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Transfer the blog of "kn-zheng" from Blog Garden
Since FFmpeg 3.0, many new interfaces have been used, which are as follows:

1. The original decoding function of avcodec_decode_video2() is decomposed into two functions avcodec_send_packet() and avcodec_receive_frame(). The specific usage is as follows:

old:
avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, pPacket);
new:
avcodec_send_packet(pCodecCtx, pPacket);
avcodec_receive_frame(pCodecCtx, pFrame);
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2. The encoding function corresponding to avcodec_encode_video2() is also split into two functions avcodec_send_frame() and avcodec_receive_packet(). The specific usage is as follows:

old:
avcodec_encode_video2(pCodecCtx, pPacket, pFrame, &got_picture);
new:
avcodec_send_frame(pCodecCtx, pFrame); avcodec_receive_packet(pCodecCtx, pPacket);
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3. avpicture_get_size() is now replaced by av_image_get_size(). The specific usage is as follows:

old:
avpicture_get_size(AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height);
new: //最后一个参数align这里是置1的，具体看情况是否需要置1
av_image_get_buffer_size(AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);
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4. avpicture_fill() is now changed to use av_image_fill_arrays. The specific usage is as follows:

old:
avpicture_fill((AVPicture *)pFrame, buffer, AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height);
new: //最后一个参数align这里是置1的，具体看情况是否需要置1
av_image_fill_arrays(pFrame->data, pFrame->linesize, buffer, AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height,1);
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5. Regarding the codec problem, some can be directly changed to codecpar, but sometimes this is not right, so I am still exploring. Here is a record of a change in the assignment method of pCodecCtx and pCodec

old:
pCodecCtx = pFormatCtx->streams[video_index]->codec; 
pCodec = avcodec_find_decoder(pFormatCtx->streams[video_index]->codec->codec_id);
new:
pCodecCtx = avcodec_alloc_context3(NULL)；avcodec_parameters_to_context(pCodecCtx,pFormatCtx->streams[video_index]->codecpar)；
pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
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6、PIX_FMT_YUV420P -> AV_PIX_FMT_YUV420P

7. 'AVStream::codec': declared deprecated:

old:
pCodecCtx = pFormatCtx->streams[videoindex]->codec;
new:
pCodecCtx = avcodec_alloc_context3(NULL)；
avcodec_parameters_to_context(pCodecCtx, pFormatCtx->streams[videoindex]->codecpar);
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8. 'avpicture_fill': declared deprecated:

old:
avpicture_fill((AVPicture *)pFrameYUV, out_buffer, AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height);
new:
av_image_fill_arrays(pFrameYUV->data, pFrameYUV->linesize, out_buffer, AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);
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9. 'avcodec_decode_video2': declared deprecated:

old:
ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet); //got_picture_ptr Zero if no frame could be decompressed
new:
ret = avcodec_send_packet(pCodecCtx, packet);
got_picture = avcodec_receive_frame(pCodecCtx, pFrame); //got_picture = 0 success, a frame was returned //注意：got_picture含义相反
或者：
int ret = avcodec_send_packet(aCodecCtx, &pkt);
if (ret != 0)
{
prinitf("%s/n","error");
return;
} 
while( avcodec_receive_frame(aCodecCtx, &frame) == 0)
{
//读取到一帧音频或者视频 //处理解码后音视频 frame
}
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10. 'av_free_packet': Declared as deprecated:

old:
av_free_packet(packet);
new:
av_packet_unref(packet);
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11. avcodec_decode_audio4: Declared as deprecated:

old:
result = avcodec_decode_audio4(dec_ctx, out_frame, &got_output, &enc_pkt);
new:
int ret = avcodec_send_packet(dec_ctx, &enc_pkt);
if (ret != 0)
{
prinitf("%s/n","error");
} 
while( avcodec_receive_frame(dec_ctx, &out_frame) == 0)
{
//读取到一帧音频或者视频
//处理解码后音视频 frame
}
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旧接口av_register_all()------------新版不需要注册
PKT_FLAG_KEY ---------------->AV_PKT_FLAG_KEY
AV_CODEC_CAP_DELAY----->AV_CODEC_CAP_DELAY
guess_format ------------>av_guess_format 
av_alloc_format_context ---------->avformat_alloc_output_context 
CODEC_TYPE_VIDEO ----------------->AVMEDIA_TYPE_VIDEO
CODEC_TYPE_AUDIO ---------------->AVMEDIA_TYPE_AUDIO
audio_resample_init ----------------->av_audio_resample_init 
PIX_FMT_YUV420P -> AV_PIX_FMT_YUV420P
AVStream::codec     被声明为已否决
‘avpicture_get_size’: 被声明为已否决
新的API中将AVStream结构体中codec作了遗弃处理，当需要解码器上下文的时候，需要用AVCodecParameters去转化，解决方案是如下
av_free_packet(packet)--------------------> av_packet_unref(packet);
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