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You use the same BitmapPropertySet for any properties you might pass to an encoder’s bitmap-
Properties.setPropertiesAsync call. Here’s we’re just using the same mechanism for encoder options.
As for custom codecs, this simply means that the first argument to BitmapEncoder.createAsync (as
well as BitmapDecoder.createAsync) is the GUID (a class identifier or CLSID) for that codec, the
implementation of which must be provided by a DLL. Details on how to write one of these is provided in
How to Write a WIC-Enabled Codec. The catch is that including custom image codecs in your package is
not presently supported. If the codec is already on the system (that is, installed via the desktop), it will
work. However, the Windows Store policies do not allow apps to be dependent on other apps, so it’s
unlikely that you can even ship such an app unless it’s preinstalled on some specific OEM device and the
DLL is part of the system image. (An app written in C++ can do more here, but that’s beyond the scope
of this book.)
In short, for apps written in JavaScript and HTML, you’re really limited, for all practical purposes, to
image formats that are inherently supported in the system.
Do note that these restrictions do not exist for custom audio and video codecs. The Media extensions
sample shows how to do this with a custom video codec, as we’ll see in the next section.
Manipulating Audio and Video
As with images, if all we want to do is load the contents of a StorageFile into an audio or video
element, we can just pass that StorageFile to URL.createObjectUrl and assign the result to a src
attribute. Similarly, if we want to get at the raw data, we can just use the StorageFile.openAsync or
openReadAsync methods to obtain a file stream.
To be honest, opening the file is probably the farthest you’d ever go in JavaScript with raw audio or
video, if even that. While chewing on an image is a marginally acceptable process in the JavaScript
environment, churning on audio and especially video is really best done in a highly performant C++
DLL. In fact, many third-party, platform-neutral C/C++ libraries for such manipulations are readily
available that you should be able to directly incorporate into such a DLL. In this case you might as well
just let the DLL open the file itself!
That said, WinRT does provide for transcoding (converting) between different media formats and
provides an extensibility model for custom codecs, effects, and scheme handlers. In fact, we’ve already
seen how to apply custom video effects through the Media extensions sample, and the same DLLs can
also be used within an encoding process, where all that the JavaScript code really does is glue the right
components together (which it’s very good at doing). Let’s see how this works with transcoding video
first and then with custom codecs.
Transcoding
Transcoding both audio and video is accomplished through the Windows.Media.Transcoding.-
MediaTranscoder class, which supports output formats of mp3 and wma for audio, and mp4, wmv, and
m4a for video. The transcoding process also allows you to apply effects and to trim start and end times.
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