Android MediaCodec 向后查找

我正在尝试使用以下方法实现视频的精确搜索MediaCodec and MediaExtractor。通过关注 Grafika 的电影播放器 https://github.com/google/grafika/blob/master/app/src/main/java/com/android/grafika/MoviePlayer.java，我已经成功地实现了前瞻性。但是我仍然在向后查找方面遇到问题。相关代码在这里：

public void seekBackward(long position){
    final int TIMEOUT_USEC = 10000;
    int inputChunk = 0;
    long firstInputTimeNsec = -1;

    boolean outputDone = false;
    boolean inputDone = false;

    mExtractor.seekTo(position, MediaExtractor.SEEK_TO_PREVIOUS_SYNC);
    Log.d("TEST_MEDIA", "sampleTime: " + mExtractor.getSampleTime()/1000 + " -- position: " + position/1000 + " ----- BACKWARD");

    while (mExtractor.getSampleTime() < position && position >= 0) {

        if (VERBOSE) Log.d(TAG, "loop");
        if (mIsStopRequested) {
            Log.d(TAG, "Stop requested");
            return;
        }

        // Feed more data to the decoder.
        if (!inputDone) {
            int inputBufIndex = mDecoder.dequeueInputBuffer(TIMEOUT_USEC);
            if (inputBufIndex >= 0) {
                if (firstInputTimeNsec == -1) {
                    firstInputTimeNsec = System.nanoTime();
                }
                ByteBuffer inputBuf = mDecoderInputBuffers[inputBufIndex];
                // Read the sample data into the ByteBuffer.  This neither respects nor
                // updates inputBuf's position, limit, etc.
                int chunkSize = mExtractor.readSampleData(inputBuf, 0);
                if (chunkSize < 0) {
                    // End of stream -- send empty frame with EOS flag set.
                    mDecoder.queueInputBuffer(inputBufIndex, 0, 0, 0L,
                            MediaCodec.BUFFER_FLAG_END_OF_STREAM);
                    inputDone = true;
                    if (VERBOSE) Log.d(TAG, "sent input EOS");
                } else {
                    if (mExtractor.getSampleTrackIndex() != mTrackIndex) {
                        Log.w(TAG, "WEIRD: got sample from track " +
                                mExtractor.getSampleTrackIndex() + ", expected " + mTrackIndex);
                    }
                    long presentationTimeUs = mExtractor.getSampleTime();
                    mDecoder.queueInputBuffer(inputBufIndex, 0, chunkSize,
                            presentationTimeUs, 0 /*flags*/);
                    if (VERBOSE) {
                        Log.d(TAG, "submitted frame " + inputChunk + " to dec, size=" + chunkSize);
                    }
                    inputChunk++;
                    mExtractor.advance();
                }
            } else {
                if (VERBOSE) Log.d(TAG, "input buffer not available");
            }
        }

        if (!outputDone) {
            int decoderStatus = mDecoder.dequeueOutputBuffer(mBufferInfo, TIMEOUT_USEC);
            if (decoderStatus == MediaCodec.INFO_TRY_AGAIN_LATER) {
                // no output available yet
                if (VERBOSE) Log.d(TAG, "no output from decoder available");
            } else if (decoderStatus == MediaCodec.INFO_OUTPUT_BUFFERS_CHANGED) {
                // not important for us, since we're using Surface
                if (VERBOSE) Log.d(TAG, "decoder output buffers changed");
            } else if (decoderStatus == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED) {
                MediaFormat newFormat = mDecoder.getOutputFormat();
                if (VERBOSE) Log.d(TAG, "decoder output format changed: " + newFormat);
            } else if (decoderStatus < 0) {
                throw new RuntimeException(
                        "unexpected result from decoder.dequeueOutputBuffer: " +
                                decoderStatus);
            } else { // decoderStatus >= 0
                if (firstInputTimeNsec != 0) {
                    // Log the delay from the first buffer of input to the first buffer
                    // of output.
                    long nowNsec = System.nanoTime();
                    Log.d(TAG, "startup lag " + ((nowNsec-firstInputTimeNsec) / 1000000.0) + " ms");
                    firstInputTimeNsec = 0;
                }
                boolean doLoop = false;
                if (VERBOSE) Log.d(TAG, "surface decoder given buffer " + decoderStatus +
                        " (size=" + mBufferInfo.size + ")");
                if ((mBufferInfo.flags & MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0) {
                    if (VERBOSE) Log.d(TAG, "output EOS");
                    if (mLoop) {
                        doLoop = true;
                    } else {
                        outputDone = true;
                    }
                }

