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Edge-Enhancement Filters

The Filter Stylize submenu offers access to a triad of filters that enhance the edges in an image. The most popular of these is undoubtedly Emboss, which adds dimension to an image by making it look as if it were carved in relief. The other two, Find Edges and Trace Contour, are less commonly applied but every bit as capable and deserving of your attention.

Embossing an image

The Emboss filter works by searching for high-contrast edges (just like the Sharpen Edges and High Pass filters), highlighting the edges with black or white pixels, and then coloring the low-contrast portions with medium gray. When you choose Filter Stylize Emboss, Photoshop displays the Emboss dialog box shown in Figure 11-8. The dialog box offers three options:

Figure 11-8: The Emboss dialog box lets you control the depth of the filtered image and the angle from which it is lit.

Angle: The value in this option box determines the angle at which Photoshop lights the image in relief. For example, if you enter a value of 90 degrees, you light the relief from the bottom straight upward. The white pixels therefore appear on the bottom sides of the edges, and the black pixels appear on the top sides. Figure 11-9 shows four reliefs lit from different angles and with different Height settings.

Figure 11-9: Reliefs lit with four different Height settings and from four different angles, in increments that are multiples of 45 degrees. Imagine a light source in the center of the grouping of four images.

Height: The Emboss filter accomplishes its highlighting effect by displacing one copy of an image relative to another. Using the Height option, you specify the distance between the copies, which can vary from 1 to 100 pixels. Lower values produce crisper effects, as demonstrated in Figure 11-9. Values above 4 goop up things pretty well unless you also enter a high Amount value. Together, the Height and Amount values determine the depth of the image in relief.

Amount: Enter a value between 1 and 500 percent to determine the amount of black and white assigned to pixels along the edges. Values of 50 percent and lower produce almost entirely gray images. Higher values produce sharper edges, as if the relief were carved more deeply.

As a stand-alone effect, Emboss is only so-so. It's one of those filters that makes you gasp with delight the first time you see it but never quite lends itself to any practical application after you become acquainted with Photoshop. But if you think of Emboss as an extension of the High Pass filter, it takes on new meaning. You can use it to edit selection outlines in the quick mask mode, just as you might use the High Pass filter. You also can use it to draw out detail in an image.

Figure 11-10 shows the result of using the Fade command immediately after applying the Emboss filter. First, I applied the Emboss filter at an Angle of 135 degrees, a Height of 2 pixels, and an Amount of 500 percent. Then I pressed Ctrl+Shift+F (z -Shift-F on the Mac) to display the Fade dialog box. To create the top-right example, I selected Darken from the Mode pop-up menu and lowered the Opacity to 65 percent. This added shadows to the edges of the image, thus boosting the texture without unduly upsetting the original brightness values. I selected the Overlay blend mode and raised the Opacity back to 100 percent to create the bottom-left example, and then switched to Pin Light at 80-percent Opacity for the bottom-right image.

Figure 11-10: After applying the Emboss filter, I used my old friend the Fade command to experiment with blend modes and Opacity levels.

Whereas this color plate chiefly demonstrates various effects you can achieve by fading the Emboss filter, the final example uses a different technique to improve the example directly above it. Rather than let the gray choke out the colors below, I adjusted the Underlying Layer settings in the Layer Style dialog box, permitting the blacks and whites to show through. For a full explanation of this feature, turn to the "Color exclusion sliders" section of Chapter 13.

Tracing around edges

Photoshop provides three filters that trace around pixels in your image and accentuate the edges. All three filters live on the Filter Stylize submenu:

Find Edges: This filter detects edges similarly to High Pass. Low-contrast areas become white, medium-contrast edges become gray, and high-contrast edges become black, as in the first example in Figure 11-11. Hard edges become thin lines; soft edges become fat ones. The result is a thick, organic outline that you can overlay on an image to give it a waxy appearance. To achieve the bottom-left effect in the figure, I chose Edit Fade Find Edges and applied the Overlay mode.

