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Introduction to SVG

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This chapter is from the book

This chapter is from the book

XML and SVG

As we keep saying, SVG is written in XML, so familiarity with XML's syntax is very helpful when you want to create or edit SVG code.

Having already looked at XML code, let's take a look at some simple SVG code. Using the Jasc WebDraw program (a drawing application for SVG, which we discuss further in Chapter 10), we made the following graphic in SVG (see Figure 1–8) and exported it into a raster bitmap file for the purposes of display in this book.

Figure 1-8FIGURE 1–8 triangle.svg drawn in Jasc WebDraw.


The code behind the SVG graphic is:

Example 1–2 triangle.svg

1.  <?xml version="1.0" standalone="no"?>
2.  <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
3.  "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
4.  <svg width="500" height="500">
5.  <path d="M114.286 260 L320 222.857" transform="translate(0 0)"
6.  style="fill:none;stroke:rgb(171,17,17);stroke-width:5"/>
7.  <polygon points="211.429,148.571 208.571,400 325.714,97.1429"
8.  style="fill:rgb(0,0,255);stroke:rgb(0,0,0);stroke-width:1"/>
9.  </svg>
  1. The first line shows the XML declaration, as we've already seen. But something new has been added—standalone. Standalone refers to whether the SVG document "stands alone," i.e., it has no reference to an external file, or whether it does contain a reference to an external file. In the above case, standalone is set to "no", so we have a reference to an external file—in this case, the actual DTD.

  2. In our XML discussion, we talked a bit about the DTD. Well, in this SVG code, the second line and the third line refer to the SVG DTD. Notice that these two lines reference an external DTD.

  3. Just where is that external DTD located? At the URL http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd. It is part of the W3C, the standards body of the Web. Here is the great news: SVG, unlike XML, has its own DTD already figured out by a team of experts, so it is not necessary to create a DTD for your SVG files. (And you thought this would be hard!) What does this DTD contain? All allowable SVG elements are defined in the SVG DTD, which resides at the W3C URL above. (For a complete list of SVG elements and attributes, see Appendices A and B. And, for the very brave, check out the W3C URL to see what the SVG DTD looks like. If you think you want to add elements or attributes to the SVG DTD, just take a look at the real thing!)

  4. The SVG code actually begins with the <svg> element—which consists of the opening <svg> tag and the closing </svg> tag—and includes everything in the document. This is the root element. Width and height are attributes of the <svg> element, and they set the width and height of the SVG document (as you might have guessed).

  5. Path is also an SVG element. In the above picture, the path is actually the red line going behind the triangle. The <path> element here has two attributes: d and transform. The d attribute refers to path data, and the values of d are what appear after the equal (=) sign. The numbers you see are x and y coordinates defining the path. Notice the letters M and L. These further define the path data. M means moveto, which tells the browser at which coordinates to start the line. L means lineto, which tells the browser where to draw the line. Therefore, M114.286 260 means moveto the coordinates 114.286 x and 260 y. L320 222.857 means draw a lineto coordinates 320 x and 222.857 y. We will be covering the path element in greater detail in Chapter 4 . The transform attribute is further defined by its value translate, which we won't cover here but will cover in detail in Chapter 2 .

  6. Style is yet another attribute of the <path> element. The style attribute can be used to describe a lot of the presentational aspects of our path. Our path element here defines just one single line, not a closed shape. Because the fill value refers to color inside of a shape, here we set the fill value to none because we don't have a shape, only a line. So how do we color the line? The color of the line is described by the stroke value as the RGB number (171,17,17), which renders as red, and the width of the red line is set by the stroke-width value to be 5 pixels wide. The stroke and stroke-width values control how the outline of a shape appears and/or how a line appears. Because here we have only a line and not a shape, stroke tells us how to color that line.

