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Symbols in SVG

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  1. Symbols
  2. Creating Symbols and Definitions
  3. Summary
  4. Q&A
  5. Workshop
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This chapter is from the book

This chapter is from the book

Creating Symbols and Definitions

As far as most Web developers are concerned, symbols came onto the scene when Flash was introduced. By saving an object into Flash's symbol library, the same item could be used over and over again with very little impact on file size.

Obviously, in the case of Web graphics, smaller is generally better, as many users still access the Web through dial-up modems. Thus, a technology that intelligently recycles its content becomes a popular tool in the online world.

SVG's use of symbols offers the same value that Flash does: 1) graphics become smaller to download as more of their visual components are replicated using symbols, and 2) graphics become easier to modify when their content comprises symbols.

With such benefits, why not make everything a symbol? For starters, if an object does not appear more than once, making it a symbol increases the amount of your code. As you go through this chapter, you will see that every instance (the term used to describe the application of a symbol within the content of the document) requires a reference element. When an object appears more than once, each reference element will likely be smaller in size than the actual artwork it represents; when the object appears only once, adding a reference only adds to the file size.

Second, if an object appears only once, making it a symbol can produce confusing results for a developer. Symbols are kept apart from the content area inside an SVG document. When several instances of the symbol occur, the efficiency of the coding and file size justify the inconvenience of looking elsewhere to understand the content being referenced. If only one instance exists, most developers will scratch their heads trying to understand why you made a "one-trick pony" a symbol.

The application of symbols is even easier than the concept and the rationale. To create a symbol, you need two items:

  • The artwork or item to be referenced, contained within a definitions library

  • A reference element

Artwork to be used as a symbol can be stored within either a defs or a symbol element. For a symbol to be referenced, however, it must have been named with the id attribute (first introduced in Hour 2). Once named, the symbol can be referenced with the use element. The use element can have a variety of attributes added to it, such as x and y to determine the symbol's placement on the page, but the most important attribute references the symbol. This attribute uses the syntax xlink:href=#symbolName, where symbolName is the name of the symbol to be referenced.

TIP

An SVG viewer will never render any content placed within a defs or symbol element. Such content will appear only when referenced by the use element.

To illustrate the symbol concept, follow Listing 8.1 to store your cloud illustrations (from the news center weather graphic example begun in Hour 1) in the defs element. First, begin by copying the code from Listing 7.1 and pasting it into a new document. This will provide you with a basic illustration of the sun and sky.

Next, create a defs element (line 15 in Listing 8.1) to store your cloud symbol in. In Listing 7.2, you began by using ellipses in place of the actual cloud artwork. If you were successful with the first exercise at the end of Hour 5, however, you likely produced art similar to lines 17 through 22. To make this a usable symbol, group the artwork (lines 16 and 23) and provide an id attribute value to name the symbol (Cloud will do just fine).

Add a use element (line 27) to reference the new Cloud symbol, and position the instance of the symbol with the x and y attributes. Add a style rule (line 9) for a white fill, and apply the class to the use element. Once positioned and stylized, copy this element and paste it on the line below (line 28) to create the second cloud; be sure to modify the x and y attribute values so that the two instances aren't overlapping.

By keeping the actual Cloud symbol devoid of style information, you can use the same artwork for both the "Sunny" and "Rainy" illustrations. (You will be building the rainy illustration later.) With these changes in place, your resulting graphic will look like Figure 8.1.

Listing 8.1 Storing the Cloud Illustration As a Symbol

01: <?xml version="1.0" standalone="no"?>
02: <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
03:  "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
04:
05: <svg width="500" height="300">
06:
07:   <style type="text/css">
08:     <![CDATA[
09:       .FillFFFFFF{fill:#FFFFFF;}
10:       .Fill99CCFF{fill:#99CCFF;}
11:       .FillFFFF00{fill:#FFFF00;}
12:     ]]>
13:   </style>
14:   
15:   <defs>
16:     <g id="Cloud">
17:       <circle cx="24" cy="36" r="15"/>
18:       <circle cx="41" cy="26" r="17"/>
19:       <circle cx="90" cy="40" r="13"/>
20:       <circle cx="105" cy="31" r="13"/>
21:       <ellipse cx="75" cy="20" rx="27" ry="20"/>
22:       <ellipse cx="56" cy="50" rx="25" ry="18"/>
23:     </g>
24:   </defs>
25:
26:   <rect id="Sky" x="10" y="45" width="200" height="245" class="Fill99CCFF"/>
27:   <use id="SunCloud1" xlink:href="#Cloud" x="–20" y="20" class="FillFFFFFF"/>
28:   <use id="SunCloud2" xlink:href="#Cloud" x="150" y="210" class="FillFFFFFF"/>
29:   <circle id="Sun" cx="105" cy="160" r="56" class="FillFFFF00"/>
30:
31: </svg>

Figure 8.1 Using symbols to reference the cloud illustrations.

As you develop the news center further throughout the book, you'll notice that the same symbol is used to create the thunderstorm clouds. By the end of the example, you will have saved 28 lines of code simply by using the symbol concept for your cloud illustrations.

