Android视频播放数据读取的流程.doc

Android视频播放数据读取的流程/s/blog_645b74b90101aezn.html这里分析Android4.0.1本地视频数据读取的流程，其他过程类似 当播放条件准备妥当之后，就要循环进行读取视频的原始数据放到MediaBuffer，将MediaBuffer中的数据输送到解码器中解码，解码后的数据放到MediaBuffer中，在将这MediaBuffer中的数据进行render显示。本文主要侧重读取原始数据的流程，主要是代码跟踪，不夹杂个人分析，有些mpeg4的算法不懂。1：onVideoEvent中开始读取数据，具体代码如下：void AwesomePlayer:onVideoEvent() if (!mVideoBuffer) MediaSource:ReadOptions options; if (mSeeking != NO_SEEK) LOGV(seeking to %lld us (%.2f secs), mSeekTimeUs, mSeekTimeUs / 1E6); options.setSeekTo( mSeekTimeUs, mSeeking = SEEK_VIDEO_ONLY ? MediaSource:ReadOptions:SEEK_NEXT_SYNC : MediaSource:ReadOptions:SEEK_CLOSEST_SYNC); for (;) status_t err = mVideoSource-read(&mVideoBuffer, &options); 蓝色为核心代码，如果是正常读取，options为null，否则这个结构体中包含了seek到的时间和seek的模式，用于具体从文件中哪里开始读取，传入的mVideoBuffer引用用来装解码后的数据2：蓝色部分调用的是OMXCodec:read函数，这个函数中核心的代码如下：status_t OMXCodec:read( MediaBuffer *buffer, const ReadOptions *options) status_t err = OK; *buffer = NULL; bool seeking = false; int64_t seekTimeUs; ReadOptions:SeekMode seekMode; if (options & options-getSeekTo(&seekTimeUs, &seekMode) seeking = true; if (seeking) CODEC_LOGV(seeking to %lld us (%.2f secs), seekTimeUs, seekTimeUs / 1E6); CHECK(seekTimeUs = 0); mSeekTimeUs = seekTimeUs; mSeekMode = seekMode; drainInputBuffers(); size_t index = *mFilledBuffers.begin(); / A list of indices into mPortStatuskPortIndexOutput filled with data. mFilledBuffers.erase(mFilledBuffers.begin(); BufferInfo *info = &mPortBufferskPortIndexOutput.editItemAt(index); CHECK_EQ(int)info-mStatus, (int)OWNED_BY_US); info-mStatus = OWNED_BY_CLIENT; info-mMediaBuffer-add_ref(); *buffer = info-mMediaBuffer; return OK;两点：a，drainInputBuffers开始了数据的读取；b，mFilledBuffers从这个队列中读取已经解码后的数据放入到传入的MediaBuffer中，mFilledBuffers队列中的MediaBuffer就是drainInputBuffers中写进去的3：跟到drainInputBuffer中看看bool OMXCodec:drainInputBuffer(BufferInfo *info) CODEC_LOGV(calling emptyBuffer with codec specific data); status_t err = mOMX-emptyBuffer( mNode, info-mBuffer, 0, size, OMX_BUFFERFLAG_ENDOFFRAME | OMX_BUFFERFLAG_CODECCONFIG, 0); CHECK_EQ(err, (status_t)OK); info-mStatus = OWNED_BY_COMPONENT; status_t err; bool signalEOS = false; int64_t timestampUs = 0; size_t offset = 0; int32_t n = 0; for (;) MediaBuffer *srcBuffer; if (mSeekTimeUs = 0) MediaSource:ReadOptions options; options.setSeekTo(mSeekTimeUs, mSeekMode); mSeekTimeUs = -1; mSeekMode = ReadOptions:SEEK_CLOSEST_SYNC; err = mSource-read(&srcBuffer, &options); if (err = OK) int64_t targetTimeUs; if (srcBuffer-meta_data()-findInt64( kKeyTargetTime, &targetTimeUs) & targetTimeUs = 0) CODEC_LOGV(targetTimeUs = %lld us, targetTimeUs); mTargetTimeUs = targetTimeUs; else mTargetTimeUs = -1; CODEC_LOGV(Calling emptyBuffer on buffer %p (length %d), timestamp %lld us (%.2f secs), info-mBuffer, offset, timestampUs, timestampUs / 1E6); err = mOMX-emptyBuffer( mNode, info-mBuffer, 0, offset, flags, timestampUs); info-mStatus = OWNED_BY_COMPONENT; return true;两点：a，调用err = mSource-read(&srcBuffer, &options);从原始文件中读取原始数据，b，往srcBuffer中读取数据前后，都调用omx转移已经读取到该info中的数据，目的是解码，解码后的数据就房子了mFilledBuffers这个队列中；4：针对mpeg4类型的视频，上面的read函数调用的是MPEG4Source的read函数，核心代码如下：status_t MPEG4Source:read( MediaBuffer *out, const ReadOptions *options) *out = NULL; int64_t seekTimeUs; ReadOptions:SeekMode mode; if (options & options-getSeekTo(&seekTimeUs, &mode) if分支是用于有seek的流程 