Brightness Curve

This transformation lets you adjust the brightness curve of an image. The brightness curve controls the gray scale of the image; by adjusting its curve you can make an image lighter or darker, stretch or compress any part of the tone scale, or create special effects like solarization and posterization.

The brightness curve dialog box is resizable; the larger you make it, the larger its curve control (see below) gets.

Amount

The amount control lets you control how much of the brightness curve transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

Color Space

If the input image is color then this control lets you select whether you want to work in the HSV, HSL or 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 red, green and blue color components. When you modify an image using RGB, a combined histogram is displayed and the same curve is applied to each channel.

Curve

The curve control occupies most of the dialog box.

  Base Histogram on Masked Area Only

The curve control displays both a histogram and a curve. If this button is raised, the histogram is based on the entire input image. If depressed, the histogram is based on just the currently masked area as defined by the amount mask. If there is no mask, the histogram is based on the entire image. If there is a mask but it is entirely black, the histogram is not displayed. Changing this setting only affects the way histograms are displayed and has no effect on the output image.

If you are using a mask to limit the action of the transformation to just part of the image, this control lets you see the histogram of just the part you are adjusting.

Tips

To make clipped pixels visible while adjusting curves, turn on Show Clipped Highlights or Show Clipped Shadows in the main tool bar .

Brightness Curve Examples

The following examples illustrate the relationship between a curve and its effect.

No-Change

This curve makes the output level the same as the input level; applying this curve leaves the input image unchanged; any curve near this curve will have only subtle effects.

Negative

This curve makes the output level the negative of the input level; applying this curve makes low levels high and high levels low; any curve near this curve will have similar effects.

Boost Middle Levels

Applying this curve increases all levels, especially the middle levels. High and low levels are changed less.

Decrease Middle Levels

Applying this curve decreases all levels, especially the middle levels. High and low levels are changed less.

Increase Mid-tone Contrast

Applying this curve increases all high levels and decreases low levels; this increases contrast in the middle of the range and reduces contrast at high and low levels.

Decrease Mid-tone Contrast

Applying this curve decreases all high levels and increases low levels; this decreases contrast in the middle of the range and increases contrast at high and low levels.

Posterize

Applying this curve maps the full range of input levels into just a few different output levels.

Solarize

Applying this curve makes an image that is part negative, part positive. Any curve that reverses its slope from positive to negative or vice versa will produce the distinctive solarization effect.

Advanced Mode

Advanced mode is selected from the Settings menu – it provides you with additional controls to deal with specialized situations.

Detail, Radius and Threshold

For parts of the tone curve that are relatively flat, local contrast is reduced and detail is lost. These three advanced mode sliders help you recover this lost detail using a three-step process:

1)The image is split into low pass and high pass components. First the image is blurred using a bilateral filter based on the Radius and Threshold settings which you can use to adjust the boundary between low and high pass. The blurred (low pass) image is then subtracted from the original image to produce a detail (high pass) image.

2)The curve is applied to the low pass image.

3)The high pass image is added back in with an adjustable scale factor controlled by the Detail slider.

The result is to adjust the overall tonality of the image while preserving detail that would otherwise be lost.

Antialias

This advanced mode checkbox toggles antialiasing on and off. When antialiasing is on and advanced mode is activated, the input image is up-sampled by a factor of 4, the curve is applied, and the result is down-sampled by a factor of 4, returning the image to its original size. This can be helpful if you are applying a curve that posterizes the output image and creates jagged edges at the boundaries between regions of solid color. Antialiasing is not recommended if the curve is smooth as it has little effect except for some minor blurring a performance penalty.

Detail Recovery Example

Original Image

Darkened Using a Curve

Shadow detail recovered using advanced mode controls