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Conditional Curve
This transformation increases or decreases one channel of a color image based on another. For example, you can use Conditional Curve to increase the saturation of the lighter parts of an image while reduce the saturation of the darker parts. Or, you could lighten or darken pixels with certain hues.
The neutral curve for this transformation is a horizontal line through the center of the graph, called the zero-line, rather than the more familiar diagonal line from lower left to upper right. The curve represents the change, positive for values above the zero-line or negative for values below the zero line, to be made to the channel on the y-axis. The transformation examines every pixel in the input image and determines the value of the channel displayed on the x-axis. The point on the curve corresponding to the x-axis value is then used to determine how much to increase to decrease the channel displayed on the y-axis.
Adding control points to the curve that raise or lower the curve above the zero-line increases or decreases the value of the channel on the y-axis, depending on the channel on the x-axis. For example, if the x-axis is set to Hue and the y-axis is set to Brightness, then the brightness of each pixel in the image is adjusted up or down by an amount that depends of its hue. The further above or below the zero-line the curve goes, the bigger the adjustment.
The example above increases the brightness of the input image where it is red.
The conditional curve dialog box is resizable; the larger you make it, the larger its curve control and the sliders become.
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.
The curve control occupies most of the dialog box. Where the curve lies above the horizontal midline, the channel associated with the Y-axis is increased. Where the curve lies below the midline, the channel is decreased.
Strength
The strength slider amplifies or attenuates the adjustments made by the curve. It is easier to make file adjustments if the strength is reduced below 100%. For dramatic effects, increase the strength.
If the input image is color then this control lets you select whether you want to work in the HSV or HSL 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.
Adjustments to hue simply apply a positive or negative offset to the input hue value. Adjustments to saturation or brightness move the value towards 100% if the curve is above the zero-line or towards 0% if the curve is below the zero-line.
Y-Axis
This control selects which channel (Hue, Saturation or Brightness) is assigned to the y-axis.
X-Axis
This control selects which channel (Hue, Saturation or Brightness) is assigned to the x-axis.
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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 .png){kind=small}
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