which is published in full in "Real World Adobe Illustrator CS2" by Mordy Golding.
What makes the Live Trace feature in Adobe® Illustrator® CS2 a joy to use is the ability to make adjustments to the settings while you see the results update on your screen. When live tracing Illustrator produces a dialog box of settings you can use to ensure that you get the results you need.
To access these settings, select the object you wish to trace and click the Tracing Options command under OBJECT in the top menu bar (Object > Live Trace > Tracing Options). The Options dialog appears on your screen; you’ll notice that it’s split into several different sections.
The Tracing Options dialog box offers a variety of settings to achieve the perfect trace.
First, a Preview check box appears on the far right of the dialog box, which allows you to see results update as you make changes to the different settings. Directly underneath the Preview checkbox is a list of important details about your traced object. The values for the number of paths, anchor points, colors, distinct closed areas, and image resolution update as you make adjustments to the settings in the dialog. Keeping an eye on these values helps you make decisions as you edit your trace settings. At the top of the dialog is a Preset pop-up menu, similar to what you see in the Control palette when a Live Trace object is selected. Stepping through the different presets in the Tracing Options dialog allows you to see the settings for each of the presets.
The rest of the Tracing Options dialog box is separated into three sections called Adjustments, Trace Settings, and View. The View section allows you to specify how the traced object appears on your artboard.
The following “Modifying the raster adjustments” and “Adjusting the vector trace settings” sections will help you clearly understand the two-step process that the Live Trace feature performs when converting raster images into vector form.
MODIFYING THE RASTER ADJUSTMENTS
The Adjustments settings found on the left side of the Tracing Options dialog box apply to the raster conditioning that occurs before the image is traced.
Mode. Live Trace converts a bitmap image to either 1-bit black and white, 8-bit grayscale, or 8-bit color, which you can choose from the Mode pop-up menu.
Threshold. The Threshold setting determines the boundaries between pixels when using the Black and White trace setting. For example, in a gray bitmap, a high threshold setting results in more gray pixels becoming black vector objects, and thus a heavier appearance. In that same image, a low Threshold setting results in more gray pixels ignored, making for more white-colored objects and an overall lighter or more delicate appearance. Too low of a Threshold setting may also result in a loss of image detail. The Threshold setting is also available in the Control palette when a Black and White Live Trace object is selected.
Making adjustments to the Threshold setting can have a large impact on the overall appearance of the traced result. The above examples show an image with a variety of different Threshold settings.
The above examples show another image with a variety of different Threshold settings.
Palette. By default, Illustrator uses the selective color reduction method to choose the best colors to fit the image (the maximum colors available). However, you can limit these (132 is a good rule of thumb) or choose specific colors from a custom palette. To do the second option, you must first load a custom swatch library. When a custom swatch library is opened in your document, the Palette pop-up menu displays all of the available palettes. Live Trace then only uses the colors that appear within the custom swatch library that you choose.
Loading several different custom libraries allows you to quickly experiment with a variety of different color schemes.
Max Colors. Live Trace uses the selective color reduction method to reduce the number of colors in the raster image to match this setting during the conditioning process. The Threshold setting is also available in the Control palette when a Grayscale or Color Live Trace object is selected.
Blur. The Blur setting applies a Gaussian Blur to the image, which helps remove noise from the raster image. This reduces the number of anchor points in the tracing result, especially when you are tracing photographic images.
Resample. The Resample setting lets you change the resolution of the bitmap image to help obtain a better traced result. Resampling a high resolution image to a lower resolution greatly enhances the speed performance of Live Trace.
ADJUSTING THE VECTOR TRACE SETTINGS
The Trace Settings found on the right side of the Tracing Options dialog apply to the actual tracing of the image and determine how the final vector paths are drawn.
Fills. When you have Fills selected, Live Trace creates close and filled vector paths for all resulting vector objects. Fill tracing produces results that more closely match the original image, including variable-width lines that are common in marker or ink renderings. Fill tracing also results in more complex vectors because it needs more anchor points.
When you choose the Fills setting, the traced paths appear with thick and thin edges, closely matching the original image.
Strokes. With the Strokes setting selected, Live Trace creates stroked open paths for all areas that fall within the Max Stroke Weight setting. Areas that exceed this setting result in unfilled areas outlined with a 1-point stroke. Stroke tracing results in paths with fewer anchor points.
When you choose the Strokes setting, the traced paths appear consistent and results in a less complex traced image overall.
Max Stroke Weight. The Max Stroke Weight setting determines the heaviest stroke weight Live Trace can use when tracing the image. This setting is only available when the Strokes trace setting is used.
Min Stroke Length. The Min Stroke Length setting determines the shortest path that Live Trace can use when tracing the image. This setting is only available when the Strokes trace setting is used.
Path Fitting. Path Fitting determines how closely Live Trace follows the shape of the original raster image. A lower Path Fitting setting results in paths that closely match the original raster image, yet might also reveal imperfections or irregular paths that aren’t smooth. A higher setting produces smoother paths with fewer anchor points but might not match the raster image as closely.
This figure shows examples of a variety of path fitting settings for the same image. Notice how the paths get smoother as the number is increased but that the result doesn’t match the original sketch as much.
Minimum Area. The Minimum Area setting sets a threshold for how large a section of the raster image has to be in order to traced into a vector object. By setting a minimum area, you can have Live Trace only trace areas of pixels that meet a minimum size. For example, if the Minimum Area is set to 9 pixels, Live Trace ignores regions of pixels that are less than 3 by 3 pixels in size. (The Minimum Area setting is also available in the Control palette when a Live Trace object is selected.)
Corner Angle. The Corner Angle setting defines the sharpness of the angles used in the resulting vector objects. This setting is measured in degrees, not pixels. If you think of 0 degrees as perfectly flat and 180 degrees as a hard corner (rather than a rounded one), anything sharper than the Corner Angle setting (the default is 20) is converted to a corner anchor point rather than a smooth anchor point.
Excerpted from "Real World Adobe Illustrator CS2" by Mordy Golding. © 2005.To purchase this book, visit www.adobepress.com.
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