Autorange Transformation

This transformation adjusts the dynamic range of the input image by automatically detecting the black and white points of the image and then applying a curve to set the black and white points to new values.

Amount

The amount control determines how much of the autorange 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

This control (only displayed for color images) determines the color space (HSV or HSL) in which the brightness of the input image will be adjusted.

Auto Black

If this box is checked, the black point of the image will be detected. If not, the black level of the input image is assumed to be pure black.

Black Threshold

The black threshold is used to determine what part of the input image is the darkest. The image color to be mapped to pure black is determined by averaging the darkest pixels in the image. If the threshold is set to zero, the darkest pixel in the input image becomes the black point. This can cause problems if there are isolated dark specks in the image or perhaps a sliver of a black border around the edge. To deal with this, you can increase the threshold. The percentage of pixels used is set by the threshold slider. The default value of 0.05% means that the darkest part of the image is determined as the average of the darkest 0.05% of the pixels in the image.

Auto White

If this box is checked, the white point of the image will be detected. If not, the white level of the input image is assumed to be pure white.

White Threshold

The white threshold is used to determine what part of the input image is the lightest. The image color to be mapped to pure white is determined by averaging the lightest pixels in the image. If the threshold is set to zero, the lightest pixel in the input image becomes the white point. This can cause problems if there are isolated light specks in the image or perhaps a sliver of a white border around the edge. To deal with this, you can increase the threshold. The percentage of pixels used is set by the threshold slider. The default value of 0.05% means that the lightest part of the image is determined as the average of the lightest 0.05% of the pixels in the image.

Dynamic Range

This dynamic range slider lets you set the dynamic range of the output image.

The tic marks below the gray bar show where the automatically detected black and white points of the image are located. By dragging the black and white markers you can set new black and white points. If you leave them in their default locations at the ends of the tonal range, the dynamic range of the output image is adjusted to run from pure black to pure white. If you move them, the dynamic range is compressed accordingly.

Curve

This shows the curve Autorange is applying to the input image to locate its black and white points. Changes to the Dynamic Range slider are also reflected in the curve. The curve also shows the input image HSV-V or HSL-L histogram and shows you how much is being cut off at each end. This curve is generated automatically from the threshold and dynamic range slider values and cannot be modified, although you can use the histogram expansion button to magnify the smaller histogram values.

Tips

Turning on Show Clipped Highlight or Show Clipped Shadows from the main tool bar  lets you see how much clipping of black or white pixels is occurring in the output image and where. The higher you set the thresholds, the more clipping you are likely to see.

If you want, you can use the Histogram Mode button  from the main tool bar to view the output image histogram and see how it changes as you adjust the Autorange settings.