An Introduction to Scalable Vector Graphics
If you're a web designer who's worked with graphics, you may have heard of Scalable Vector Graphics (SVG). You may even have downloaded a plug-in to view SVG files in your browser. The first and most important thing to know about SVG is that it isn't a proprietary format. On the contrary, it's an XML language that describes two-dimensional graphics. SVG is an open standard, proposed by the W3C:
SVG is a language for describing two-dimensional graphics in XML. SVG allows for three types of graphic objects: vector graphic shapes (e.g., paths consisting of straight lines and curves), images and text. Graphical objects can be grouped, styled, transformed and composited into previously rendered objects. The feature set includes nested transformations, clipping paths, alpha masks, filter effects and template objects.
SVG drawings can be interactive and dynamic. Animations can be defined and triggered either declaratively (i.e., by embedding SVG animation elements in SVG content) or via scripting.
This article gives you all the basic information you need to start putting SVG to use. You'll learn enough to be able to make a handbill for a digital camera that's on sale at the fictitious MegaMart. (Any resemblance of this camera to a real product is slightly coincidental.)
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We want the handbill to be the size of a half sheet of U.S. letter
paper. We specify that in the <svg> tag below.
You should always include <title> and
<desc> elements. SVG display programs use the title
to display a tooltip, and the description is useful for search
engines. Additionally your document will be more easily accessible to
visually impaired users.
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN"
"http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="21cm" height="13.5cm">
<title>MegaMall Handbill</title>
<desc>
Handbill for the fictitious MegaMall
</desc>
<!-- graphic specifications go here -->
</svg>
The DOCTYPE declaration above is not final yet because SVG is not a recommendation, although it is expected to gain that status soon.
When specifying width and height, you are
actually establishing a viewport in which your drawing will be
displayed. You may use em, ex,
px, pt, pc, cm,
mm, or in to specify dimensions. If you
don't use a measurement unit, then the numbers are assumed to be
pixels (px).
| Unit | Meaning |
|---|---|
em
| a unit equal to the current font size |
ex |
the x-height, usually the height
of a lower case letter x |
px |
pixels (the default) |
pt |
points (one point = 1/72 inch) |
pc |
picas (one pica = 1/6 inch) |
cm | centimeters |
mm | millimeters |
in | inches |
All numbers used to measure coordinates are assumed to be the same
unit-type used to establish width and height. In our handbill, we'll
set the width="21cm" and height="13.5cm".
In order to avoid having to use decimals for all of the coordinates,
we will establish a viewBox. A viewBox sets up a user
coordinate system which is mapped into the viewport bounds, stretching
or shrinking a graphic if the viewBox and viewport aren't
proportional. This stretching and shrinking happens only if the
preserveAspectRatio is set to none (which is
not the default). In the following specification, we set up a system
with ten "units" per centimeter.
<svg id="body" width="21cm" height="13.5cm"
viewBox="0 0 210 135">
Let's start off by adding the red-bordered rectangle with the light blue interior.
<svg id="body" width="21cm" height="13.5cm"
viewBox="0 0 210 135">
<title>Example 1</title>
<desc>
Rectangle with red border and light blue interior.
</desc>
<rect x="10" y="20" width="150" height="70"
fill="#eeeeff" stroke="red" stroke-width="1" />
</svg>
You specify a rectangle by giving the x and
y coordinates of its upper left corner and its
width and height. When specifying
coordinates, the positive x direction is to the right, and the
positive y direction is downwards.
In this case, we've also set the fill color,
stroke (line) color, and stroke-width as
separate attributes. It's also possible to set all these properties,
in a way similar to CSS, by way of a style attribute, an
internal style sheet, or an external style sheet. You can look at the SVG file for the style attribute
or for the internal style sheet.
Here's the result, shown at half size:
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If you'd like to try this yourself, download the Apache Software Foundation's Batik tool and install it according to its instructions. You will need a Java VM version 1.2 or greater. You can invoke the viewer tool with a simple UNIX command line like
# use path appropriate to where you've installed batik java -jar /usr/local/xml-batik/batik/batik-svgviewer.jar
Let's add the gray drop-shadow rectangle. We want it three units to
the right and below the red rectangle. Rather than do the addition of
the x and y coordinates ourselves, we can
specify that SVG should perform a transform on the
graphic; it should translate the rectangle by three units
in both dimensions. Here's the SVG, followed by the resulting image.
<svg id="body" width="21cm" height="13.5cm"
viewBox="0 0 210 135">
<title>Example 2</title>
<desc>
Rectangle with red border and light blue interior,
with (intended) gray shadow rectangle.
</desc>
<rect x="10" y="20" width="150" height="70"
fill="#eeeeff" stroke="red" stroke-width="1" />
<rect x="10" y="20" width="150" height="70"
transform="translate(3, 3)"
fill="#999999" stroke="#999999" stroke-width="1" />
</svg>
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Not exactly what we had in mind. This demonstrates one of the rules of SVG: if object B is specified after object A in the source file, it appears above object A. We simply reverse the order in which the rectangles appear to get the desired result.
