关于ffmpeg硬解码，其实就是不使用ffmpeg自身的解码器，而是从系统查找硬解码器，在Android上就是通过反射调用系统的解码器中间件MediaCodec。网络上关于ffmpeg硬解码的文章很多，ffmpeg官方demo里面也有很详细的写法。但是这些公开的资料，大部分都是针对播放视频文件（AVCC），极少针对AnnexB视频流的硬解码。

众所周知，Android中使用MediaCodec硬解码前，需要先通过MediaFormat设置视频流的参数，之后才能正确开启解码器解码。ffmpeg中，如果是从视频文件硬解码，ffmpeg可以自动从视频文件头读取包含视频的sps、pps等信息等的extradata，用于设置AVCodecContext->extradata和AVCodecContext->extradata_size，从而正确打开硬件解码器avcodec_open2，AVCodecContext->extradata中存储的就是sps、pps、width、height等MediaFormat中存储的信息。但是如果是AnnexB的视频流，没有文件头来读取视频参数来设置extradata，就会导致avcodec_open2失败，相当于Java中使用MediaCodec前，没有通过MediaFormat设置参数。

1. 硬解码

AnnexB视频流虽然没有文件头，但是每一帧有包含sps和pps信息的nalu，可以在打开解码器前，先通过av_read_frame拆出一个帧的AVPacket，解出AVPacket中的extradata，填充进AVCodecContext，来实现成功avcodec_open2。

int extract_extradata(AVCodecContext *pCodecCtx, AVPacket *packet, uint8_t **extradata_dest, int *extradata_size_dest)
{
    const AVBitStreamFilter *bsf;
    int ret;
    if( (bsf = av_bsf_get_by_name("extract_extradata")) == NULL)
    {
        LOGD("failed to get extract_extradata bsf\n");
        return 0;
    }
    printf("\nfound bsf\n");

    AVBSFContext *bsf_context;
    if( (ret=av_bsf_alloc(bsf, &bsf_context) ) < 0)
    {
        LOGD("failed to alloc bsf contextx\n");
        return 0;
    }

    printf("alloced bsf context\n");

    if( (ret=avcodec_parameters_from_context(bsf_context->par_in, pCodecCtx) ) < 0)
    {
        LOGD("failed to copy parameters from contextx\n");
        av_bsf_free(&bsf_context);
        return 0;
    }

    printf("copied bsf params\n");

    if( (ret = av_bsf_init(bsf_context)) < 0 )
    {
        LOGD("failed to init bsf contextx\n");
        av_bsf_free(&bsf_context);
        return 0;
    }

    printf("initialized bsf context\n");

    AVPacket *packet_ref = av_packet_alloc();
    if(av_packet_ref(packet_ref, packet) < 0 )
    {
        LOGD("failed to ref packet\n");
        av_bsf_free(&bsf_context);
        return 0;
    }

    //make sure refs are used corectly
    //this probably resests packet
    if((ret = av_bsf_send_packet(bsf_context, packet_ref)) < 0)
    {
        LOGD("failed to send packet to bsf\n");
        av_packet_unref(packet_ref);
        av_bsf_free(&bsf_context);
        return 0;
    }

    printf("sent packet to bsf\n");

    int done=0;

    while (ret >= 0 && !done) //!h->decoder_ctx->extradata)
    {
        int extradata_size;
        uint8_t *extradata;

        ret = av_bsf_receive_packet(bsf_context, packet_ref);
        if (ret < 0)
        {
            if (ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
            {
                LOGD("bsf error, not eagain or eof\n");
                return 0;
            }
            continue;
        }

        extradata = av_packet_get_side_data(packet_ref, AV_PKT_DATA_NEW_EXTRADATA, &extradata_size);

        if (extradata)
        {
            LOGD("got extradata, %d size!\n", extradata_size);
            done=1;
            *extradata_dest = (uint8_t *) av_mallocz(extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
            memcpy(*extradata_dest, extradata, extradata_size);
            *extradata_size_dest = extradata_size;
            av_packet_unref(packet_ref);
        }
    }

    av_packet_free(&packet_ref);
    av_bsf_free(&bsf_context);

    return done;
}

2. 截图JPEG

如果是软解后的YUV420P格式，在通过mjpeg编码器编码成jpeg的时候，可以直接指定输入格式为AV_PIX_FMT_YUVJ420P，此处不是YUV420P而是YUVJ420P的原因是，YUVJ420P的color_range是JPEG，才可以编码JPEG，见https://stackoverflow.com/a/33939577 。那么问题来了，大部分Android设备，以及NVIDIA显卡，硬解码后的YUV，并不是YUV420P，而是NV12，与YUVJ420P之间就不只是color_range的不同，如果强行指定AV_PIX_FMT_YUVJ420P为输入格式，编码的时候会崩溃。所以需要编码JPEG前，先通过sws_scale转换为AV_PIX_FMT_YUVJ420P。

