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Capturing and Synchronizing Slides with WMV |
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Introduction
This page describes a methodology and a set of custom tools used for capturing
high-res slide images from a lecture, and preparing them for playback,
synchronized with a Windows Media video.
The basic elements described are:
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Capturing high resolution images from a VGA source
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Encoding WMV with embedded timing information
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Post-processing, including slide image filtering and automated synchronization
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Synchronizing system time between the capture and encoding systems
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Basic HTML/CGI support for embedded video and slides.
Capturing Images from VGA Source
The image capture was done using the
Epiphan
VGA2USB. This is an inexpensive USB2 framegrabber. We installed the
Epiphan driver and a simple capture application on a image capture computer
system, plugged the USB2 interface from the framegrabber to the image capture
system, and plugged the VGA signal from the lecturer laptop into the
framegrabber VGA interface. We used an old PIII Windows laptop for the
image capture system. The capture application was configured to capture
one image every 3 seconds from the lecturer's laptop. The time of the
capture of each file was preserved in the Windows File Modification Time.
The images were compressed to jpg format automatically by the Epiphan
software. To make the modification time meaningful, it was important that
the system time on the image capture system was synchronized with the Windows
Media Encoder system (see below).
Encoding WMV with Timing Information
To permit the slides and the video to be synchronized automatically, we
implemented a custom tool to insert timing information into one of our Windows
Media archives. One goal in the collection of this timing information was
that it should persist even after the archive file had been cropped to remove
the dead time at the beginning and end. The approach taken was to insert
a custom Windows Media script command containing the system time into the WMV
at a regular interval. Provided that the system clock on the encoder
system is correct, this approach allows us to automatically synchronize any
portion of the resulting WMV with the relevant slides.
The tool used for this process is here:
This is a Windows console application. To install it, simply copy the
Debug or Release directory containing the exe to the target system. The
single required argument is the WME file, defining the desired encoding
properties. The WME file must include provisions for a script stream so
that the tool can insert the SYNC script commands. When it runs, the tool
runs the encoder and loads the specified WME file. If credentials are
required for connection to a server using the Push method, the tool will prompt
for them.
After starting the encoder, click the encode button on the WME interface as
normal. While the encoder is encoding, the tool will report when a SYNC
script command has been successfully inserted. To quit the encoder, press
'enter' at the command prompt.
Post-processing
The first step in the post processing is to remove the dead time from the
beginning and end of the WMV file. This may be done with the Windows
Media File Editor, among other tools.
To complete the post processing, we created a custom Windows application the
basic functions of which are:
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Parse a WMV, and using the SYNC script commands, determine its start time and
duration.
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Parse a directory of slide image files. Remove those files which do not
fall between the WMV start and end time, and remove duplicate and near
duplicate files.
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Insert URL script commands into a copy of the WMV, one for each of the set of
filtered slides. These represent slide transitions.
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Upload the resulting slide set to a web server, and the resulting WMV to a
Windows Media server.
The post-processing tool is available here:
To use this tool, first crop and name the WMV as desired. Put the WMV in
a directory with a subdirectory containing the raw slide captures. There
should be no other files or directories in the directory. Specifiy this
as the input directory in the SyncSlides tool's UI. Fill in the other
desired fields on the tool's UI, and click the Process button.
Time Synchronization
In a Windows domain environment, a simple way to synchronize clocks is to use
'net time'. Our arrangement of systems included several networks with no
domain which caused some problems with the use of 'net time'. Instead, we
used a custom time sync tool which used the daytime protocol to synchronize
clocks.
This is a Windows console application. To install, simply copy the Debug
or Release directory containing the executable to the target system. It
requires one or more arguments. The arguements are names or IP addresses
of servers supporting the Daytime protocol. The tool will progressively
try each until one succeeds. A list of DayTime servers was supplied by
http://www.nist.gov.
Basic HTML/CGI Support for Embedded Video and Slides
Our goal in designing HTML/CGI support was to provide one simple solution which
would function on multiple browsers. The basics of embedding Windows
Media Player for IE and Firefox browsers is well described elsewhere, for
example the Windows Media Player SDK, and will not be discussed here. A
key observation is that the simple cross-browser solution can be created using
the DefaultFrame parameter on the Windows Media Player embedded object.
The frame identified as the default frame will automatically be the target of
all URL script commands embedded in the Windows Media stream with no additional
scripting. One drawback with this approach is that one must use a
frameset which imposes many limitations on page layout.
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Computer Science & Engineering
University of Washington
Box 352350
Seattle, WA 98195-2350
(206) 543-1695 voice, (206) 543-2969 FAX
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