You can convert a grayscale image or color image to a bitonal image.
Converting an image to bitonal can help isolate features in an image, such as the contour lines in a USGS map. After the image is converted to a bitonal image, you can use the vectorization tools to convert the raster objects to vectors.
You use a threshold value to determine which pixels become black and which pixels become white. AutoCAD Raster Design toolset uses Single Image Optimization to speed up the editing commands.