+// delete the buffer 'out' from the chain of buffers whose head is 'buf_out'.
+// out & buf_out must be non-null (checks prior to anywhere this routine
+// can be called guarantee this) and out must be on buf_out's chain
+// (all places that call this get 'out' while traversing the chain).
+// 'out' is freed and its predecessor is returned.
+static hb_buffer_t *delete_buffer_from_chain( hb_buffer_t **buf_out, hb_buffer_t *out)
+{
+ hb_buffer_t *succ = out->next;
+ hb_buffer_t *pred = *buf_out;
+ if ( pred == out )
+ {
+ // we're deleting the first buffer
+ *buf_out = succ;
+ }
+ else
+ {
+ // target isn't the first buf so search for its predecessor.
+ while ( pred->next != out )
+ {
+ pred = pred->next;
+ }
+ // found 'out' - remove it from the chain
+ pred->next = succ;
+ }
+ out->next = 0;
+ hb_buffer_close( &out );
+ return succ;
+}
+
+// insert buffer 'succ' after buffer chain element 'pred'.
+// caller must guarantee that 'pred' and 'succ' are non-null.
+static hb_buffer_t *insert_buffer_in_chain( hb_buffer_t *pred, hb_buffer_t *succ )
+{
+ succ->next = pred->next;
+ pred->next = succ;
+ return succ;
+}
+
+// This section of the code implements video frame rate control.
+// Since filters are allowed to duplicate and drop frames (which
+// changes the timing), this has to be the last thing done in render.
+//
+// There are three options, selected by the value of job->cfr:
+// 0 - Variable Frame Rate (VFR) or 'same as source': frame times
+// are left alone
+// 1 - Constant Frame Rate (CFR): Frame timings are adjusted so that all
+// frames are exactly job->vrate_base ticks apart. Frames are dropped
+// or duplicated if necessary to maintain this spacing.
+// 2 - Peak Frame Rate (PFR): job->vrate_base is treated as the peak
+// average frame rate. I.e., the average frame rate (current frame
+// end time divided by number of frames so far) is never allowed to be
+// greater than job->vrate_base and frames are dropped if necessary
+// to keep the average under this value. Other than those drops, frame
+// times are left alone.
+//
+
+static void adjust_frame_rate( hb_work_private_t *pv, hb_buffer_t **buf_out )
+{
+ hb_buffer_t *out = *buf_out;
+
+ while ( out && out->size > 0 )
+ {
+ // this frame has to start where the last one stopped.
+ out->start = pv->out_last_stop;
+
+ // compute where this frame would stop if the frame rate were constant
+ // (this is our target stopping time for CFR and earliest possible
+ // stopping time for PFR).
+ double cfr_stop = pv->frame_rate * ( pv->count_frames + 1 );
+
+ if ( cfr_stop - (double)out->stop >= pv->frame_rate )
+ {
+ // This frame stops a frame time or more in the past - drop it
+ // but don't lose its chapter mark.
+ if ( out->new_chap )
+ {
+ pv->chapter_time = out->start;
+ pv->chapter_val = out->new_chap;
+ }
+ ++pv->drops;
+ out = delete_buffer_from_chain( buf_out, out );
+ continue;
+ }
+
+ // at this point we know that this frame doesn't push the average
+ // rate over the limit so we just pass it on for PFR. For CFR we're
+ // going to return it (with its start & stop times modified) and
+ // we may have to dup it.
+ ++pv->count_frames;
+ if ( pv->job->cfr > 1 )
+ {
+ // PFR - we're going to keep the frame but may need to
+ // adjust it's stop time to meet the average rate constraint.
+ if ( out->stop <= cfr_stop )
+ {
+ out->stop = cfr_stop;
+ }
+ }
+ else
+ {
+ // we're doing CFR so we have to either trim some time from a
+ // buffer that ends too far in the future or, if the buffer is
+ // two or more frame times long, split it into multiple pieces,
+ // each of which is a frame time long.
+ double excess_dur = (double)out->stop - cfr_stop;
+ out->stop = cfr_stop;
+ for ( ; excess_dur >= pv->frame_rate; excess_dur -= cfr_stop )
+ {
+ /* next frame too far ahead - dup current frame */
+ hb_buffer_t *dup = hb_buffer_init( out->size );
+ memcpy( dup->data, out->data, out->size );
+ hb_buffer_copy_settings( dup, out );
+ dup->new_chap = 0;
+ dup->start = cfr_stop;
+ cfr_stop += pv->frame_rate;
+ dup->stop = cfr_stop;
+ out = insert_buffer_in_chain( out, dup );
+ ++pv->dups;
+ ++pv->count_frames;
+ }
+ }
+ pv->out_last_stop = out->stop;
+ out = out->next;
+ }
+}
+