Introduction
to Synchronized Multimedia Integration Language (SMIL)?
SMIL
known as Synchronized Multimedia Integration Language is used for time
based delivery of multimedia content over web. It is possible to mix different
multimedia elements such as text, video, graphics, audio, and vector based
animation and synchronizes them to a timeline for delivery. SMIL is a
World Wide Web Consortium recommendation.
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Using
SMIL a user can describe the behavior of the presentation, describe the
layout of the presentation and associate the media objects in the presentation
with hyperlinks. Basically SMIL is a XML document with the root element
being the <smil> element. The <smil> element can have child
elements like <head> and <body>. The code given below is a
sample SMIL document which is used to display an image in a layout specified.
<smil>
<head>
<switch>
<layout>
<region id="a" left="50" top="50" />
</layout>
<layout type="text/css">
[region="a"] {top: 50px; left: 50px }
</layout>
</switch>
</head>
<body>
<seq>
<img region="a" src="--give image url here--" dur="10s"
/>
</seq>
</body>
</smil>
The above
code gives you an idea of the structure of the SMIL document. You can
use the <switch> element within the <head> element to specify
a layout for the presentation. The switch element is used to select one
of the alternative layouts given. In the above code both the alternative
point to the same region. The purpose of the switch element is to select
an alternative child element. The start point of the region where the
image is to be displayed is given in the <region> tag with the help
of the attributes like ‘top’ and ‘left’. These attributes
give the ‘x’ and the ‘y’ coordinates in pixels. The
region in which the image is displayed is identified by and ‘id’
attribute. The <body> tag encloses the actual media content that
is to be displayed. The <body> can have child elements like <a>,
<animation>, <audio>, <img>, <par>, <ref>,
<seq>, <switch>, <text>, <textstream>, and <video>.
The <par>
element is one of the synchronization elements that can synchronize different
media content over a timeline. The children of the par element can overlap
in the timeline. Consider the code snippet given below for the par element.
<par>
<audio id="a" begin="6s" src=”audio url here”
.../>
<img begin="id(a)(4s)" src=”image url here” .../>
</par>
The above
code of the par element synchronizes an audio and image together. If you
look at the audio tag you see that the begin attribute has a value of
‘6s’, which means that the audio will start after a delay of
6 seconds. The img tag has a delay of “id(a)(4s)”, which clearly
indicates that the image has to be displayed after 4 seconds from the
start of the audio. This attribute clearly indicates a cumulative delay
of 10 seconds altogether. The elements of the <par> tag can be in
any order and the order of the elements will not affect the timeline and
the way the media content are displayed. Suppose instead of a <par>
element you have a <seq> element as given below:
<seq>
<audio src=”audio url here” .../>
<img begin="4s" src=”image url here” .../>
</seq>
In the above
code you can see that the media content are enclosed in a <seq>
element which clearly states that the audio will be first played and then
the image will be displayed after a delay of 4 seconds, in that order.
Synchronization
of the continuous media elements within the <par> element is implementation
independent. The different media of the par element can be either synchronized
to a common clock or the individual child element may have its own clock.
It is implementation independent. If the elements are bound to a common
clock then it is called Hard Synchronization and if the elements have
their own clocks then it is called Soft Synchronization. The par element
can have child elements like <a>, <animation>, <audio>,
<img>, <par>, <ref>, <seq>, <switch>, <text>,
<textstream>, and <video>. These child elements can appear
multiple times within the par element.
Thus with
the help of SMIL you can integrate different media contents in a presentation
and control the appearance of each of the media content in a timeline.
For more details on the specification recommended by the W3C you can visit
the link http://www.w3.org/TR/REC-smil/.
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