Figure 11-11: The top row of images demonstrates the effect of the three edge-tracing commands in the Filter Stylize submenu. After applying each command, I used the Fade command to apply the blend modes and Opacity values demonstrated in the bottom row.

Glowing Edges: This Gallery Effects filter is a variation on Find Edges, with two important differences. Glowing Edges produces an inverted effect, changing low-contrast areas to black and edges to white, as in the top-middle image in Figure 11-11. This filter also enables you to adjust the width, brightness, and smoothness of the traced edges. For example, the top-middle image in Figure 11-11 uses an Edge Width of 2, an Edge Brightness of 10, and a Smoothness of 5. Glowing Edges is a great backup command. If you aren't satisfied with the effect produced by the Find Edges filter, choose Glowing Edges instead and adjust the options as desired. If you want black lines against a white background, press Ctrl+I (Win) or z -I (Mac) to invert the effect.

Trace Contour: Illustrated on the right side of Figure 11-11, Trace Contour is a little more involved than the others and slightly less interesting. The filter traces a series of single-pixel lines along the border between light and dark pixels. Choosing the filter displays a dialog box containing three options: Level, Upper, and Lower. The Level value indicates the lightness value above which pixels are considered to be light and below which they are dark. For example, if you enter 128 — medium gray, the default setting used in Figure 11-11 — Trace Contour draws a line at every spot where an area of color lighter than medium gray meets an area of color darker than medium gray. The Upper and Lower options tell the filter where to position the line — inside the lighter color's territory (Upper) or inside the space occupied by the darker color (Lower).

Like Mezzotint, Trace Contour applies itself to each color channel independently and renders each channel as a 1-bit image. A collection of black lines surrounds the areas of color in each channel. The RGB, Lab, or CMYK composite view shows these lines in the colors associated with the channels. When you work in RGB, a cyan line indicates a black line in the red channel (no red plus full-intensity green and blue becomes cyan). A yellow line indicates a black line in the blue channel, and so on. You get a single black line when working in the grayscale mode.

Creating a metallic coating

The edge-tracing filters are especially fun to use in combination with Edit Fade. I became interested in playing with these filters after trying out the Chrome filter included with the first Gallery Effects collection. Now included with Photoshop as Filter Sketch Chrome, this filter turns an image into a melted pile of metallic goo. No matter how you apply Chrome, it completely wipes out your image and leaves a ton of jagged color transitions in its wake. It's really only useful with color images, and then only if you follow up with the Fade command and a blend mode. Even then, I've never been particularly satisfied with the results.

But all that experimenting got me thinking: How can you create a metallic coating, with gleaming highlights and crisp shadows, without depending on Chrome? Find Edges offers a way. First, copy your image to a separate layer by pressing Ctrl+J (z -J on the Mac). Then apply the Gaussian Blur filter. A Radius value between 1.0 and 4.0 produces the best results, depending on how gooey you want the edges to be. Next, apply the Find Edges filter. Because the edges are blurry, the resulting image is light, so I recommend you darken it using Image Adjustments Levels (raise the first Input Levels value to 100 or so, as explained in Chapter 17). The blurry edges appear in the top-left example in Figure 11-12.

Figure 11-12: After applying Gaussian Blur and Find Edges to a layered version of the image (top left), I composited the filtered image with the original using the Overlay mode (bottom left). The second and third columns show similar effects achieved using the Bas Relief and Chrome effects filters.

To produce the bottom-left image, I mixed the layer with the underlying original using the Overlay blend mode. The result is a shiny effect that produces a metallic finish without altogether destroying the detail in the image.

If you decide you like this effect, there's more where it came from. The second and third columns of Figure 11-12 show the results of applying Filter Sketch Bas Relief and (for comparison) Filter Sketch Chrome, respectively. After applying each filter, I chose Edit Fade and selected the Overlay mode, repeating the effect I applied to the Gaussian Blur and Find Edges layer. In this case, the Overlay mode actually makes the Chrome filter look quite respectable.