Tonal Adjustment Tutorial

This tutorial shows you several ways to improve images by adjusting tone with histograms and contrast curves.

Tonal adjustment in AutoCAD Raster Design toolset uses a contrast curve to change image brightness and contrast. Unlike the simpler Brightness and Contrast controls, the contrast curve can adjust ranges of tones in an image independently of others. For example, dark shadowed areas can be brightened without significantly affecting the brighter parts of the image. Curve definitions can be exported and imported so the same curve can be applied to other images or other parts of the same image.

The exercises in this tutorial use an aerial photo. Part of the photo appears washed out, possibly due to cloud cover or ground fog when the photo was taken. You can use tonal adjustment to enhance the photo, making it appear more uniform. The tutorial includes the following activities:

• Open the drawing and insert the pre-correlated image.
• See how the histogram looks for a “normal” part of the image.
• Select a problem area and create a piecewise linear contrast curve to correct it.
• Select a roadway segment in the image to make it appear more uniform.
• Switch to a fitted curve and adjust the tone. Export the curve.
• Apply the curve to this and other roadway segments.
• See the overall results of your tonal enhancement.