To Adjust the Image Contrast Curve

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 Tonal Adjustment 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 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 select a color channel to manipulate. You can manipulate the contrast curve for the combined RGB channels, or you can select a single channel (red, green, or blue).
5. Select a curve type from the Select Curve Type list:
   • Choose Gamma correction to create an exponential curve by defining either the single exponent (gamma) value or using an interactive slider which determines the gamma value.
   • Choose Fitted to draw a curve smoothly through points you specify by clicking and dragging on curve segments or on existing points.
   • Choose Piecewise Linear to draw straight segments between points you specify. You can create or modify points by clicking and dragging on curve segments or on existing points.
6. To delete a point on a Piecewise Linear or Fitted curve, select the point, and press Delete, or right-click and click Delete Point.
7. To import a curve, click the Import button, or right-click in the contrast curve window and click Import. In the Import dialog box, simply navigate to the Gamma Point List (*.gpl) file and click Open.
8. To export a curve, click the Export button, or right-click in the contrast curve window and click Export. In the Export dialog box, navigate to the folder where you want to save the Gamma Point List (*.gpl) file and click Save.
9. 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 are more visible and the highest frequency values are truncated by the top of the window.
10. To restore the default settings for the curve, click the Reset button, or right-click in the contrast curve window and click Reset.
11. Click Apply.
12. 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.
13. If the sub-region is associated with an 8-bit color image, select a palette option from the Sub-Region Options 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 the pixels inside the sub-region over pixels outside the sub-region. Because this image changes the palette, pixels outside the sub-region may be affected by this option.
14. Click Close to exit the Histogram dialog box.

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