Brightness/Contrast Tab (Histogram Dialog Box)

Use this tab to adjust the brightness and contrast of color and grayscale images.

Brightness

You can move the slider to the right to increase the brightness or the left to decrease brightness.

In color images, increasing brightness causes colors to become lighter shades. In grayscale images, increasing brightness causes more grays to become white.

In color images, decreasing brightness causes colors to become darker. In grayscale images, decreasing brightness causes more grays to become black.

Contrast

You can move the slider to the right to decrease the tonal gradation or to the left to increase the tonal gradation between the highlights, midtones, and shadows in an image.

When contrast is increased to an extreme in a grayscale image, the shades of gray can be polarized to just black and white. In a color image, an extreme increase in contrast causes the selected color channel to become more prominent. For example, if the selected color channel is red, then more shades of red appear in the image.

RGB

Specifies the color channel to change in a color image. For a grayscale image, you can change only RGB, which represents the gray channel.

Index

Displays an index for a shade of the color channel that is currently selected.

When you place your pointer over a bar in the histogram, the Index value displays the intensity of the shade that the bar represents (0 to 255).

Freq

Display a frequency value for a shade of the color channel that is currently selected.

When you place your pointer over a bar in the histogram, the Freq value displays the number of pixels in the image that are the shade that the bar represents.

If you selected a grayscale image, then this value represents the number of pixels that are the shade that corresponds to the bar.

If you selected a color image, and you choose a color channel, then the frequency value represents the number of pixels that are the shade of the selected color channel.

Apply Changes To

• Entire Image. Applies the changes to the selected image.
• Sub-Region. Applies the changes to the sub-region only.

Sub-Region Options

Displays palette options for sub-regions associated with an Index Color (8-bit) image:

• 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 does 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.

Apply

Changes the color values in the image file and generates a new histogram that reflects the changes.

Reset Controls

Resets the brightness and contrast on all of the color channels to zero.

Note: If you have already clicked Apply, clicking Reset Controls does not undo your changes. To undo your changes, exit the dialog box and use the UNDO command.

Preview

Displays a preview of the image showing the modifications.

Click an image name to view the preview for that image. Histogram changes apply to all images you have selected, not just to the image that is currently previewed.

This preview does not show any display-only adjustments you may have made to the image using the AutoCAD IMAGEADJUST command. If you apply changes to the image using the Histogram, then all display-only adjustments are reset to the defaults.