To further maximize your coding efficiency, symbols can be made up of other symbols. By reusing symbols within each other, grouping one with another and then creating a new symbol with this content, you can create complex but easy to change graphics.

To attempt this symbol-within-a-symbol effect, take a break from the news center example and picture a pegboard with alternating white and red pegs in its holes. Creating such an image will require code (Listing 8.2) that builds up from the lowest common denominator.

First, create a symbol based on a circle named peg (line 7 in Listing 8.2); this will serve as the shape for all the pegs on the board, regardless of color. To distinguish the pegs by color, use the use element to create two new symbols, pegRed and pegWhite, that reference the original peg symbol for shape; then apply paint information (lines 8 and 9).

Next, create two more symbols (lines 10 and 16) to define pegboard columns that start with red and white pegs respectively. Each column symbol should contain references to the colored peg symbols with appropriate coordinate information for placement. This ends your symbol creation. Finally, create six references to the column symbols with coordinate information to correctly arrange their positions (lines 24–29). When you're finished, your symbol-within-a-symbol example will render similar to Figure 8.2.

Listing 8.2 Using Symbols within Symbols

01: <?xml version="1.0" standalone="no"?>
02: <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20010904//EN"
    "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
03:
04: .<svg width="500" height="300">
05:
06:   <defs>
07:     <circle id="peg" r="15"/>
08:     <use id="pegRed" xlink:href="#peg" style="fill:red; stroke:black;"/>
09:     <use id="pegWhite" xlink:href="#peg" style="fill:white; stroke:black;"/>
10:     <g id="columnRed">
11:       <use xlink:href="#pegRed" x="100" y="50"/>
12:       <use xlink:href="#pegWhite" x="100" y="100"/>
13:       <use xlink:href="#pegRed" x="100" y="150"/>
14:       <use xlink:href="#pegWhite" x="100" y="200"/>
15:     </g>
16:     <g id="columnWhite">
17:       <use xlink:href="#pegWhite" x="150" y="50"/>
18:       <use xlink:href="#pegRed" x="150" y="100"/>
19:       <use xlink:href="#pegWhite" x="150" y="150"/>
20:       <use xlink:href="#pegRed" x="150" y="200"/>
21:     </g>
22:   </defs>
23:
24:   <use xlink:href="#columnRed" x="0" y="0"/>
25:   <use xlink:href="#columnWhite" x="0" y="0"/>
26:   <use xlink:href="#columnRed" x="100" y="0"/>
27:   <use xlink:href="#columnWhite" x="100" y="0"/>
28:   <use xlink:href="#columnRed" x="200" y="0"/>
29:   <use xlink:href="#columnWhite" x="200" y="0"/>
30:
31: </svg>

Figure 8.2 Creating a pegboard illustration to demonstrate symbol-within-symbol creation.

By containing symbols within symbols, you can maximize the use of your objects. In this example, one circle became a multi-colored pegboard. However, due to its modular creation, the same pegboard could become a checkerboard in seconds simply by swapping out the circle with a rectangle, as shown in line 7 of Listing 8.3. Results are shown in Figure 8.3.

Listing 8.3 Demonstrating the Efficiency of Symbols within Symbols

01: <?xml version="1.0" standalone="no"?>
02: <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20010904//EN"
     "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
03:
04: <svg width="500" height="300">
05:
06:   <defs>
07:     <rect id="peg" width="50" height="50"/>
08:     <use id="pegRed" xlink:href="#peg" style="fill:red; stroke:black;"/>
09:     <use id="pegWhite" xlink:href="#peg" style="fill:white; stroke:black;"/>
10:     <g id="columnRed">
11:       <use xlink:href="#pegRed" x="100" y="50"/>
12:       <use xlink:href="#pegWhite" x="100" y="100"/>
13:       <use xlink:href="#pegRed" x="100" y="150"/>
14:       <use xlink:href="#pegWhite" x="100" y="200"/>
15:     </g>
16:     <g id="columnWhite">
17:       <use xlink:href="#pegWhite" x="150" y="50"/>
18:       <use xlink:href="#pegRed" x="150" y="100"/>
19:       <use xlink:href="#pegWhite" x="150" y="150"/>
20:       <use xlink:href="#pegRed" x="150" y="200"/>
21:     </g>
22:   </defs>
23:
24:   <use xlink:href="#columnRed" x="0" y="0"/>
25:   <use xlink:href="#columnWhite" x="0" y="0"/>
26:   <use xlink:href="#columnRed" x="100" y="0"/>
27:   <use xlink:href="#columnWhite" x="100" y="0"/>
28:   <use xlink:href="#columnRed" x="200" y="0"/>
29:   <use xlink:href="#columnWhite" x="200" y="0"/>
30:
31: </svg>

Figure 8.3 Similar code to Listing 8.2 suddenly produces a checkerboard image.

By using this efficient method to design your documents, you will be able to make sweeping changes to your imagery with the replacement of one object. Although more code was used to create this checkerboard/pegboard than if you drew each element by hand, the rule of "less is more" can often be overruled by "efficiency is the key." By intelligently planning your artwork to use symbols when possible or practical, you can significantly reduce the amount of effort necessary to modify the work later.

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