AVFrame *pFrameYUVJ420;

if (pix_fmt != AV_PIX_FMT_YUVJ420P) {
    pFrameYUVJ420 = av_frame_alloc();
    if (pFrameYUVJ420 == NULL) {
        LOGD("Could not allocate video frame: pFrameYUVJ420.");
        return -1;
    }

    // Determine required buffer size and allocate buffer
    // buffer中数据就是用于编码的,且格式为YUVJ420
    int numBytes = av_image_get_buffer_size(AV_PIX_FMT_YUVJ420P, pFrame->width, pFrame->height,
                                            1);
    uint8_t *buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));
    av_image_fill_arrays(pFrameYUVJ420->data, pFrameYUVJ420->linesize, buffer,
                         AV_PIX_FMT_YUVJ420P,
                         pFrame->width, pFrame->height, 1);

    // 由于解码出来的帧格式不是YUVJ420的,在编码之前需要进行格式转换
    struct SwsContext *sws_ctx = sws_getContext(pFrame->width,
                                                pFrame->height,
                                                pix_fmt,
                                                pFrame->width,
                                                pFrame->height,
                                                AV_PIX_FMT_YUVJ420P,
                                                SWS_BILINEAR,
                                                NULL,
                                                NULL,
                                                NULL);

    // 格式转换
    sws_scale(sws_ctx, (uint8_t const *const *) pFrame->data,
              pFrame->linesize, 0, pFrame->height,
              pFrameYUVJ420->data, pFrameYUVJ420->linesize);

    pFrameYUVJ420->format = AV_PIX_FMT_YUVJ420P;
    pFrameYUVJ420->width = pFrame->width;
    pFrameYUVJ420->height = pFrame->height;

    av_frame_unref(pFrame);
    av_free(pFrame);
} else {
    pFrameYUVJ420 = pFrame;
}

3. OpenGL渲染

由于软解码出来的YUV420P和硬解码出来的NV12，数据交织方式不同，所以创建的纹理层数不同，使用到的片元着色器也不同。YUV420P需要3层纹理，NV12只需要2层纹理。由于视频播放器一般要同时支持硬解和软解，所以这边着色器就支持了多种格式。

顶点着色器和片元着色器：

//顶点着色器glsl
#define GET_STR(x) #x
static const char *vertexShader = GET_STR(
        attribute vec4 aPosition; //顶点坐标
        attribute vec2 aTexCoord; //材质顶点坐标
        varying vec2 vTexCoord;   //输出的材质坐标
        void main(){
            vTexCoord = vec2(aTexCoord.x,1.0-aTexCoord.y);
            gl_Position = aPosition;
        }
);

//片元着色器
static const char *fragYUV420P = GET_STR(
        precision mediump float;    //精度
varying vec2 vTexCoord;     //顶点着色器传递的坐标
uniform sampler2D yTexture; //输入的材质（不透明灰度，单像素）
uniform sampler2D uTexture;
uniform sampler2D vTexture;
uniform int u_ImgType;// 1:RGBA, 2:NV21, 3:NV12, 4:I420
void main(){
    if(u_ImgType == 1) //RGBA
    {
        gl_FragColor = texture2D(yTexture, vTexCoord);
    }
    else if(u_ImgType == 2) //NV21
    {
        vec3 yuv;
        vec3 rgb;
        yuv.r = texture2D(yTexture,vTexCoord).r;
        yuv.g = texture2D(uTexture,vTexCoord).a - 0.5;
        yuv.b = texture2D(uTexture,vTexCoord).r - 0.5;
        rgb = mat3(1.0,     1.0,    1.0,
                   0.0,-0.39465,2.03211,
                   1.13983,-0.58060,0.0)*yuv;
        //输出像素颜色
        gl_FragColor = vec4(rgb,1.0);
    }
    else if(u_ImgType == 3) //NV12
    {
        vec3 yuv;
        vec3 rgb;
        yuv.r = texture2D(yTexture,vTexCoord).r;
        yuv.g = texture2D(uTexture,vTexCoord).r - 0.5;
        yuv.b = texture2D(uTexture,vTexCoord).a - 0.5;
        rgb = mat3(1.0,     1.0,    1.0,
                   0.0,-0.39465,2.03211,
                   1.13983,-0.58060,0.0)*yuv;
        //输出像素颜色
        gl_FragColor = vec4(rgb,1.0);
    }
    else if(u_ImgType == 4) //I420
    {
        vec3 yuv;
        vec3 rgb;
        yuv.r = texture2D(yTexture,vTexCoord).r;
        yuv.g = texture2D(uTexture,vTexCoord).r - 0.5;
        yuv.b = texture2D(vTexture,vTexCoord).r - 0.5;
        rgb = mat3(1.0,     1.0,    1.0,
                   0.0,-0.39465,2.03211,
                   1.13983,-0.58060,0.0)*yuv;
        //输出像素颜色
        gl_FragColor = vec4(rgb,1.0);
    }
    else
    {
        gl_FragColor = vec4(1.0);
    }
}
    );

