Posterization Techniques
Julieanne Kost. As the Senior Digital Imaging Evangelist at Adobe Systems, Julieanne is a master of Photoshop where her role includes supporting product development, contributing to several publications, and teaching at conferences, tradeshows and workshops around the world. She is also the author behind Software Cinema’s Photoshop Quickstart and Advanced Photoshop Techniques training CDs.
This tip and technique demonstrates how to reduce the color palette of an image to create a posterized effect with the most control possible. Although Adobe® Photoshop® CS has a filter that automatically creates posterized images, it works on all channels simultaneously. The unique approach described here, allows for more flexibility by converting an image to grayscale so that the number of posterized levels can be specifically applied and colors assigned as desired.
1. Open the color image to be posterized.
If there are distracting elements in the background, you might choose to remove them since this effect is more dramatic when applied to bold subjects. In this example, the dog was selected with the Lasso tool. Then the background was selected by choosing Select > Inverse. Finally Edit > Fill was used to fill the background with black.
2. Convert the image to grayscale.
Although there are many ways to do this in Photoshop, the most flexible is to add a Channel Mixer adjustment layer. This will “convert” the image to grayscale, while retaining the color information that can be used later to fine tune the cutoff for each level of color. Select Layer > New Adjustment Layer > Channel Mixer and in the New Layer dialog box, click OK. In the Channel Mixer dialog box, click the “Monochrome” checkbox at the bottom and use the Red, Green, and Blue sliders to add or subtract amounts of each of the channels until you achieve the desired Grayscale conversion. Then click OK.
Note: At this point you are only estimating the grayscale values, you can always return to this dialog box later in this technique to further refine your settings, if necessary.
3. Posterize the image.
Select Layer > New Adjustment Layer > Posterize. In the New Layer dialog box, click OK and select the number of Levels (colors) desired in the image. In this example I selected 4. If you had used the Posterize filter on the original color image, creating only 4 levels of gray isn’t possible because even when entering the lowest value available in the posterized option (2), it would work on each channel resulting in 6 levels of gray.
4. Refine the adjustment layer.
If you aren’t happy with the breaks that the posterized adjustment layer made, you can refine them by double-clicking on the thumbnail for the Channel Mixer adjustment layer in the Layers palette. Adjust the sliders to change where the image divides into levels of posterization.
5. Add color to the image.
Target the topmost layer in the Layers palette, and select Layer > New Adjustment Layer > Gradient Map. In the New Layer dialog box, click OK. In the Gradient Map dialog box, click in the gradient area to access the Gradient Editor. Create a custom gradient with the same number of color stops as posterization levels entered in the previous step and evenly space them along the ramp.
Each color will be mapped to one of the gray values in the posterized image, depending on its placement in the gradient stop. For a more realistic image, try assigning a darker color to the gradient stops on the left, and lighter ones for the stops towards the right. The advantage to using a gradient is that changes in the color stops will change the colors that are added to the posterized image, making different color combinations easy to try.
6. Conclusion.
As a result of using this method for creating posterized images it is easy to see how using adjustment layers (to convert the image, apply posterization and colorize) results in an infinitely more flexible document—easily updated or changed based on your desired result.
