To Adjust the Brightness and Contrast of an Image

1. Click Raster menu Image ProcessingHistogram.

   The following prompt appears:

   Press Enter for entire image or specify sub-region option [Existing/Clip region/Window/Polygon]:

2. Do one of the following:
   • Press Enter to use the histogram on the entire image.
   • Enter e, then click a closed vector entity to use an existing entity to define the region.
   • Enter c to use an existing image clip.
   • Enter w, then enter two points to define a rectangular sub-region.
   • Enter p, then enter several points to define a polygonal sub-region.

   The Histogram dialog box is displayed.

3. Click the Brightness/Contrast tab.

   A preview of the image is displayed. If you selected more than one image, the histogram displays a composite of the images' values. You can view each image by clicking its name in the list.

   Note: The preview reflects the image as it was read from the disk. If you have made any display-only adjustments to the image using the Image Adjust color map or the AutoCAD IMAGEADJUST command, then those adjustments are not displayed in the image preview. If you make adjustments to the image using the Histogram, the adjustments made with IMAGEADJUST are no longer applied to the image; instead the new settings are applied.
4. If you selected a color image, then choose a color channel from the RGB box.
5. Do one of the following to manipulate the image:
   • To make the image lighter, drag the Brightness slider to the right.
   • To make the image darker, drag the Brightness slider to the left.
   • To increase the distinction between the light and dark, drag the Contrast slider to the right.
   • To decrease the distinction between the light and dark, drag the Contrast slider to the left.
6. To adjust the scale in the histogram window, move the slider up and down. Moving the slider up increases the scale such that low frequency values become more visible and the highest frequency values become truncated by the top of the window.
7. Click Apply to change the color values in the image file and generate a new histogram that reflects the changes.
   Note: After you click Apply and generate the new histogram, the Brightness and Contrast sliders are reset to 0.
8. If you specified a sub-region in step 2, select an option from the Apply Changes To section:
   • Sub-Region applies the changes to the sub-region only.
   • Entire Image applies the changes to the selected image.
9. If you selected sub-region in step 6, and if the sub-region is associated with an 8-bit color image, select a palette option from the drop-down list:
   • Retain Current Palette applies your changes to the sub-region by mapping to the closest colors from the current palette. This option maintains the current palette and will not affect pixels outside the selected region.
   • Rebuild Palette With All Colors creates a new palette to best accommodate all colors in the entire image. Because this image changes the palette, pixels inside and outside the sub-region may be affected by this option.
   • Rebuild Palette With New Colors also creates a new palette, but this option guarantees that colors in the sub-region are included in the palette by giving preference to those pixels over pixels outside the sub-region. Because this image changes the palette, pixels outside the sub-region may be affected by this option.
10. Click Close to exit the Histogram dialog box.

    If you are not satisfied with the results, you can use the AutoCAD UNDO command to restore the image to the original settings.

Note: You can reset the brightness and contrast to zero for all color channels by clicking Reset Controls. This does not undo any edits you have already applied to the image.