Exercise P4: Enhancing Grayscale Images

In this exercise you use three image processing functions to improve the image clarity of a grayscale aerial photograph.

Several editing filters are available in AutoCAD Raster Design toolset. Those that use the histogram include functions to convert color images to grayscale or bitonal (black and white) images, and to convert grayscale images to bitonal images.

In this lesson, you experiment with the Brightness, Contrast, and Equalize functions. You can observe the effects of each function on the image by using both the preview and histogram display before you apply the changes.

In this exercise you process the whole image. AutoCAD Raster Design toolset also allows only a specified portion of the image to be processed.

Related Exercises

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 \Tutorial5 folder, open the drawing file Map_04.dwg.
2. If the vector line work is not displayed on top of the aerial photo, enter REGEN to restore the display order.

   Adjust the contrast and the brightness

3. Click Raster menu Image ProcessingHistogram. Press Enter for the whole image. The Histogram dialog box appears.

   Ensure that Brightness/Contrast is the active tab.

4. Move the pointer over the histogram display until the index value equals 69.

   The frequency value identifies the number of pixels in the image assigned to this shade of gray. In the next few steps, you will see the effects of the contrast and brightness controls.

5. Move the Contrast slider to the right to increase the contrast, observing the effect in the preview window.

   Increasing the contrast displays mid-tone image values in more extreme light and dark shades. Increasing the contrast to an extreme value in a grayscale image polarizes the shades of gray to display in black and white.

6. Move the Contrast slider to the left to decrease the contrast, observing the effect in the preview window.

   Decreasing the contrast in a grayscale image displays more image values in midrange shades. At extreme low contrast, all the image values display as a single shade of gray.

7. Move the Brightness slider to 128 to increase the brightness, observing the effect in the preview window.

   In grayscale images, increasing the brightness displays gray image values in lighter shades. At extreme brightness, all image values display as white.

8. Move the Brightness slider to the left to decrease the brightness, observing the effect in the preview window.

   In grayscale images, decreasing the brightness displays image values in darker shades. In the extreme case, all image values display as black.

9. Click Reset. The Contrast and Brightness sliders return to center.

   Equalize the grayscale of the image

10. Click the Equalize tab.

    Equalization is useful for images in which a large percentage of pixels are approximately the same color. In a grayscale image, equalization changes the darkest pixels to black and the lightest pixels to white and then reassigns the remaining pixels to use all shades in between.

11. Click Apply to equalize the shades of gray across the full range of grays and observe the effect in the preview window.
12. Click Close.

    Note that image details are more visible in the equalized image.

13. Close the drawing without saving changes.