<svg id="body" width="21cm" height="13.5cm"
viewBox="0 0 210 135">
<title>Example 3</title>
<desc>
Rectangle with red border and light blue interior,
with gray shadow rectangle.
</desc>
<rect x="10" y="20" width="150" height="70"
transform="translate(3, 3)"
fill="#999999" stroke="#999999" stroke-width="1" />
<rect x="10" y="20" width="150" height="70"
fill="#eeeeff" stroke="red" stroke-width="1" />
</svg>
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Next we'll put in the green loops above and
below the rectangle. If you look closely, you'll see that it's just
one loop repeated over and over again. By putting the description of
the single loop inside the <defs> element, you
specify that you are defining a graphic for later use, but do not want
it displayed immediately. So you add this immediately after the
</desc>. (Line numbers are shown for reference
only.)
1 <defs> 2 <polyline id="loop" 3 points= 4 1.00, 0.00 0.93, 0.16 5 0.72, 0.26 0.43, 0.25 6 0.13, 0.11 -0.11, -0.13 7 -0.25, -0.43 -0.26, -0.72 8 -0.16, -0.93 0.00, -1.00 9 0.16, -0.93 0.26, -0.72 10 0.25, -0.43 0.11, -0.13 11 -0.13, 0.11 -0.43, 0.25 12 -0.72, 0.26 -0.93, 0.16 13 -1.00, 0.00" 14 transform="scale(5, 5)" 15 stroke="green" stroke-width="0.1" fill="none" /> 16 </defs>
polyline defines a set of connected
straight line segments. The id attribute gives
the object a unique name by which you can refer to it.
points attribute lists the
(x, y) coordinate pairs for each point in the
line segment. These points happen to be a graph of the polar
equation r = 2cos(a) where a ranges
from 0° to 180°, generated by this
Java program. That's why the coordinates all range from -1 to
1 in the x and y directions. You may use commas and/or whitespace
to separate coordinate values.
scale
transformation to multiply coordinates by 5 in both the x
and y directions. Important: all coordinates
for this object are now scaled by a factor of five.
Then, after drawing the rectangles, you <use> the
object that you defined, translating it to the proper place on the
canvas. The <use> element's xlink:href
will be the URI of the resource to use. It produces the result shown
below. See source code
<rect x="10" y="20" width="150" height="70"
fill="#eeeeff" stroke="red" stroke-width="1" />
<use xlink:href="#loop" transform="translate(20,100)"/>
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To make the series of loops below the rectangle, we use the
<g> element, which lets you group graphic objects
together. While you may group together any objects you wish, and may
nest groups to any depth you desire, in this case we'll just group
together multiple <use>s of the loop
we defined earlier. Inside the <defs> element, add
<g id="multiloop">
<use xlink:href="#loop"/>
<use xlink:href="#loop" transform="translate(10, 0)"/>
<use xlink:href="#loop" transform="translate(20, 0)"/>
<use xlink:href="#loop" transform="translate(30, 0)"/>
<use xlink:href="#loop" transform="translate(40, 0)"/>
<use xlink:href="#loop" transform="translate(50, 0)"/>
<use xlink:href="#loop" transform="translate(60, 0)"/>
<use xlink:href="#loop" transform="translate(70, 0)"/>
<use xlink:href="#loop" transform="translate(80, 0)"/>
<use xlink:href="#loop" transform="translate(90, 0)"/>
<use xlink:href="#loop" transform="translate(100, 0)"/>
<use xlink:href="#loop" transform="translate(110, 0)"/>
<use xlink:href="#loop" transform="translate(120, 0)"/>
</g>
Then, after the rectangle, replace the old <use>
with one that refers to multiloop, producing the result
shown below. See source code
<use xlink:href="#multiloop" transform="translate(20, 100)"/>
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To put the upside-down loops at the top of the page, we'll need
to use the rotate transform, which comes in two versions.
rotate(angle)rotate(angle, cx, cy)
In many cases it's more convenient to use the second form of
rotate, especially if an object is not already centered
around the origin, as the single loop is. In the case of the
multiloop, its center point is (60,0). See the entire source.
<!-- loops at bottom -->
<use xlink:href="#multiloop" transform="translate(20, 100)"/>
<!-- loops at top -->
<use xlink:href="#multiloop"
transform="translate(20, 10) rotate(180, 60, 0)"/>
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Note that the angle specified in a rotate transformation
is measured in degrees, with positive numbers indicating a clockwise
direction; 0 degrees is east, 90 degrees is south, etc.
Adding the circular bullets to the handbill is easy; we use the
<circle> element, which has these atributes:
cx, the center x coordinate; cy, the
center y coordinate; and r, the radius. We'll add
the circle to the <defs> section. There's no
scaling or color information; that leaves us free to change the color
as we like.
<circle id="bullet" cx="0" cy="0" r="1.5"/>
The other SVG shapes such as
<rect>, <polyline>, and
<polygon> are shorthand for a more generic
path object. A path represents the outline of a shape which can
be stroked (have its outline drawn) or filled. We could represent the
star-shaped bullet as a <polygon>, but instead,
we'll represent it as a path.