纹理着色：

switch (pCodecCtx->pix_fmt) {
    case AV_PIX_FMT_RGBA:
    {
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, texts[0]);
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width,height,GL_RGBA,GL_UNSIGNED_BYTE,pFrame->data[0]);
    }
        break;
    case AV_PIX_FMT_NV21:
    case AV_PIX_FMT_NV12:
    {
        //激活第1层纹理,绑定到创建的opengl纹理
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D,texts[0]);
        //替换纹理内容
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width,height,GL_LUMINANCE,GL_UNSIGNED_BYTE,pFrame->data[0]);

        //update UV plane data
        glActiveTexture(GL_TEXTURE0+1);
        glBindTexture(GL_TEXTURE_2D, texts[1]);
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width/2,height/2,GL_LUMINANCE_ALPHA,GL_UNSIGNED_BYTE,pFrame->data[1]);
    }
        break;
    case AV_PIX_FMT_YUV420P:
    {
        //激活第1层纹理,绑定到创建的opengl纹理
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D,texts[0]);
        //替换纹理内容
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width,height,GL_LUMINANCE,GL_UNSIGNED_BYTE,pFrame->data[0]);

        //激活第2层纹理,绑定到创建的opengl纹理
        glActiveTexture(GL_TEXTURE0+1);
        glBindTexture(GL_TEXTURE_2D,texts[1]);
        //替换纹理内容
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width/2,height/2,GL_LUMINANCE,GL_UNSIGNED_BYTE,pFrame->data[1]);


        //激活第3层纹理,绑定到创建的opengl纹理
        glActiveTexture(GL_TEXTURE0+2);
        glBindTexture(GL_TEXTURE_2D,texts[2]);
        //替换纹理内容
        glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width/2,height/2,GL_LUMINANCE,GL_UNSIGNED_BYTE,pFrame->data[2]);
    }
        break;
}

4. 编码成MP4

编码成MP4后如果发现无法播放，或者只有VLC可以播放，同时手机或者pc或者mac无法对MP4文件加载出缩略图，可能是MP4文件头不对，关键在于要设置输出流AVStream->codecpar的extradata和extradata_size，以及AV_CODEC_FLAG_GLOBAL_HEADER。软解的话，ffmpeg自动给AVCodecContext加上extradata和extradata_size了，这边可以直接从AVCodecContext中读取，再拷贝给AVStream->codecpar，硬解的话，需要按照第一节中的方式，先从一个AVPacket中提取extradata和extradata_size，再拷贝给AVStream->codecpar。

AVStream *in_stream = ifmt_ctx_v->streams[i];
AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);
videoindex_v=i;
if (!out_stream) {
    LOGD( "Failed allocating output stream");
    ret = AVERROR_UNKNOWN;
    goto end;
}
videoindex_out=out_stream->index;
//Copy the settings of AVCodecContext
ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);

// extra_data to write file header
out_stream->codecpar->extradata = (uint8_t *) av_mallocz(pCodecCtx->extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
memcpy(out_stream->codecpar->extradata, pCodecCtx->extradata, pCodecCtx->extradata_size);
out_stream->codecpar->extradata_size = pCodecCtx->extradata_size;
LOGD("got extradata, %d size!\n", out_stream->codecpar->extradata_size);

if (ret < 0) {
    LOGD( "Failed to copy context from input to output stream codec context");
    goto end;
}
out_stream->codec->codec_tag = 0;
if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER) {
    out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    LOGD( "AV_CODEC_FLAG_GLOBAL_HEADER");
}

参考文献：

https://blog.csdn.net/yue_huang/article/details/75126155
https://blog.csdn.net/special00/article/details/82533768
https://github.com/bmegli/hardware-video-decoder/issues/5#issuecomment-469857880
https://github.com/bmegli/hardware-video-decoder/blob/2b9bf0f053/hvd.c
https://github.com/githubhaohao/AudioVideo/blob/main/app/src/main/cpp/player/render/video/VideoGLRender.cpp
https://stackoverflow.com/a/33939577
https://blog.csdn.net/Kennethdroid/article/details/108737936
https://www.jianshu.com/p/65d926ba1f1c/
https://qincji.gitee.io/2021/02/01/afplayer/03_mediacodec/index.html