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transcoding: optimized video encoder settings
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@stbenz @perexg
stbenz authored and perexg committed May 23, 2015
1 parent ce2b077 commit 644f7ca
Showing 1 changed file with 51 additions and 77 deletions.
128 changes: 51 additions & 77 deletions src/plumbing/transcoding.c
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Expand Up @@ -1026,8 +1026,6 @@ transcoder_stream_video(transcoder_t *t, transcoder_stream_t *ts, th_pkt_t *pkt)
// Common settings
octx->width = vs->vid_width ? vs->vid_width : ictx->width;
octx->height = vs->vid_height ? vs->vid_height : ictx->height;
octx->gop_size = 25;
octx->has_b_frames = ictx->has_b_frames;

// Encoder uses "time_base" for bitrate calculation, but "time_base" from decoder
// will be deprecated in the future, therefore calculate "time_base" from "framerate" if available.
Expand All @@ -1038,111 +1036,87 @@ transcoder_stream_video(transcoder_t *t, transcoder_stream_t *ts, th_pkt_t *pkt)
octx->time_base = ictx->time_base;
#endif

// set default gop size to 1 second
octx->gop_size = ceil(av_q2d(av_inv_q(av_div_q(octx->time_base, av_make_q(1, octx->ticks_per_frame)))));

switch (ts->ts_type) {
case SCT_MPEG2VIDEO:
octx->codec_id = AV_CODEC_ID_MPEG2VIDEO;
octx->pix_fmt = PIX_FMT_YUV420P;
octx->flags |= CODEC_FLAG_GLOBAL_HEADER;

// Default settings for quantizer. Best quality unless changed by the streaming profile.
octx->qmin = 1;
octx->qmax = FF_LAMBDA_MAX;

if (t->t_props.tp_vbitrate == 0) { // "Auto"
octx->bit_rate = 2 * octx->width * octx->height;
octx->rc_max_rate = 4 * octx->bit_rate;
}

if (t->t_props.tp_vbitrate > 0 && t->t_props.tp_vbitrate < 64) { // CRF
octx->qmin = t->t_props.tp_vbitrate;
}

if (t->t_props.tp_vbitrate >= 64) { // CBR
octx->rc_max_rate = t->t_props.tp_vbitrate * 1000;
octx->bit_rate = ceil(octx->rc_max_rate / 1.15);
if (t->t_props.tp_vbitrate < 64) {
// encode with specified quality and optimize for low latency
// valid values for quality are 2-31, smaller means better quality, use 5 as default
octx->flags |= CODEC_FLAG_QSCALE;
octx->global_quality = FF_QP2LAMBDA *
(t->t_props.tp_vbitrate == 0 ? 5 : MAX(2, MIN(31, t->t_props.tp_vbitrate)));
} else {
// encode with specified bitrate and optimize for high compression
octx->bit_rate = t->t_props.tp_vbitrate * 1000;
octx->rc_max_rate = ceil(octx->bit_rate * 1.25);
octx->rc_buffer_size = octx->rc_max_rate * 3;
// use gop size of 5 seconds
octx->gop_size *= 5;
// activate b-frames
octx->max_b_frames = 3;
}

if (octx->rc_max_rate > 0)
octx->rc_buffer_size = 2 * octx->rc_max_rate;

break;

case SCT_VP8:
octx->codec_id = AV_CODEC_ID_VP8;
octx->pix_fmt = PIX_FMT_YUV420P;

// setting quality to realtime will use as much CPU for transcoding as possible,
// while still encoding in realtime
av_dict_set(&opts, "quality", "realtime", 0);

octx->qcompress = 0.6;

if (t->t_props.tp_vbitrate == 0) {
octx->qmin = 10;
octx->qmax = 20;
octx->rc_max_rate = 6 * octx->width * octx->height;
}

