MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Transcoding happens either from one StorageFile to another or one RandomAccessStream to
another, and in each case happens according to a MediaEncodingProfile. To set up a transcoding
operation you call the MediaTranscoder prepareFileTranscodeAsync or prepareStream-
TranscodeAsync method, which returns back a PrepareTranscodeResult object. This represents the
operation that’s ready to go, but it won’t happen until you call that result’s transcodeAsync method. In
JavaScript, each result is a promise, allowing you to provide completed and progress handlers for a
single operation but also allowing you to combine operations with WinJS.Promise.join. This allows
them to be set up and started later, which is useful for batch processing and doing automatic uploads to
a service like YouTube while you’re sleeping! (And at times like these I’ve actually pulled ice packs from
my freezer and placed them under my laptop as a poor-man’s cooling system….)
The Transcoding media sample provides us with a couple of transcoding scenarios. In Scenario 1
(js/presets.js) we can pick a video file, pick a target format, select a transcoding profile, and turn the
machine loose to do the job (with progress being reported), as shown in Figure 10-7.
FIGURE 10-7 The Transcoding media sample cranking away on a video of my then two-year-old son discovering the
joys of a tape measure.
The code that’s executed when you press the Transcode button is as follows (some bits omitted; this
sample happens to use nested promises, which again isn’t recommended for proper error handling
unless you want, as this code would show, to eat any exceptions that occur prior to the transcode-Async
call):
function onTranscode() {
// Create transcode object.
var transcoder = null;
transcoder = new Windows.Media.Transcoding.MediaTranscoder();
// Get transcode profile.
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