Interactive Contour Plot

The Interactive Contour Plot command opens the Interactive Contour Plot Toolbar which has commands for interactively marking an intensity contour on an image. The contour follows a path of constant luminance, perpendicular to the local gradient. This is a point & click tool in which you click the mouse on the point having the intensity at which you want to create a contour. Be sure to read "Contouring Regions with High Noise Level", below.

The contour may be generated inside the Image Cursor or you can contour the entire image. There are a number of preferences that control generation and display of the contours. Be advised that contouring the entire image by clicking on noise can take a very long time to compute.


You can terminate the contour computation at any time by hitting the [Esc] key. This is especially handy if you start a contour computation after accidentally clicking on a level far into the noise.

Toolbar Commands

The Interactive Contour Toolbar controls the contouring task.

To interactively mark contours, simply enable marking mode by pressing the top button in the toolbar, then click on the image to mark the contour at the desired luminance level.

Contouring Regions of High Noise Level

Below is shown a magnified view of the top-right part of the contour region from above. This contour was created using Smoothing = low, which creates many wiggles as the contour tracks through noisy, low level pixels. For more information about contour plotting in noisy areas of an image, see Contour Plotting with Various Levels of Smoothing.


It is usually advisable to constrain the contour to a rectangle inside the image cursor. Contours require drawing many short lines, and a complex contour containing a large number of line segments may cause the computer to lag every time the image window needs to be redrawn.

With many complex contours visible, display updating is sluggish. You can see this when you adjust the palette or move the image cursor. To speed up window re-drawing try the following solutions: increase smoothing, confine the contours to a rectangle of smaller size, or delete the contour using the (delete all) button on the toolbar.

Related Topics

Interactive Contour Preferences, Contour Plot, Plotting Commands