MediaCodec 介绍

MediaCodec 是 Android 提供的用于对音视频进行编解码的类，它通过访问底层的 codec 来实现编解码的功能，是 Android media 基础框架的一部分，通常和 MediaExtractor, MediaSync, MediaMuxer, MediaCrypto, MediaDrm, Image, Surface和AudioTrack 一起使用。

详细描述可参见官方文档：https://developer.android.com/reference/android/media/MediaCodec.html

AMediaCodec 是 MediaCodec 的 native 接口，Google 从 Android 5.0 开始提供，Native 代码编译时需要引入 mediandk 库，官方 demo :

https://github.com/android/ndk-samples/tree/main/native-codec

FFmpeg + ANativeCodec

在 Android 没有在 Native 层开放 ModecCodec 接口之前，FFmpeg 实现硬解码需要将视频和音频数据拷贝到 Java 层，在 Java 层调用 MediaCodec (通过 JNI 调用 Java 对象方法)。

本文将实现 FFmpeg 和 AMediaCodec 结合使用， FFmpeg 负责解复用和音频解码，MediaCodec 负责视频解码并输出到 Surface（ANativeWindow）对象，其中解复用、音频解码、视频解码分别在一个子线程进行，利用队列管理音视频数据包。

注意：本 demo 处理的视频是 H.264 编码，由于 AVPacket data 不是标准的 NALU ，需要利用 av_bitstream_filter_filter 将 AVPacket 前 4 个字节替换为 0x00000001 得到标准的 NALU 数据，这样保证 MediaCodec 解码正确。

配置 AMediaCodec 对象，只解码视频流：

        m_MediaExtractor = AMediaExtractor_new();
        media_status_t err = AMediaExtractor_setDataSourceFd(m_MediaExtractor, fd,static_cast<off64_t>(outStart),static_cast<off64_t>(outLen));
        close(fd);
        if (err != AMEDIA_OK) {
            result = -1;
            LOGCATE("HWCodecPlayer::InitDecoder AMediaExtractor_setDataSourceFd fail. err=%d", err);
            break;
        }

        int numTracks = AMediaExtractor_getTrackCount(m_MediaExtractor);

        LOGCATE("HWCodecPlayer::InitDecoder AMediaExtractor_getTrackCount %d tracks", numTracks);
        for (int i = 0; i < numTracks; i++) {
            AMediaFormat *format = AMediaExtractor_getTrackFormat(m_MediaExtractor, i);
            const char *s = AMediaFormat_toString(format);
            LOGCATE("HWCodecPlayer::InitDecoder track %d format: %s", i, s);
            const char *mime;
            if (!AMediaFormat_getString(format, AMEDIAFORMAT_KEY_MIME, &mime)) {
                LOGCATE("HWCodecPlayer::InitDecoder no mime type");
                result = -1;
                break;
            } else if (!strncmp(mime, "video/", 6)) {
                // Omitting most error handling for clarity.
                // Production code should check for errors.
                AMediaExtractor_selectTrack(m_MediaExtractor, i);
                m_MediaCodec = AMediaCodec_createDecoderByType(mime);
                AMediaCodec_configure(m_MediaCodec, format, m_ANativeWindow, NULL, 0);
                AMediaCodec_start(m_MediaCodec);
            }
            AMediaFormat_delete(format);
        }

FFmpeg 在一个线程中解复用，分别将音频和视频的编码数据包放入 2 个队列。

int HWCodecPlayer::DoMuxLoop() {
    LOGCATE("HWCodecPlayer::DoMuxLoop start");

    int result = 0;
    AVPacket avPacket = {0};
    for(;;) {
        double passTimes = 0;

		......

        if(m_SeekPosition >= 0) { // seek 操作
            //seek to frame
            LOGCATE("HWCodecPlayer::DoMuxLoop seeking m_SeekPosition=%f", m_SeekPosition);
			......
        }

        result = av_read_frame(m_AVFormatContext, &avPacket);
        if(result >= 0) {
            double bufferDuration = m_VideoPacketQueue->GetDuration() * av_q2d(m_VideoTimeBase);
            LOGCATE("HWCodecPlayer::DoMuxLoop bufferDuration=%lfs", bufferDuration);
            //防止缓冲数据包过多
            while (BUFF_MAX_VIDEO_DURATION < bufferDuration && m_PlayerState == PLAYER_STATE_PLAYING && m_SeekPosition < 0) {
                bufferDuration = m_VideoPacketQueue->GetDuration() * av_q2d(m_VideoTimeBase);
                usleep(10 * 1000);
            }

			//音频和视频的编码数据包分别放入 2 个队列。
            if(avPacket.stream_index == m_VideoStreamIdx) {
                m_VideoPacketQueue->PushPacket(&avPacket);
            } else if(avPacket.stream_index == m_AudioStreamIdx) {
                m_AudioPacketQueue->PushPacket(&avPacket);
            } else {
                av_packet_unref(&avPacket);
            }
        } else {
            //解复用结束，暂停解码器
            std::unique_lock<std::mutex> lock(m_Mutex);
            m_PlayerState = PLAYER_STATE_PAUSE;
        }
    }
    LOGCATE("HWCodecPlayer::DoMuxLoop end");
    return 0;
}

