Live Trace: Converting Rasters to Vectors

Certain Illustrator features, like Pathfinder, are incredibly useful and as a result, are used many times a day. Features like 3D are also extremely cool, but they aren't used as often. Every once in a while, a feature comes along in Illustrator that is cool and fun to use, but that is also practical enough that you use it on a regular basis. Illustrator's Live Trace is such a feature.

The concept is simple enough: take a raster-based image and convert it into a vector-based image. You would want to do this to get around the limitations of a raster-based file. For example, if you want to scale artwork up in size, or if you want to edit the artwork easily and use spot colors, you want to work with a vector-based file.

There are separate applications (like Adobe Streamline) and Illustrator plugins (like Free Soft's Silhouette) that have the ability to convert raster content into vectors, but Illustrator CS2's Live Trace is a far step above and beyond what those tools are capable of. One of the main reasons for this is because of how Live Trace works.

Live Trace uses a two-step process when converting rasters to vectors. In the first step, Live Trace conditions the raster image for optimal tracing. This means that Illustrator makes adjustments to the raster image, like adjusting contrast or blurring jagged edges. In the second step, Live Trace draws vector paths, creating highly accurate vector art (Figure 8.14). Although the tracing is theoretically done at that point, Illustrator retains a link to the original raster image so that you can make adjustments to the tracing settings. As you update the different raster conditioning and vector tracing settings, you can preview the results immediately. This makes it easy to get just the right tracing result that suits your needs best.

Figure 8.14. Illustrator's Live Trace feature starts with the original raster image (top), conditions the image (center), and then converts it to clean vectors (bottom). The first two examples in this 3-page spread use the Black and White setting for tracing, whereas the third shows the Grayscale tracing setting.

Be sure to check out Scott Crouse's Live Trace example in the color insert.

Tracing an Image

Tracing an image is simple. Select any raster image in your Illustrator document and click the Live Trace button in the Control palette. Alternatively, you can choose Object > Live Trace > Make. This action traces the image using Illustrator's default trace preset. Illustrator actually ships with 13 different tracing presets, each optimized for different kinds of images and desired result. Once the image is traced, it maintains a live link to the raster image and you can customize the tracing settings. For example, once the image is traced, the Control palette changes to reflect different settings, including a Preset pop-up menu (Figure 8.15). Choose from any of Illustrator's thirteen different presets to see a different traced result on your screen.

Figure 8.15. The Preset pop-up menu in the Control palette lets you quickly experiment with different tracing presets.

If you know which tracing preset you want to use before you trace your image, you can select it directly by clicking the upside-down triangle that appears just to the right side of the Live Trace button and choosing it from the list that pops up (Figure 8.16).

Figure 8.16. It doesn't look like a pop-up menu, but it is. Clicking the upside-down triangle allows you to apply a specific tracing preset when you first choose to trace an image.

As long as Illustrator maintains a live link to the raster image, you won't be able to edit the actual vector paths that were created during the tracing process. In order to do so, you either have to expand the traced object or convert it to a Live Paint group. We discuss both of these options later, but for now, we're going to focus on how to customize the tracing settings so that you can get the bests results from the Live Trace feature.

If you're happy with the traced results, there's no need to expand the object at all. Illustrator can print the vector paths just fine (it expands the paths in the print stream).

Exploring the Live Trace Preview Options

Once you've traced an image, Illustrator displays the traced result on your artboard so that you can see the results. However, Illustrator offers a variety of different settings that you can use to control how both the raster image and the traced vector result appear on your artboard.

When a traced image is selected on the artboard, the Control palette updates to contain two icons that, at first glance, look like triangles. Upon closer inspection, you'll notice that one icon features a jagged edge; this icon is used to control how the raster image is previewed. The icon on the right, which has a smooth edge, is used to control how the traced vector result is previewed (Figure 8.17). Each of these settings is separate and can be chosen independently of the other.

Figure 8.17. Once you've traced an image, you can use the two icons in the Control palette to control how the artwork appears on the artboard.

Previewing the Original Raster Image

In the Control palette, the jagged triangle on the left controls how the raster image is viewed. Click once on the icon and choose from one of the four available settings (Figure 8.18).

Figure 8.18. You can preview the raster image with the Original Image setting (left), the Adjusted Image setting (center), or the Transparent Image setting (right). The No Image option is not shown for obvious reasons.

• No Image. This setting completely hides the raster image from the screen (and is the default setting).

• Original Image. This setting displays the original raster image in your document, which can be useful when you're comparing the original image to the traced result.

• Adjusted Image. This setting displays the raster image as it appears after Live Trace has applied the raster conditioning adjustments. This preview mode is great for seeing how Live Trace works and it makes it easier to preview any adjustments that you make to the raster image settings.

• Transparent Image. This setting displays a dimmed preview of the bitmap image beneath the traced result, letting you see the traced results as compared to the original raster image.