The <path> element's main attribute is the
d attribute. The d stands for the path
data. Path data consists of path command letters and
coordinate information. Some of the more important path command
letters are listed in the following table.
| Letter | Meaning |
|---|---|
M |
move to the following x,y coordinate |
L |
Draw a line to the following x,y coordinate |
A |
Draw an elliptical arc using the information that describes the x and y radius, x-axis rotation, direction and size of the arc's sweep, and the ending x, y coordinates of the arc. |
Z |
Close the path; that is, return to the most recent Moveto point. |
SVG provides several shortcuts to minimize the amount of space needed to describe path. When you have a repeated command letter (for example, a series of lines or arcs), you don't have to repeat the letter. You also can eliminate any unnecessary whitespace. Thus, the following three paths describe the same parallelogram.
<path d="M 5,3 L 10,3 L 7,6 L 2,6 Z"/> <path d="M 5,3 L 10,3 7,6 2,6 Z"/> <path d="M5 3L10 3 7 6 2 6Z"/>
Upper-case letters indicate absolute coordinates; lower-case letters indicate coordinates relative to the previous coordinates.
Here's the non-minimized path for the star-shaped bullet.
<path id="star"
d=
"M -0.951,-0.309
L 0.951,-0.309
-0.588, 0.809
0.000,-1.000
0.588, 0.809
Z"
transform="scale(3,3)" stroke="none" />
And here are the references to them in the main SVG, along with the result. See the entire source.
<use xlink:href="#bullet" fill="#990000" transform="translate(5, 120)"/> <use xlink:href="#star" fill="#009900" transform="translate(10, 120)"/> <use xlink:href="#star" fill="#990099" transform="translate(180, 120)"/> <use xlink:href="#bullet" fill="#000099" transform="translate(185, 120)"/>
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SVG also provides command letters for cubic and quadratic Bézier curves, but they are beyond the scope of this article. The path for the camera (which you may see in the source) uses quadratic curves. Here's the result of the camera's path specifications:
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For full information about paths, see the SVG specification.
The <text> element is a container element; the
text you wish to display is between the opening and closing tags.
The most important attributes are the x and y
location for the text, the font-family,
font-size, and fill color.
If you need to change the attributes of a word or two somewhere in
your text, you should use the <tspan> element
rather than splitting your text into multiple
<text> elements. The
camera body's text is grouped so that we don't have to repeat the
font-family and font-size on each text
element.
<text font-family="sans-serif" font-size="10pt" fill="black"
x="18" y="123">Cameras cost
<tspan fill="#990000" font-style="italic"
text-decoration="underline">less</tspan>
at <tspan font-family="serif">MegaMart</tspan>!
</text>
<g font-family="sans-serif" font-size="6pt" fill="black">
<text x="54" y="59">
<tspan fill="red">S</tspan><tspan
fill="green">V</tspan><tspan fill="blue">G</tspan>
</text>
<text x="53" y="66">Cam</text>
</g>
We'll also put some scaled, rotated text at the right side of the page. See the source.
<text font-family="serif" font-size="12pt" fill="black"
x="0" y="0"
transform="rotate(-90) translate(-100, 180) scale(1.5, 1)">
MegaMart
</text>
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The final task is to place the yellow glow behind the
rotated text by using the SVG <filter> element.
The filter element allows you to apply one or more filters,
or graphic operations, to a graphic object. Unlike transforms,
which change an object's size, shape, and orientation, filters
usually affect an object's visual aspect. SVG has, among others,
filters that blur an image, change its color saturation or
characteristics, add or overlap it with another object,
produce a beveled or embossed effect, and lighting effects.
Because we'll be using the MegaMart words twice, we'll put
it into the <defs> section:
<text id="megaText" fill="black" x="0" y="0"
font-family="serif" font-size="12pt">
MegaMart
</text>
To get the yellow glow from black text, you have to blur the image and change the resulting grayscale to yellow. It looks like this, with line numbers added for reference.
1 <filter id="glowShadowFilter"> 2 <feGaussianBlur stdDeviation="0.7 0.7"/> 3 <feColorMatrix 4 type="matrix" 5 values= 6 "1 0 0 0 0 7 0 1 0 0 0 8 0 0 0 0 0 9 0 0 0 1 0"/> 10 <feOffset dx="2" dy="2"/> 11 </filter>
dx and
dy distances. This would usually be in the middle of
a filter sequence; here we use it to avoid having to do a
transform=translate() later on.
Now, we group both text items, and filter the first one, which must appear before the second one so that it shows up underneath. Here's the SVG, and the final product. See the source.
<g transform="rotate(-90) translate(-100,180) scale(1.5,1)">
<use xlink:href="#megaText" fill="whitw"
filter="url(#glowShadowFilter)"/>
<use xlink:href="#megaText"/>
</g>
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In this brief introduction, you've learned how to
If you wish to learn more about the many things SVG can do, read the SVG specification.
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