// Stream profile vbitrate 1-63 is used for user specified qmin quantizer (CRF mode).
if (t->t_props.tp_vbitrate > 0 && t->t_props.tp_vbitrate < 64) {
octx->qmin = t->t_props.tp_vbitrate;
octx->qmax = octx->qmin + 30 <= 63 ? octx->qmin + 30 : 63;
octx->rc_max_rate = 16 * octx->width * octx->height;
}

if (t->t_props.tp_vbitrate >= 64) { // CBR mode.
octx->rc_max_rate = t->t_props.tp_vbitrate * 1000;
octx->bit_rate = ceil(octx->rc_max_rate / 1.15);
if (t->t_props.tp_vbitrate < 64) {
// encode with specified quality and optimize for low latency
// valid values for quality are 1-63, smaller means better quality, use 15 as default
av_dict_set_int(&opts, "crf", (t->t_props.tp_vbitrate == 0 ? 15 : t->t_props.tp_vbitrate), 0);
// bitrate setting is still required, as it's used as max rate in CQ mode
// and set to a very low value by default
octx->bit_rate = 25000000;
} else {
// encode with specified bitrate and optimize for high compression
octx->bit_rate = t->t_props.tp_vbitrate * 1000;
octx->rc_buffer_size = octx->bit_rate * 3;
// use gop size of 5 seconds
octx->gop_size *= 5;
}

if (octx->rc_max_rate > 0)
octx->rc_buffer_size = 8 * 1024 * 224;

break;

case SCT_H264:
octx->codec_id = AV_CODEC_ID_H264;
octx->pix_fmt = PIX_FMT_YUV420P;
octx->flags |= CODEC_FLAG_GLOBAL_HEADER;

// Qscale difference between I-frames and P-frames.
// Note: -i_qfactor is handled a little differently than --ipratio.
// Recommended: -i_qfactor 0.71
octx->i_quant_factor = 0.71;

// QP curve compression: 0.0 => CBR, 1.0 => CQP.
// Recommended default: -qcomp 0.60
octx->qcompress = 0.6;
octx->flags |= CODEC_FLAG_GLOBAL_HEADER;

// Default = "medium". We gain more encoding speed compared to the loss of quality when lowering it _slightly_.
av_dict_set(&opts, "preset", "faster", 0);
// Use main profile instead of the standard "baseline", we are aiming for better quality.
// Older devices (iPhone <4, Android <4) only supports baseline. Chromecast only supports >=4.1...
av_dict_set(&opts, "profile", "main", 0); // L3.0
av_dict_set(&opts, "tune", "zerolatency", 0);

// If we are encoding HD, upgrade the profile to high.
if (octx->height >= 720 && t->t_props.tp_resolution >= 720) {
av_dict_set(&opts, "profile", "high", 0); // L3.1
}

// Default "auto" CRF settings. Aimed for quality without being too agressive.
if (t->t_props.tp_vbitrate == 0) {
octx->qmin = 10;
octx->qmax = 30;
}

// Stream profile vbitrate 1-63 is used for user specified qmin quantizer (CRF mode).
if (t->t_props.tp_vbitrate > 0 && t->t_props.tp_vbitrate < 64) {
octx->qmin = t->t_props.tp_vbitrate; // qmax = 51 in all default profiles, let's stick with it for now.
}
// All modern devices should support "high" profile
av_dict_set(&opts, "profile", "high", 0);

if (t->t_props.tp_vbitrate >= 64) { // Bitrate limited encoding (CBR mode).
octx->rc_max_rate = t->t_props.tp_vbitrate * 1000;
octx->bit_rate = ceil(octx->rc_max_rate / 1.15);
if (t->t_props.tp_vbitrate < 64) {
// encode with specified quality and optimize for low latency
// valid values for quality are 1-51, smaller means better quality, use 15 as default
av_dict_set_int(&opts, "crf", (t->t_props.tp_vbitrate == 0 ? 15 : MIN(51, t->t_props.tp_vbitrate)), 0);
// tune "zerolatency" removes as much encoder latency as possible
av_dict_set(&opts, "tune", "zerolatency", 0);
} else {
// encode with specified bitrate and optimize for high compression
octx->bit_rate = t->t_props.tp_vbitrate * 1000;
octx->rc_max_rate = ceil(octx->bit_rate * 1.25);
octx->rc_buffer_size = octx->rc_max_rate * 3;
// force-cfr=1 is needed for correct bitrate calculation (tune "zerolatency" also sets this)
av_dict_set(&opts, "x264opts", "force-cfr=1", 0);
// use gop size of 5 seconds
octx->gop_size *= 5;
}

if (octx->rc_max_rate > 0)
octx->rc_buffer_size = 8 * 1024 * 224;

break;

default:
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