英文:
libavcodec/libx264 isn't respecting keyint_min
问题
使用 libavcodec 57.107.100，我有一些用于将帧编码为 H.264 的代码，并正在调整设置以准备将生成的视频用 DASH 流式传输，这需要一致的关键帧定时。因此，我将 GOP 设置为帧速率的 4 倍，并将 keyint_min 设置为与 GOP 相同，以确保关键帧间隔与 4 秒段边界对齐。
我正在使用以下代码构建 AVCodec 和 AVCodecContext：
AVCodec* codec = NULL;
codec = avcodec_find_encoder_by_name("libx264");
AVCodecContext* avcodec_context = avcodec_alloc_context3(codec);
avcodec_context->bit_rate = 200000;
avcodec_context->height = 640;
avcodec_context->width = 480;
avcodec_context->keyint_min = 100;
avcodec_context->gop_size = 100;
avcodec_context->qmax = 31;
avcodec_context->qmin = 2;
avcodec_context->pix_fmt = AV_PIX_FMT_YUV420P;
avcodec_context->time_base = (AVRational) {1, 25};
显然，这里的 keyint_min 设置为 100，但当我执行我的程序时，来自 libavcodec 的 stderr 输出在编码开始时显示：
[libx264 @ 0x5577bde61f20] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2
AVX2
[libx264 @ 0x5577bde61f20] profile High, level 3.0
[libx264 @ 0x5577bde61f20] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=100 keyint_min=51 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=200 ratetol=1.0 qcomp=0.60 qpmin=2 qpmax=31 qpstep=4 ip_ratio=1.40 aq=1:1.00
为了强调，在上述文字的第三行，编码器显示 keyint=100 keyint_min=51。因此，显然我的 GOP 设置正确为 100，但 keyint_min 没有被尊重。由于 FFMPEG 文档相对难以理解，我无法找到答案。有人知道我漏掉了什么吗？
英文:
Using libavcodec 57.107.100, I have some code to encode frames into H.264 and I'm working on tweaking the settings to prep resulting videos to be streamed with DASH, which requires consistent keyframe timings. So I'm setting the GOP to 4x the framerate and setting the keyint_min to the same as the GOP so that I can guarantee that the keyframe interval lines up with the 4 second segment boundaries.
I'm using the following code to build the AVCodec and AVCodecContext:
AVCodec* codec = NULL;
codec = avcodec_find_encoder_by_name("libx264");
AVCodecContext* avcodec_context = avcodec_alloc_context3(codec);
context->bit_rate = 200000;
context->height = 640;
context->width = 480;
context->keyint_min = 100;
context->gop_size = 100;
context->qmax = 31;
context->qmin = 2;
context->pix_fmt = AV_PIX_FMT_YUV420P;
context->time_base = (AVRational) {1, 25};
Clearly, the keyint_min is 100 here, yet when I execute my program, the stderr output from libavcodec says this when the encoding starts:
[libx264 @ 0x5577bde61f20] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2
AVX2
[libx264 @ 0x5577bde61f20] profile High, level 3.0
[libx264 @ 0x5577bde61f20] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=100 keyint_min=51 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=200 ratetol=1.0 qcomp=0.60 qpmin=2 qpmax=31 qpstep=4 ip_ratio=1.40 aq=1:1.00
For emphasis, in the third line of that blurb, the encoder says keyint=100 keyint_min=51. So obviously my GOP is being set correctly at 100, but the keyint_min is not being respected. Since the FFMPEG docs are pretty difficult to approach, I haven't been able to find an answer to this. Does anyone know what I'm missing?
答案1
得分: 2
x264编码器将keyint_min限制为keyint/2+1。
直接传递一个选项给x264，以避免过早的IDR帧。
例如：
av_opt_set(context->priv_data, "x264opts", "no-scenecut", 0);
或者
context->scenechange_threshold = 0;
英文:
The x264 encoder will clamp keyint_min to keyint/2+1
Pass an option directly to x264 to avoid premature IDR frames
e.g.
av_opt_set(context->priv_data, "x264opts", "no-scenecut", 0);
or
context->scenechange_threshold = 0;
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