Posterizing With Cutout Filter
Photoshop CS3 Tutorial
How to Posterize an Image with the Photoshop Cutout Filter
Posterizing takes a continuous tone photograph and converts it to a simplified version with limited shapes and colors.
Open an image by choosing File > Open.
From the Filter menu, choose Artistic > Cutout.
By clicking the eye ball you can turn the preview on or off. Make sure the preview is on.
Adjust the sliders to enhance your image and press OK.