Finding the Natural Mask Performing a selection requires separating the subject, the part of the image that interests us, from the background. Often the subject has colorspace features that differentiate it from the background, and the goal of this section is to explain how to exploit this fact. Since the techniques described in this section depend on using an image's natural color features to make the selection, I call this finding the natural mask . The Threshold tool allows you to specify a range of values in an image. All the pixels that are in the range of the selected values are mapped to white, and the rest are mapped to black.
Threshold is a powerful tool for automatically creating masks.
This is illustrated in the following example. Pasting the Image into Its Own Channel Mask illustrates the first step of using Threshold to create a natural mask. Since a channel mask is only 8 bits deep, pasting the color image into the channel mask immediately converts it to a grayscale. The Threshold dialog works by clicking and dragging out a part of the range of values in the histogram. The pixels having values outside the swept range are mapped to black.