1：首先找到seektime附近的sampleIndex； 2：然后找到sampleIndex附近的关键帧的syncSampleIndex； 3：然后syncSampleIndex找到具体的sampleTime，sampleTime就是目前需要播放到的位置； 4：mSampleTable-getMetaDataForSample调用这个函数找到sampleTime时间的offset和size； 5：有了offset和size之后剩下就是调用mDataSource-readAt(offset, (uint8_t *)mBuffer-data(), size);读取数据放到buffer中去了； uint32_t findFlags = 0; switch (mode) case ReadOptions:SEEK_PREVIOUS_SYNC: findFlags = SampleTable:kFlagBefore; break; case ReadOptions:SEEK_NEXT_SYNC: findFlags = SampleTable:kFlagAfter; break; case ReadOptions:SEEK_CLOSEST_SYNC: case ReadOptions:SEEK_CLOSEST: findFlags = SampleTable:kFlagClosest; break; default: CHECK(!Should not be here.); break; uint32_t sampleIndex; status_t err = mSampleTable-findSampleAtTime( seekTimeUs * mTimescale / 1000000, &sampleIndex, findFlags); uint32_t syncSampleIndex; if (err = OK) err = mSampleTable-findSyncSampleNear( sampleIndex, &syncSampleIndex, findFlags); uint32_t sampleTime; if (err = OK) err = mSampleTable-getMetaDataForSample( sampleIndex, NULL, NULL, &sampleTime); if (mode = ReadOptions:SEEK_CLOSEST) targetSampleTimeUs = (sampleTime * 1000000ll) / mTimescale; mCurrentSampleIndex = syncSampleIndex; off64_t offset; size_t size; uint32_t cts; bool isSyncSample; bool newBuffer = false; if (mBuffer = NULL) newBuffer = true; status_t err = mSampleTable-getMetaDataForSample( mCurrentSampleIndex, &offset, &size, &cts, &isSyncSample); if (err != OK) return err; err = mGroup-acquire_buffer(&mBuffer); if (err != OK) CHECK(mBuffer = NULL); return err; if (!mIsAVC | mWantsNALFragments) if (newBuffer) ssize_t num_bytes_read = mDataSource-readAt(offset, (uint8_t *)mBuffer-data(), size); CHECK(mBuffer != NULL); mBuffer-set_range(0, size); mBuffer-meta_data()-clear(); mBuffer-meta_data()-setInt64( kKeyTime, (int64_t)cts * 1000000) / mTimescale); if (isSyncSample) mBuffer-meta_data()-setInt32(kKeyIsSyncFrame, 1); +mCurrentSampleIndex; if (!mIsAVC) *out = mBuffer; mBuffer = NULL; return OK; / Each NAL unit is split up into its constituent fragments and / each one of them returned in its own buffer. CHECK(mBuffer-range_length() = mNALLengthSize); const uint8_t *src = (const uint8_t *)mBuffer-data() + mBuffer-range_offset(); size_t nal_size = parseNALSize(src); MediaBuffer *clone = mBuffer-clone(); CHECK(clone != NULL); clone-set_range(mBuffer-range_offset() + mNALLengthSize, nal_size); CHECK(mBuffer != NULL); mBuffer-set_range( mBuffer-range_offset() + mNALLengthSize + nal_size, mBuffer-range_length() - mNALLengthSize - nal_size); if (mBuffer-range_length() = 0) mBuffer-release(); mBuffer = NULL; *out = clone; return OK; 蓝色部分为主要的流程5：后续就是开始调用SampleTable.cpp和SampleIterator.cpp这两个类的相关函数解析文件和读取数据，最主要的函数时通过sampleIndex获取offset和size信息了，代码如下：status_t SampleTable:getMetaDataForSample( uint32_t sampleIndex, off64_t *offset, size_t *size, uint32_t *compositionTime, bool *isSyncSample) Mutex:Autolock autoLock(mLock); status_t err; if (err = mSampleIterator-seekTo(sampleIndex) != OK) return err; if (offset) *offset = mSampleIterator-getSampleOffset(); if (size) *size = mSampleIterator-getSampleSize(); if (compositionTime) *compositionTime = mSampleIterator-getSampleTime(); if (isSyncSample) *isSyncSample = false; if (mSyncSampleOffset 0) / Every sample is a sync sample. *isSyncSample = true; else