Building Web Applications with SVG - Cdn.oreilly.com

The xmlns attribute (which appears not to have been a part of the language originally, because the
Adobe and Opera viewers are unique in not requiring it) is necessary for most browsers to be able to
display the code as SVG.
Essentially, the xmlns attribute merely tells the browser that it will be speaking a new dialect of
XML. This is because most browsers of the 20th century assumed that the only language they would
need to know was HTML. Writing isn’t sufficient to let the browser know this, because the XML
specification requires a namespace. It is rather unfortunate, from the perspective of teachers and
learners, that the computer languages we learn are filled with mysteries that have no apparent purpose
until one becomes a guru. However, you can think of the code xmlns=”http://www.w3.org/2000/
svg” within the element as just that: a mysterious incantation probably placed in the language
to make sure that casual learners know to be on their guard. It turns out that it is not all that important
to understand.
Screen Coordinates
Before beginning the second exercise, in which you’ll begin experimenting with SVG, consider the
drawing space itself—the browser window. Each point within the drawing space (also known as the
Cartesian plane) is identified by a pair of coordinates (x and y). The upper-left corner of the screen
is the point (0,0) and—depending on screen resolution and the current size of the window—the
lower-right corner could have coordinates such as (800,640), (951,651), or (1440,900). The number of
pixels determines the resolution of the screen. The resolution on mobile devices varies considerably;
240×320 pixels is a popular size for smaller and older devices.
CHAPTER 1 SVG Basics 23