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    <p>Your question is very broad in perspective and hence, I will refer you to corresponding paths to get an understanding of the overall system.</p> <p>First, the overall processing of <code>media files</code> starts from the <code>Gallery</code> application. Though not the entry point, <a href="http://androidxref.com/4.2.2_r1/xref/packages/apps/Gallery2/src/com/android/gallery3d/data/ImageCacheRequest.java#46" rel="nofollow"><code>ImageCacheRequest</code></a> could be a good reference point to start the study, which shows how a cached image is read and rendered onto the screen.</p> <p>Next, to understand how this thumbnail is generated, you will have to refer to <a href="http://androidxref.com/4.2.2_r1/xref/packages/apps/Camera/src/com/android/camera/Thumbnail.java" rel="nofollow"><code>ThumbnailManager.java</code></a> which is invoked from <code>camera</code>, <code>mms</code> etc. </p> <p><code>Thumbnail</code> class internally employs <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/base/media/java/android/media/MediaMetadataRetriever.java" rel="nofollow"><code>MediaMetadataRetriever</code></a> which is the main class for retrieving the thumbnail data.</p> <p><code>MediaMetadataRetriever</code> has a corresponding <code>JNI</code> implementation as shown <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/base/media/jni/android_media_MediaMetadataRetriever.cpp" rel="nofollow">here</a>. The main function of interest is <code>getFrameAtTime</code>.</p> <p>The most common implementation is <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/av/media/libstagefright/StagefrightMetadataRetriever.cpp" rel="nofollow"><code>StagefrightMetadataRetriever</code></a>, which works on a simple principle. First, a <code>MediaExtractor</code> i.e. a parser is created which will <em>sniff</em> out the media-file type and create a corresponding parser entity as shown <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/av/media/libstagefright/StagefrightMetadataRetriever.cpp#321" rel="nofollow">here</a>. Next, the parser will then invoke a codec to decode the corresponding <strong>key frame</strong> at the requested time stamp and provide the image back as can be observed in <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/av/media/libstagefright/StagefrightMetadataRetriever.cpp#extractVideoFrameWithCodecFlags" rel="nofollow"><code>extractVideoFramewithFlags</code></a>. </p> <p>Inside this function, a <a href="http://androidxref.com/4.2.2_r1/xref/frameworks/av/media/libstagefright/StagefrightMetadataRetriever.cpp#156" rel="nofollow">codec is created</a>, frame is decoded, color converted and returned back to the caller which will transfer the same to the higher application.</p> <p>In a nutshell, I feel your player will not come into picture and as long as the corresponding parsers and codecs are registered with the system, the thumbnails will be generated .</p>
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    1. CO+1 for the easy to understand explanation! I also learned that when I analyzed the android source code.
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    2. CO@artsylar. Thanks for the kind words
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