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Demote scheduled B-frames without an L1 reference to P #2

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Demote scheduled B-frames without an L1 reference to P #2

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32 changes: 32 additions & 0 deletions encoder/encoder.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2340,6 +2340,33 @@ static inline void reference_filter_spiral_exact_layers( x264_t *h )
}
}

/* A Spiral-scheduled B/B-ref frame must have an available backward (L1)
* reference by the time ratecontrol runs. In a partial final GOP the forward
* anchor a B references may never be submitted (its display position lies past
* end-of-stream), so it is never decoded and never enters the DPB. That leaves
* the B with an empty L1 list, and rate_estimate_qscale() unconditionally
* dereferences h->fref_nearest[1] / h->fref[1][h->i_ref[1]-1] for B slices ->
* SIGSEGV on flush (CRF only; fixed-QP frames skip rate_estimate_qscale).
* Mirror x264's normal end-of-stream behaviour and demote such a frame to P;
* its past/L0 anchor is always present, so P ratecontrol is well-defined. */
static void x264_spiral_demote_unreferenced_bframe( x264_t *h )
{
if( !IS_X264_TYPE_B( h->fenc->i_type ) )
return;
const x264_spiral_frame_props_t *props = x264_spiral_frame_props( h->fenc );
if( !props )
return;
for( int i = 0; h->frames.reference[i]; i++ )
{
x264_frame_t *ref = h->frames.reference[i];
if( ref->b_corrupt || ref->i_poc <= h->fenc->i_poc )
continue;
if( x264_spiral_ref_allowed( props, x264_spiral_frame_props( ref ) ) )
return; /* a usable future reference exists; keep it a B */
}
h->fenc->i_type = X264_TYPE_P;
}

static inline void reference_build_list( x264_t *h, int i_poc )
{
int b_ok;
Expand Down Expand Up @@ -3562,6 +3589,11 @@ int x264_encoder_encode( x264_t *h,
h->fdec->i_poc =
h->fenc->i_poc = 2 * ( h->fenc->i_frame - X264_MAX( h->frames.i_last_idr, 0 ) );

/* Spiral: a scheduled B whose forward anchor was never submitted (partial
* final GOP at flush) has no L1 reference; demote it to P before its slice
* type is derived so ratecontrol never dereferences a NULL backward ref. */
x264_spiral_demote_unreferenced_bframe( h );

/* ------------------- Setup frame context ----------------------------- */
/* 5: Init data dependent of frame type */
if( h->fenc->i_type == X264_TYPE_IDR )
Expand Down