Previewing the Traced Vector Result

In the Control palette, the smooth triangle on the right controls how the traced vector result is viewed. Click once on the icon and choose from one of the four available settings (Figure 8.19).

Figure 8.19. You can preview the vector result with the Tracing Result setting (left), the Outlines setting (center), and the Outlines With Tracing setting (right). The No Tracing Result option is not shown.

• No Tracing Result. This setting hides the traced vector objects from the screen.

• Tracing Result. This setting displays the vector result of the tracing (and is the default setting).

• Outlines. This setting highlights the actual Bézier paths that were created when the image was traced.

• Outlines With Tracing. This setting highlights the Bézier paths as semi-transparent, enabling you to compare filled areas of the traced vector result with the original bitmap image. The color of the outlines will match the color specified for guides in the Guides & Grid preferences panel.

Tweaking to Get the Perfect Trace

What makes the Live Trace feature a joy to use is the ability to make adjustments to the settings while you see the results update on your screen. Aside from the different presets that you can apply, Illustrator contains a dialog box chock full of settings that you can use to ensure that you get the results you need from the Live Trace feature.

To access these settings, select a Live Trace object and click the Tracing Options dialog button in the Control palette. Alternatively, you can choose Object > Live Trace > Tracing Options. Once the Tracing Options dialog appears on your screen, you'll notice that it's split into several different sections (Figure 8.20).

Figure 8.20. The Tracing Options dialog offers a smorgasbord of settings to achieve the perfect trace.

First of all, a Preview check box appears on the far right of the dialog, which allows you to see results update as you make changes to the different settings. Directly underneath the Preview check box 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 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, as discussed in the earlier section, "Exploring the Live Trace Preview Options."

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 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 (Figure 8.21). 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.

  Figure 8.21. Making adjustments to the Threshold setting can have a large impact on the overall appearance of the traced result. Here are examples of an 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 (based on the Max Colors value). However, you can choose specific colors that Illustrator should use when tracing your image. To do so, you must first load a custom swatch library (refer to Chapter 1 for instruction on how to define 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 (Figure 8.22). Live Trace then only uses the colors that appear within the custom swatch library that you choose.

  Figure 8.22. Loading several different custom libraries allows you to quickly experiment with a variety of different color schemes.

  
  
  

• Max Colors. The Max Colors setting determines the maximum number of colors that can be used in the final traced result. This setting is not available for the Black and White Mode setting. 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.

  

  Check the Output to Swatches option to have Illustrator add each color that is used during the tracing process as a global process color in your Swatches palette.

  
  
  
  

• 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 closed 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 (Figure 8.23). Fill tracing also results in more complex vectors because it needs more anchor points.

  Figure 8.23. 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 (Figure 8.24).

  Figure 8.24. 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 (Figure 8.25, next page).

  Figure 8.25. 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.

Editing Live Trace Paths

Once you've achieved a trace result that you're satisfied with, you might want to edit the Bézier paths, either to delete portions of the image, or to apply your own colors, gradients, or patterns. In order to edit the vector paths of the traced object, you will need to either expand the trace, or convert the traced object to a Live Paint group.

Once you've specified your settings in the Tracing Options dialog, you can click the Save Preset button to define your own tracing presets.

Expanding a Live Trace Object

With a Live Trace object selected, click the Expand button in the Control palette. Alternatively, you can choose Object > Live Trace > Expand. You can then use the Direct Selection tool to edit anchor points and Bézier paths (Figure 8.26). At this point, the traced object is no longer linked to the original raster image, and the traced result can no longer be adjusted using any of the Live Trace options.

Figure 8.26. Once you've expanded a Live Trace object, you can edit the paths as you would with any vector object.

If you select both the Fills and the Strokes options, Live Trace converts the raster to a vector using a combination of both stroked and filled paths.

For performance reasons, Gap Detection is turned off for Live Paint groups that are created directly from a Live Trace object. You can either turn Gap Detection on manually, or you can create smaller Live Paint groups to get better performance.

Converting Traced Images to Live Paint Groups

In Chapter 4, Advanced Vectors, you learned about Illustrator's new Live Paint feature, which allows you to apply fill attributes to areas, even if they aren't fully enclosed shapes. If you've traced an image because you want to fill regions of the image with color, converting the Live Trace object to a Live Paint group makes a lot of sense.

With a Live Trace object selected on the artboard, click the Live Paint button that appears in the Control palette. This action expands the traced object and converts all of the resulting vector objects into a Live Paint group in a single step. You can then use the Live Paint Bucket tool to fill your art with color without any additional steps (Figure 8.27). For more information on Live Paint groups, refer to Chapter 4.

Figure 8.27. What started out as a pencil sketch quickly turns into final art when you combine the Live Trace and the Live Paint features in Illustrator.

You can press Option (Alt) while clicking the Live Trace button or when choosing a Live Trace preset from the Control palette to both trace and expand an image in one step.