Tracing Contour Lines in Photographic Images
This Demonstration draws contour lines of equal gray-level intensity in a black-and-white photographic image. The slider with multiple locators can be used to set the gray levels of the contours. Additional contours can be added or deleted by pressing the ALT key while clicking inside the slider. Different color schemes can be selected for contour shading. You can also click the checkbox to show the contour lines only.
The multiSlider program is adapted from a Demonstration by Brett Champion, "Effects of Bin Width and Height in a Histogram".