Layers
Using the simple coordinate system, you may choose to define regions
that overlap. If so, you also need to indicate the layer order of
all of overlapping regions. If you don't specify a layer order,
the media player default is to display the list of clips from bottom
to top (that is, the one listed first will be on the bottom and
the one listed last will be on top).
The z-index parameter allows you to assign a numerical value to
each region, where 0 is the layer displayed on the "bottom"
and an infinite number of regions can be created on top by assigning
playback regions values such as z-index="1"
or z-index="6".
<smil>
<head>
<layout>
<root-layout width="450" height="450"/>
<region id="image" top="5" left="5"
width="150" height="75" z-index="2"/>
<region id="text" top="50" left="50"
width="100" height="100" z-index="3"/>
</layout>
</head>
<body>
...media clip tags will go here...
</body>
</smil>
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