thumbnail的处理流程和读取缩略图thumbnail所在sample的index的方法

取得video thumbnail的处理流程如下图所示，取得image的thumbnail的处理流程类似，只是取得image thumbnail的函数变成了ThumbnailUtils.createImageThumbnail。

Thumbnail的返回类型是Bitmap。

寻找缩略图所在sample的index的方法在SampleTable::findThumbnailSample函数中，在要寻找的前numSamplesToScan个samples中，寻找size最大的sample作为缩略图的sample index。

status_t SampleTable::findThumbnailSample(uint32_t *sample_index) { Mutex::Autolock autoLock(mLock); if (mSyncSampleOffset < 0) { // All samples are sync-samples. *sample_index = 0; return OK; } uint32_t bestSampleIndex = 0; size_t maxSampleSize = 0; static const size_t kMaxNumSyncSamplesToScan = 20; // 最大查找20个samples // Consider the first kMaxNumSyncSamplesToScan sync samples and // pick the one with the largest (compressed) size as the thumbnail. size_t numSamplesToScan = mNumSyncSamples; if (numSamplesToScan > kMaxNumSyncSamplesToScan) { numSamplesToScan = kMaxNumSyncSamplesToScan; } for (size_t i = 0; i < numSamplesToScan; ++i) { uint32_t x = mSyncSamples[i]; // Now x is a sample index. size_t sampleSize; status_t err = getSampleSize_l(x, &sampleSize); if (err != OK) { return err; } if (i == 0 || sampleSize > maxSampleSize) { // 寻找size最大的一个sample当做thumbnail的sample bestSampleIndex = x; maxSampleSize = sampleSize; } } *sample_index = bestSampleIndex; return OK; }