视频解码线程中，Native 使用 AMediaCodec 对视频进行解码，从视频的 AVPacket 队列中取包进行解码。

void HWCodecPlayer::VideoDecodeThreadProc(HWCodecPlayer *player) {
    LOGCATE("HWCodecPlayer::VideoDecodeThreadProc start");
    AVPacketQueue* videoPacketQueue = player->m_VideoPacketQueue;
    AMediaCodec* videoCodec = player->m_MediaCodec;
    AVPacket *packet = av_packet_alloc();
    for(;;) {

		....
  
        ssize_t bufIdx = -1;
        bufIdx = AMediaCodec_dequeueInputBuffer(videoCodec, 0);
        if (bufIdx >= 0) {
            size_t bufSize;
            auto buf = AMediaCodec_getInputBuffer(videoCodec, bufIdx, &bufSize);
            av_bitstream_filter_filter(player->m_Bsfc, player->m_VideoCodecCtx, NULL, &packet->data, &packet->size, packet->data, packet->size,
                                       packet->flags & AV_PKT_FLAG_KEY);
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc 0x%02X 0x%02X 0x%02X 0x%02X \n",packet->data[0],packet->data[1],packet->data[2],packet->data[3]);
            memcpy(buf, packet->data, packet->size);
            AMediaCodec_queueInputBuffer(videoCodec, bufIdx, 0, packet->size, packet->pts, 0);
        }
        av_packet_unref(packet);
        AMediaCodecBufferInfo info;
        auto status = AMediaCodec_dequeueOutputBuffer(videoCodec, &info, 1000);
        LOGCATI("HWCodecPlayer::VideoDecodeThreadProc status: %d\n", status);
        uint8_t* buffer;
        if (status >= 0) {
            SyncClock* videoClock = &player->m_VideoClock;
            double presentationNano = info.presentationTimeUs * av_q2d(player->m_VideoTimeBase) * 1000;
            videoClock->SetClock(presentationNano, GetSysCurrentTime());
            player->AVSync();//音视频同步

            size_t size;
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc sync video curPts = %lf", presentationNano);
            buffer = AMediaCodec_getOutputBuffer(videoCodec, status, &size);
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc buffer: %p, buffer size: %d", buffer, size);
            AMediaCodec_releaseOutputBuffer(videoCodec, status, info.size != 0);
        } else if (status == AMEDIACODEC_INFO_OUTPUT_BUFFERS_CHANGED) {
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc output buffers changed");
        } else if (status == AMEDIACODEC_INFO_OUTPUT_FORMAT_CHANGED) {
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc output format changed");
        } else if (status == AMEDIACODEC_INFO_TRY_AGAIN_LATER) {
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc no output buffer right now");
        } else {
            LOGCATI("HWCodecPlayer::VideoDecodeThreadProc unexpected info code: %zd", status);
        }
        if(isLocked) lock.unlock();
    }

    if(packet != nullptr) {
        av_packet_free(&packet);
        packet = nullptr;
    }
    LOGCATE("HWCodecPlayer::VideoDecodeThreadProc end");
}

视频向音频同步，delay 时间根据实际帧率与目标帧率的时间差进行微调。

void HWCodecPlayer::AVSync() {
    LOGCATE("HWCodecPlayer::AVSync");
    double delay = m_VideoClock.curPts - m_VideoClock.lastPts;
    int tickFrame = 1000 * m_FrameRate.den / m_FrameRate.num;
    LOGCATE("HWCodecPlayer::AVSync tickFrame=%dms", tickFrame);
    if(delay <= 0 || delay > VIDEO_FRAME_MAX_DELAY) {
        delay = tickFrame;
    }
    double refClock = m_AudioClock.GetClock();// 视频向音频同步
    double avDiff = m_VideoClock.lastPts - refClock;
    m_VideoClock.lastPts = m_VideoClock.curPts;
    double syncThreshold = FFMAX(AV_SYNC_THRESHOLD_MIN, FFMIN(AV_SYNC_THRESHOLD_MAX, delay));
    LOGCATE("HWCodecPlayer::AVSync refClock=%lf, delay=%lf, avDiff=%lf, syncThreshold=%lf", refClock, delay, avDiff, syncThreshold);
    if(avDiff <= -syncThreshold) { //视频比音频慢
        delay = FFMAX(0,  delay + avDiff);
    }
    else if(avDiff >= syncThreshold && delay > AV_SYNC_FRAMEDUP_THRESHOLD) { //视频比音频快太多
        delay = delay + avDiff;
    }
    else if(avDiff >= syncThreshold)
        delay = 2 * delay;

    LOGCATE("HWCodecPlayer::AVSync avDiff=%lf, delay=%lf", avDiff, delay);

    double tickCur = GetSysCurrentTime();
    double tickDiff =  tickCur - m_VideoClock.frameTimer;//两帧实际的时间间隔
    m_VideoClock.frameTimer = tickCur;

    if(tickDiff - tickFrame >  5) delay-=5;//微调delay时间
    if(tickDiff - tickFrame < -5) delay+=5;

    LOGCATE("HWCodecPlayer::AVSync delay=%lf, tickDiff=%lf", delay, tickDiff);
    if(delay > 0) {
        usleep(1000 * delay);
    }
}

实现代码路径：

https://github.com/githubhaohao/LearnFFmpeg

-- END --

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Android FFmpeg + MediaCodec 实现视频硬解码

MediaCodec 介绍

FFmpeg + ANativeCodec