow contrast and can clip highlights. Darkening decreases shadow contrast and makes pure white gray.

Neither -- lightens or darkens the input image by applying a curve that adds or subtracts the same amount to the entire gray scale, affecting all levels equally. Lightening can clip highlights and darkening can clip shadows.

Tips

Using this transformation with a mask lets you lighten some parts of the image while darkening others at the same time.

Turn on Show Clipped HIghlight or Show Clipped Shadows from the main tool bar  to see what if any parts of the image are being clipped.

The Black and White option is usually the most useful choice for Preserve as it lightens or darkens the image without clipping any part of the image that was not already clipped.