  7. The next element we find in the code is the <polygon> element. In SVG, polygon can be used to describe any closed shape made with straight lines. In this case, we've described a three-sided shape (yes, a triangle) by using the attribute points, then listing the x and y coordinates of the points of the triangle. How is this possible from this seemingly strange list of numbers? If you imagine a pen on paper, it's as though the pen touches the paper on the first set of coordinates (211. 429 and 143.571). Where are those coordinates? Well, in SVG, as almost everywhere else, the convention is that you write the x coordinate first and the y coordinate second. So the pen touches the paper on coordinates 211.429 x and 143.571 y. Then our hypothetical pen draws a line to the second set of coordinates, 201.571 x and 400 y, then draws a line down to the last set of coordinates, 325.714 x and 97.1429 y. So far, we have only a two-sided, open angle, but there are no more coordinates. How does SVG know to close the line and make this a triangle?

  8. Well, remember, we made this shape a polygon element. A polygon element is always a closed shape, so if we don't provide a closing coordinate, SVG automatically draws a line from the last coordinate we define to our beginning coordinate to close the shape (Figure 1–9). If we had wanted an open angle here, we couldn't have used the polygon element; we'd have to use another element, such as <path> or <polyline>.

    Figure xxxFIGURE 1–9 The image on the left is drawn using the <polyline> element, with three points specified. The image on the right is drawn using the <polygon> element with three points specified. The <polygon> element automatically closes the shape by adding a line from the last point back to the starting point.


    Again, we use the style attribute to describe how our polygon shape is going to be colored. The style attribute again describes mostly the presentational aspects of the shape, rather than its structure. In this case, we've filled the polygon with blue by using an RGB number (0,0,255) as our fill value, and we've outlined the polygon in black, again by using the RGB number for black (0,0,0) in the stroke value. We've set a thin outline by making the stroke-width value equal to one pixel.

  9. The closing </svg> tag closes the root SVG element and the document. Note that, like XML, all opening tags must have closing tags, and the closing tag of the root element must be the one to close the file.

Changing Values and Code

It is helpful when learning SVG (as with any programming language!) to play around with the code. So open up your text editor and type in the code for the above drawing:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
"http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
<svg width="500" height="500">
<path d="M114.286 260 L320 222.857" transform="translate(0 0)"
style="fill:none;stroke:rgb(171,17,17);stroke-width:5"/>
<polygon points="211.429,148.571 208.571,400 325.714,97.1429"
style="fill:rgb(0,0,255);stroke:rgb(0,0,0);stroke-width:1"/>
</svg>

Save the drawing to your hard drive as triangle.svg or something like that, and remember to make sure to save the file with the .svg extension; otherwise, it will save as a text file. View the file in your browser. We saved the file as triangle.svg.

NOTE

Make sure you have the Adobe SVG viewer downloaded and installed before you try to see your SVG files. Otherwise, they'll just show up as code!

Let's have some fun.

Challenge 1

1.1

First, save your work under a slightly different name, for example, triangleTest.svg, in case you make a mistake in coding and can't get back to the original code.

1.2

In the new file, try changing the coordinates of the path element (the red line) to <path d="M120 100 L400 300". Save the file and view it again. See how the red line has changed?

1.3

Now take a look at the style attribute for our path element. Change the stroke value of the path element (line 6 of code) to style="fill:none; stroke:rgb(17,171,17). Again, save the file and take a look in a browser.

1.4

Now, let's set the path's stroke-width value to 20. Save the file, and take a look.


You should see something similar to the screenshot in Figure 1–10.

Figure xxxFIGURE 1–10 Original triangle.svg file, modified by changing values for the <path> element.


Try altering the code for the polygon element by changing coordinates or by changing style attribute values, such as fill color and stroke color. Remember to save your code and view it in a browser window to see whether the effect you get is the one you intended. You can learn a lot about how basic SVG operates this way!

In this chapter, you've learned to:

  • Download the SVG viewer

  • View an SVG document

  • Recognize an XML file

  • Create both a well-formed and a valid XML file using a text editor

  • Create and save a simple SVG file using a text editor

  • Edit your SVG file and view it in a browser window

  • Change some SVG attribute values and view the results

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