Consolidating Colors

The Threshold and Posterize commands let you consolidate color values in an image or selection. Besides producing interesting effects with these commands, you can use them in alpha channels to help isolate areas within an image.

If you select an area within an image, Canvas X Draw applies the adjustment only to that area. Otherwise, Canvas X Draw adjusts the selected paint objects.

Setting a Brightness Threshold

Use the Threshold command to convert any image to black and white. The Threshold command compares each pixel’s brightness value to a threshold value that you set. It changes brighter pixels to white and darker pixels to black. The threshold setting is based on a scale of brightness values from 0 (black) to 255 (white). You can’t use the Threshold command on images in Black & White or Indexed mode.

For example, if you set a threshold value of 128, pixels that are brighter than medium gray become white, while pixels darker than medium gray become black.

To Map an Image to Black and White:

1. Select one or more paint objects to adjust all the images. You can select an area in one image in Edit mode to adjust the selected area only. If you don’t make a selection, the entire image is affected.
2. Choose Image | Adjust | Threshold.
3. Enter the threshold value by dragging the slider or typing a number in the text box. If you want Canvas X Draw to convert half the pixels to black and half to white, click Auto.
4. Click OK after entering the setting you want.

To isolate selections, apply the Threshold command in conjunction with the High Pass filter to an image in an alpha channel. (See Isolating Areas Using the High Pass Filter.)

Creating High Contrast Posterized Images

You can condense the brightness variations in an image with the Posterize command. If you apply the Posterize command to a photograph, it creates a high-contrast image by compressing hundreds of brightness levels into only a few. You set the number of brightness levels you want to retain, and Canvas X Draw reduces each color channel to that number of values.

Original RGB image	Posterize 8 levels
Posterize 4 levels	Posterize 2 levels

The Posterize command’s effect depends on the mode of the image you posterize; e.g., if you apply the Posterize command with a setting of 2 levels to a grayscale-mode image, the image becomes black and white. If you apply the same setting to an RGB-mode image (even if it contains only grays), the command converts each pixel’s red, green, and blue value to either zero or full color, reducing the image to eight colors — red, green, blue, red-green, red-blue, blue-green, black, and white.

You can’t use the Posterize command on images in Black & White or Indexed mode.

To Posterize an Image:

1. Select one or more paint objects to posterize all the images. You can select an area in one image in Edit mode to posterize the selected area only. If you don’t make a selection, the entire image is affected.
2. Choose Image | Adjust | Posterize.
3. Enter a level from 2 to 255. Higher numbers produce subtle effects. Lower numbers produce high-contrast images.
4. Click OK after you enter the Levels setting.

See also:

• Using the High Pass, Maximum, and Minimum Filters