Creating line art & posterizing effects in Photoshop

The term “line art” refers to images that are exclusively black and white. Line art images use no colors or tonal grays, just solid areas of two tonal extremes to express line, shape, and contour. It focuses on outlines and simple details rather than depth, texture, or realistic lighting in order to dramatize content.

Line art is used to spike pictures with high-contrast impact. Before computer graphic software, high-contrast black and white images were made by photographing through a dense red filter with legendary Kodak TriX pan, high-speed, ISO 400, panchromatic black-and-white film, known for its classic, gritty look, high contrast, and distinctive grain structure. For even more contrast, the images were sometimes traced by hand using India ink or “bumped” by quickly exposing the image to light with a process camera, to burn out the light grays and blacken the dark grays. High contrast images were quite fashionable for promotional materials in the 1970s and 1980s, especially in the movie industry (Fig.1).

Graphics software

Nowadays, by replacing film or hand-inked line art techniques with easy-to-use graphics software, line art images can be created instantly from a photograph to produce dramatic black-and-white effects. There are several methods for converting images to black-and-white line art, and the outcome depends on how the image will ultimately be used. These techniques are straightforward, but it helps to understand how tonal characteristics are altered. Furthermore, combining line art techniques with posterization and color overlays can produce beautiful dramatic graphics.

Black or white?

Since a grayscale image contains only one channel, it has the potential being composed of 256 desaturated colors (shades of gray). The secret to converting a grayscale image to line art is to control which pixels are black and which pixels are white depending on their numerical values.

Consider a grayscale where black is assigned the number zero, and white is assigned the number 255. The midtone gray is assigned the number 127 (about half of 255). When creating line art, all pixels in the grayscale that have values from zero to 127 become black, all pixels with numerical values from 128 to 255 become white.

Bit depth

In Photoshop, this process can be accomplished a couple of different ways. One method is to reduce the bit depth of the image to one bit per pixel. A grayscale consists of pixels that have eight-bits of information. Each bit functions like a light switch. It’s either on or off. The combination of eight “light switches” produces the potential of 256 combinations that appear as shades of gray (28 = 256). By converting the image’s bit depth to one bit per pixel, its potential is reduced to two options: the switch is either on (white) or off (black).

Bitmaps

To change an RGB image’s bit depth, it first needs to be converted to grayscale. In Photoshop, (Image menu > Mode > Grayscale) and then convert it to a bitmap in the same menu. A bitmap is simply an image with one bit of information per pixel.

The Bitmap dialog box that appears offers several choices. Choosing the 50% Threshold, option creates a black and white image based on the tonal range of the original photograph (Fig. 2). It converts pixels with values from zero to 127 to black, and values from 128 to 255 to white.

There are other interesting menu options in the Bitmap dialog box that create screens of various matrixes. These screens produce black-and-white dots of variable sizes and configurations such as halftones, or densities such as Diffusion Dither based on the tonal range of the image.

Limitations

Unfortunately, bitmap file sizes are small, and they are limited in the features that they support. If you want to add special effects, or make modifications, you’ll need to convert bitmaps to grayscale or RGB modes. Then you can experiment with them and apply cool effects like color fill layers or smart filters while maintaining the image’s line art effects (Fig. 3).

Threshold

An alternative solution to creating line art images offers more control over how light and dark pixels are converted. The Threshold command can be applied directly to the image from the Adjustments menu or as an adjustment layer. The dialog box initially converts the image to black and white with the midpoint level set at 127. The slider controls the midpoint. If you want more black pixels to darken the image drag the slider to the right, increasing the numerical value of the midpoint.

Drag the slider to the left to lighten the picture and convert more pixels to white (Fig. 4). It’s unnecessary to convert the image to grayscale or bitmap to apply the Threshold adjustment. It directly affects RGB color images.

Posterization

As Threshold divides an image into black and white pixels, posterizing determines the number of colors that compose the image. Choose Image > Adjustments > Posterize, or better yet, choose the Posterize icon in the Adjustments panel to display the Posterize dialog box and create a Posterize adjustment layer. Type a Levels value from two to 255 or drag the slider.

Posterize applies the value to each channel of the image, so if you enter a levels value of two, to an RGB image with three channels, for example, you produce eight colors (23). RGB is an additive color process, so it’s difficult to predict the results. This technique can be interesting depending on how many colors you enter into the Levels field, but more often, it can be unpredictable and disappointing. (Fig. 5)

Grayscale technique

Fortunately, the Posterize feature can be used to determine the number and choice of colors and how they are distributed in the image. The following workaround creates flat areas of gray that can be tinted with your choice of color. This method achieves an effect that resembles a silkscreen print. Bright, vibrant, subtle pastel, or any combination of hues can be applied. Here is a step-by-step guide to how it’s done using adjustment layers:

Open the RGB image. From the Adjustments panel, click on the Black and White icon to produce what looks like a black and white photographic image. Adjust the color sliders to enhance contrast (Fig. 6).

From the Adjustments panel, click on the Posterize icon. Make sure the Posterize layer is above the black and white layer in the stack. Enter a Levels value of four. Any amount from two to 255 can be entered, but four produces a nice simple posterized effect. Notice that the image is now divided into four shades of gray: white, light gray, dark gray, and black that correspond to the picture’s original tonality (Fig. 7).

Select the Posterize layer. From the Tools panel, choose the Magic Wand tool. Set the Tolerance to 1 and clear the Anti-Aliased and Contiguous options in the Options bar. Click a white region to select all the white pixels in the image.

At the bottom of the Layers panel click the New Fill or Adjustment Layer icon and choose Solid Color from the menu. When the Color Picker appears choose a light color and click OK. A new Fill layer is created with a layer mask that fills the white areas with the color (Fig. 8).

Select the Posterize layer again. Choose the Magic Wand tool. Click on a region of light gray. Create a new solid color fill layer and choose a slightly darker color.

Repeat these steps on the remaining shades of gray, choosing a darker color for each corresponding shade of gray. Your image will look similar to Figure 9.

The advantage of using Fill layers is that after you’ve applied them, you can click on each layer’s color swatch, and the Color Picker is displayed. Then, you can modify each color or change it entirely at any time, making the color selection process completely dynamic.

Push the envelope

Line art and posterization techniques push the color envelope and provide interesting ways to modify photographs. The kinds of images they produce can be graphically dramatic, attracting the viewer’s attention. What the camera sees, usually resembles everyday reality. These strong, artful techniques are a moon shot beyond reality into new realms of color that can be beautiful and compelling. As usual, I encourage you to experiment with these techniques. Try adjusting the threshold slider in both dark and light directions to capture or discard greater or less detail. Experiment with more shades of gray and multiple color combinations when you posterize. You have the potential to create extraordinary images that excite and inspire, so give it a try.