{IDR(PBB)(PBB)}.
*/
-// General pts/dts strategy:
-//
-// Getting pts and dts right with variable frame rate (VFR) and B-frames can be a
-// bit tricky. We assume first of all that the frame rate never goes _above_
-// MAX_FPS, which gives us a frame period N. The decoder can always decode
-// in at least this speed, as long at dts <= pts (the frame is not attempted
-// presented before it is decoded). Furthermore, we never have longer chains of
-// B-frames than a fixed constant C. (In a B-frame chain, we say that the base
-// I/P-frame has order O=0, the B-frame depending on it directly has order O=1,
-// etc. The last frame in the chain, which no B-frames depend on, is the “tip”
-// frame, with an order O <= C.)
-//
-// Many strategies are possible, but we establish these rules:
-//
-// - Tip frames have dts = pts - (C-O)*N.
-// - Non-tip frames have dts = dts_last + N.
-//
-// An example, with C=2 and N=10 and the data flow showed with arrows:
-//
-// I B P B B P
-// pts: 30 40 50 60 70 80
-// ↓ ↓ ↓
-// dts: 10 30 20 60 50←40
-// | | ↑ ↑
-// `--|--' |
-// `----------'
-//
-// To show that this works fine also with irregular spacings, let's say that
-// the third frame is delayed a bit (something earlier was dropped). Now the
-// situation looks like this:
-//
-// I B P B B P
-// pts: 30 40 80 90 100 110
-// ↓ ↓ ↓
-// dts: 10 30 20 90 50←40
-// | | ↑ ↑
-// `--|--' |
-// `----------'
-//
-// The resetting on every tip frame makes sure dts never ends up lagging a lot
-// behind pts, and the subtraction of (C-O)*N makes sure pts <= dts.
-//
-// In the output of this function, if <dts_lag> is >= 0, it means to reset the
-// dts from the current pts minus <dts_lag>, while if it's -1, the frame is not
-// a tip frame and should be given a dts based on the previous one.
#define FRAME_P 0
#define FRAME_B 1
#define FRAME_I 2
int encoding_order, int intra_period,
int intra_idr_period, int ip_period,
int *displaying_order,
- int *frame_type, int *pts_lag)
+ int *frame_type)
{
int encoding_order_gop = 0;
- *pts_lag = 0;
-
if (intra_period == 1) { /* all are I/IDR frames */
*displaying_order = encoding_order;
if (intra_idr_period == 0)
// We have B-frames. Sequence is like IDR (PBB)(PBB)(IBB)(PBB).
encoding_order_gop = (intra_idr_period == 0) ? encoding_order : (encoding_order % (intra_idr_period + 1));
- *pts_lag = -1; // Most frames are not tip frames.
if (encoding_order_gop == 0) { /* the first frame */
*frame_type = FRAME_IDR;
*displaying_order = encoding_order;
- // IDR frames are a special case; I honestly can't find the logic behind
- // why this is the right thing, but it seems to line up nicely in practice :-)
- *pts_lag = TIMEBASE / MAX_FPS;
} else if (((encoding_order_gop - 1) % ip_period) != 0) { /* B frames */
*frame_type = FRAME_B;
*displaying_order = encoding_order - 1;
- if ((encoding_order_gop % ip_period) == 0) {
- *pts_lag = 0; // Last B-frame.
- }
} else if (intra_period != 0 && /* have I frames */
encoding_order_gop >= 2 &&
((encoding_order_gop - 1) / ip_period % (intra_period / ip_period)) == 0) {
}
}
+// General pts/dts strategy:
+//
+// Getting pts and dts right with variable frame rate (VFR) and B-frames can be a
+// bit tricky. This strategy roughly matches what x264 seems to do: We take in
+// the pts as the frames are encoded, and reuse that as dts in the same order,
+// slightly offset.
+//
+// If we don't have B-frames (only I and P), this means pts == dts always.
