The following step requires locating the darkest and lightest portions of an image. If locating these points is difficult with normal image display, the double-threshold effect displays the image as a monochrome illustration, making it easy to locate the darkest (shown in black) and lightest (shown in white) portions of the image.
If shadows and highlights are not immediately displayed, adjust the triangle sliders, positioned just under the histogram display when the Double Threshold box is checked.

The degree of shadow enhancement can be adjusted using a slider.
The same adjustment can also be made entering a luminosity value into the Black Point dialog box.

Just as with the Black Control Point, the degree of highlight enhancement can be adjusted using the slider. The same adjustment can also be made entering a luminosity value into the White Point dialog box.

When Black or White Control Points are placed over an image, those points become true black (0 at the default luminance setting) and true white (100 at the default luminance setting), respectively. Therefore, the brightness of the resulting image will vary dramatically depending upon where Control Points are placed.
To achieve optimum brightness enhancement, Black and White Control Points must be positioned over the appropriate shadow and/or highlight portions. More precise positioning of Control Points is possible if image display is enlarged to 100–200%.