RealPix: The RP File
February 1, 1999
In my
last article,
I went over how to integrate a RealPix (.RP) file into the SMIL
presentation so I won't go over it again here. Instead I'll talk
about the construction of the RP file.
The RealPix file is defined between two tags. These tags define
the beginning and end of the RealPix file.
<imfl> and </imfl>
The <head/> Tag
After the start <imfl> tag should
be the <head/> tag which contains
information on how the file is displayed. Notice that unlike HTML
the <head> tag does not have a
corresponding </head>. This is
because SMIL is an XML based language and ends in
/>. Let's look at the
code
from the example we used in my last article and examine it line
by line.
<head
width = "160"
height = "120"
start = "0"
duration = "20"
timeformat = "dd:hh:mm:ss.xyz"
bitrate = "12000"
url = "http://www.wdvl.com"/>
The height and width
are obvious, they define the size of the area in which the images
will be displayed.
The start attribute defines how many seconds
after the RP file is loaded that the images should start displaying.
Some confusion can arise here since the time that the RP file is
loaded can also can controlled in the SMIL file. I normally control
the load time in the SMIL file and leave the start time set to zero.
The presentation, with all images, will run for a
duration of 20 seconds. If you enter only
one number it will default to seconds.
The timeformat shows how time will be
presented if you use days:hours:minutes:seconds:decimals. I
normally do all my timing in seconds and avoid minutes and hours.
The bitrate is calculated using the
RealPix Bandwidth Calculator.
Careful planning using this calculator can make for much smoother
streaming and better use of bandwidth. A great guide to calculating
bitrate and using the Calculator can be found
here.
Essentially if you have a RealPix file containing graphics which
have a total size of 200KB and the presentation is two minutes long,
then you need to stream the graphics at 13.3KB/second. A 28.8 modem
allows a realistic data rate of 20KB/sec that should allow successful
streaming of the Real Pix files. However, when you start
combining this with audio that may take up an additional 10KB/second,
then you start running into problems.
The preroll parameter can help to solve these
problems. It allows for a buffering time before the presentation
plays in order to read in some of the data. This parameter is also
defined using seconds. The Calculator will also help you calculate
preroll.
The maxfps parameter, which I also did not
include, is an optional parameter. The best frame rate is
automatically calculated by the Real Player based on the computer's
CPU power. However it is occasionally useful to create lower fps
transitions in order to create a special effect.
To personalize the file you can also include the optional
title, author, and
copyright parameters.
The <image/> Tag:
Once the head tag is defined the next thing
to do is introduce the images. The
<image/> tag defines the images that
will be used in the presentation, gives them a
handle and defines their source.
<image handle="1" name="headline1.jpg"/>
<image handle="2" name="headline2.jpg"/>
<image handle="3" name="headline3.jpg"/>
The handle is used later in the presentation to reference these
files. You can name them anything you wish. I normally give
them numbers so that I know the order in which they appear. The
name parameter simply points to their
source file.
RealPix: The SMIL Graphics Format
RealPix: The SMIL Graphics Format
RealPix: The <fill/> Tag
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