                boolean doRender = (mBufferInfo.size != 0);

                // As soon as we call releaseOutputBuffer, the buffer will be forwarded
                // to SurfaceTexture to convert to a texture.  We can't control when it
                // appears on-screen, but we can manage the pace at which we release
                // the buffers.
                if (doRender && mFrameCallback != null) {
                    mFrameCallback.preRender(mBufferInfo.presentationTimeUs);
                }
                mDecoder.releaseOutputBuffer(decoderStatus, doRender);
                doRender = false;
                if (doRender && mFrameCallback != null) {
                    mFrameCallback.postRender();
                }

                if (doLoop) {
                    Log.d(TAG, "Reached EOS, looping");
                    mExtractor.seekTo(0, MediaExtractor.SEEK_TO_CLOSEST_SYNC);
                    inputDone = false;
                    mDecoder.flush();    // reset decoder state
                    mFrameCallback.loopReset();
                }
            }
        }
    }
}

基本上和MoviePlayer的一样doExtract方法。我只是添加了一个轻微的修改来返回到前一个关键帧，而不是向前解码到我想要的位置。我也关注过fadden的这里的建议 https://stackoverflow.com/a/31322473/1602807但收效甚微。

另一个附带问题，据我了解，ExoPlayer 是建立在MediaCodec那么为什么MoviePlayer的纯实现却可以正常播放iOS录制的视频MediaCodec can't?

好吧，这就是我解决问题的方法，基本上我误解了 fadden 对render旗帜。问题不在于解码，而在于仅显示最接近查找位置的最后一个缓冲区。我是这样做的：

if (Math.abs(position - mExtractor.getSampleTime()) < 10000) {
   mDecoder.releaseOutputBuffer(decoderStatus, true);
} else {
   mDecoder.releaseOutputBuffer(decoderStatus, false);
}

这是一种相当黑客的方法。优雅的方法应该是保存最后一个输出缓冲区并将其显示在while循环，但我真的不知道如何访问输出缓冲区，以便我可以将其保存到临时缓冲区。

EDIT:

这是一种不太黑客的方法。基本上，我们只需要计算关键帧和搜索位置之间的总帧数，然后我们只需要显示距离搜索位置最近的 1 或 2 帧。像这样的事情：

    mExtractor.seekTo(position, MediaExtractor.SEEK_TO_PREVIOUS_SYNC);
    int stopPosition = getStopPosition(mExtractor.getSampleTime(), position);
    int count = 0;

    while (mExtractor.getSampleTime() < position && mExtractor.getSampleTime() != -1 && position >= 0) {
    ....

        if(stopPosition - count < 2) { //just to make sure we will get something (1 frame sooner), see getStopPosition comment
           mDecoder.releaseOutputBuffer(decoderStatus, true);
        }else{
           mDecoder.releaseOutputBuffer(decoderStatus, false);
        }
        count++;
     ...
    }

/**
 * Calculate how many frame in between the key frame and the seeking position
 * so that we can determine how many while loop will be execute, then we can just
 * need to stop the loop 2 or 3 frames sooner to ensure we can get something.
 * */
private int getStopPosition(long start, long end){
    long delta = end - start;
    float framePerMicroSecond = mFPS / 1000000;

    return (int)(delta * framePerMicroSecond);
}