+// This is the simple case. Now consider the case with a single B-frame:
+//
+// I B P B P
+// pts: 30 40 50 60 70
+//
+// Since we always inherently encode P-frames before B-frames, this means that
+// we see them in this order, which we can _almost_ use for dts:
+//
+// dts: 30 50 40 70 60
+//
+// the only problem here is that for the B-frames, pts < dts. We solve this by
+// priming the queue at the very start with some made-up dts:
+//
+// I B P B P
+// pts: 30 40 50 60 70
+// dts: xx 30 50 40 70 60
+//
+// Now we have all the desirable properties: pts >= dts, successive dts delta
+// is never larger than the decoder can figure out (assuming, of course,
+// the pts has that property), and there's minimal lag between pts and dts.
+// For the made-up dts, we assume 1/60 sec per frame, which should generally
+// be reasonable. dts can go negative, but this is corrected using global_delay()
+// by delaying both pts and dts (although we probably don't need to).
+//
+// If there's more than one B-frame possible, we simply insert more of them
+// (here shown with some irregular spacing, assuming B-frames don't depend
+// on each other and simply go back-to-front):
+//
+// I B B B P B B B P
+// pts: 30 40 55 60 65 66 67 68 80
+// dts: xx yy zz 30 65 60 55 40 80 68 67 66
+class DTSReorderer {
+public:
+ DTSReorderer(int num_b_frames) : num_b_frames(num_b_frames) {}
+
+ void push_pts(int64_t pts)
+ {
+ if (buf.empty() && num_b_frames > 0) { // First frame.
+ int64_t base_dts = pts - num_b_frames * (TIMEBASE / MAX_FPS);
+ for (int i = 0; i < num_b_frames; ++i) {
+ buf.push(base_dts + i * (TIMEBASE / MAX_FPS));
+ }
+ }
+ buf.push(pts);
+ }
+
+ int64_t pop_dts()
+ {
+ assert(!buf.empty());
+ int64_t dts = buf.front();
+ buf.pop();
+ return dts;
+ }
+
+private:
+ const int num_b_frames;
+ queue<int64_t> buf;
+};
void QuickSyncEncoderImpl::enable_zerocopy_if_possible()
{
{
pthread_setname_np(pthread_self(), "QS_Encode");
+ DTSReorderer dts_reorder_buf(ip_period - 1);
+
int64_t last_dts = -1;
int gop_start_display_frame_num = 0;
for (int display_frame_num = 0; ; ++display_frame_num) {
}
}
+ dts_reorder_buf.push_pts(frame.pts);
+
// Pass the frame on to x264 (or uncompressed to HTTP) as needed.
// Note that this implicitly waits for the frame to be done rendering.
pass_frame(frame, display_frame_num, frame.pts, frame.duration);
// Now encode as many QuickSync frames as we can using the frames we have available.
// (It could be zero, or it could be multiple.) FIXME: make a function.
for ( ;; ) {
- int pts_lag;
int frame_type, quicksync_display_frame_num;
encoding2display_order(quicksync_encoding_frame_num, intra_period, intra_idr_period, ip_period,
- &quicksync_display_frame_num, &frame_type, &pts_lag);
+ &quicksync_display_frame_num, &frame_type);
if (!reorder_buffer.count(quicksync_display_frame_num)) {
break;
}
gop_start_display_frame_num = quicksync_display_frame_num;
}
- // Determine the dts of this frame.
- int64_t dts;
- if (pts_lag == -1) {
- assert(last_dts != -1);
- dts = last_dts + (TIMEBASE / MAX_FPS);
- } else {
- dts = frame.pts - pts_lag;
- }
+ const int64_t dts = dts_reorder_buf.pop_dts();
last_dts = dts;
encode_frame(frame, quicksync_encoding_frame_num, quicksync_display_frame_num, gop_start_display_frame_num, frame_type, frame.pts, dts, frame.duration, frame.ycbcr_coefficients);
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