Exercise P12: Converting Color Images to Bitonal Images

In this exercise you convert color images to bitonal (black and white) images.

You convert a color image of a USGS quad sheet to a bitonal image, preparing to trace the contours with AutoCAD Raster Design toolset Vectorization Tools.

Related Exercises

• Exercise S3: Cleaning Up Images with Invert and Mirror
• Vectorization Tutorial

Before doing this exercise, ensure that AutoCAD Raster Design toolset options are set as described in the exercise Exercise A1: Setting AutoCAD Raster Design Toolset Options.

Exercise

1. In the ..\Program Files\Autodesk\ApplicationPlugins\RasterDesign2024.Bundle\Contents\Tutorials\Tutorial5 folder, open the drawing file Map_12.dwg.

   Convert the image to bitonal

2. Isolate the TU_image_usgs layer. To do this, on the ribbon, click Home tab > Layers panel > Layer Properties . Then select the TU_image_usgs layer in the Layer Properties Manager, right-click and click Select All But Current, then freeze all the selected layers.
3. On the ribbon, click Raster Tools tab Edit panel Process Image drop-down menu Histogram . Press Enter for entire image. The Histogram dialog box is displayed.
4. Click the Threshold tab.

   Use the Threshold slider to determine which pixels will be black and which will be white after you convert the image.

5. Move the slider as close as you can to 171, observing the effect on the image in the preview window.

   This exercise uses one image. However, if you select multiple images, you can select the image name below the preview to observe the effect of the threshold value on that image. The selected images convert to black and white when you click Apply and Close.

6. Click Apply and Close to convert the image.

   Save the image

7. On the ribbon, click Raster Tools tab Insert & Write panel Save As .
8. In the Save As dialog box, enter usgs.cal for the filename, select CALS Type 1 for the file type, and click Save.

   Invert the image

9. On the ribbon, click Raster Tools tab Edit panel Cleanup drop-down menu Invert .
   

   After you convert an image (and invert it, if necessary), you typically use the editing filters, such as Despeckle and the bitonal filters, to eliminate raster data that could interfere with tracing the contours.

10. Close the drawing without saving changes.