This tool allows you to dynamically adjust levels in the image by way of selecting a pixel (or a group of pixels) as the reference point for the desired adjustment.
The following modes are supported:
Options
White-point
With this option, the selected point is taken to be the whitest point in the image, and the entire image is adjusted to ensure that this is the case.
Grey-point
With this option, the selected point is taken to be the whitest point in the image, and the entire image is adjusted to ensure that this is the case.
Black-point
With this option, the selected point is taken to be the darkest point in the image, and the entire image is adjusted to ensure that this is the case.
Clipping-point
With this option, the selected point is taken to be the point at which at least one channel in the image is at maximum intensity, and the entire image is adjusted to ensure that this is the case.
Transparent-point
With this option, the selected point is taken to be the point at which at least one channel in the image is at maximum intensity, and the entire image is adjusted to ensure that this is the case.
Opaque-point
With this option, the selected point is taken to be the point at which at least one channel in the image is at maximum intensity, and the entire image is adjusted to ensure that this is the case.
This tool allows you to dynamically adjust levels in the image by way of selecting a pixel (or a group of pixels) as the reference point for the desired adjustment. 
White-point
Grey-point
Black-point
Clipping-point
Transparent-point
Opaque-point