decoder: h264: add support for missing first field.

Try to identify missing first fields too, thus disregarding any
intermediate gaps in frames. We also assume that we keep the same
field sequence, i.e. if previous frames were in top-field-first
(TFF) order, then so are subsequent frames.

Note that insertion of dummy first fields need to operate in two
steps: (i) create the original first field that the current field
will inherit from, and (ii) submit that field into the DPB prior
to initializing the current (other) field POC values but after any
reference flag was set. i.e. copy reference flags from the child
(other field) to the parent (first field).

https://bugzilla.gnome.org/show_bug.cgi?id=745048

gst-libs/gst/vaapi/gstvaapidecoder_h264.c

@@ -286,6 +286,14 @@ gst_vaapi_picture_h264_new_field(GstVaapiPictureH264 *picture)
     return (GstVaapiPictureH264 *)gst_vaapi_picture_new_field(&picture->base);
 }
 
+static inline GstVaapiPictureH264 *
+gst_vaapi_picture_h264_new_clone(GstVaapiPictureH264 *picture)
+{
+    g_return_val_if_fail(picture, NULL);
+
+    return (GstVaapiPictureH264 *)gst_vaapi_picture_new_clone(&picture->base);
+}
+
 /* ------------------------------------------------------------------------- */
 /* --- Frame Buffers (DPB)                                               --- */
 /* ------------------------------------------------------------------------- */
@@ -471,6 +479,7 @@ struct _GstVaapiDecoderH264Private {
     guint                       decoder_state;
     GstVaapiStreamAlignH264     stream_alignment;
     GstVaapiPictureH264        *current_picture;
+    GstVaapiPictureH264        *missing_picture;
     GstVaapiParserInfoH264     *sps[GST_H264_MAX_SPS_COUNT];
     GstVaapiParserInfoH264     *active_sps;
     GstVaapiParserInfoH264     *pps[GST_H264_MAX_PPS_COUNT];
@@ -541,6 +550,9 @@ struct _GstVaapiDecoderH264Class {
 static gboolean
 exec_ref_pic_marking(GstVaapiDecoderH264 *decoder, GstVaapiPictureH264 *picture);
 
+static gboolean
+exec_ref_pic_marking_sliding_window(GstVaapiDecoderH264 *decoder);
+
 static gboolean
 is_inter_view_reference_for_next_pictures(GstVaapiDecoderH264 *decoder,
     GstVaapiPictureH264 *picture);
@@ -1153,6 +1165,7 @@ gst_vaapi_decoder_h264_close(GstVaapiDecoderH264 *decoder)
     GstVaapiDecoderH264Private * const priv = &decoder->priv;
 
     gst_vaapi_picture_replace(&priv->current_picture, NULL);
+    gst_vaapi_picture_replace(&priv->missing_picture, NULL);
     gst_vaapi_parser_info_h264_replace(&priv->prev_slice_pi, NULL);
     gst_vaapi_parser_info_h264_replace(&priv->prev_pi, NULL);
 
@@ -2939,6 +2952,80 @@ init_picture_refs(
     }
 }
 
+static GstVaapiPictureH264 *
+fill_picture_first_field_gap(GstVaapiDecoderH264 *decoder,
+    GstVaapiPictureH264 *f0)
+{
+    GstVaapiDecoderH264Private * const priv = &decoder->priv;
+    GstVaapiPictureH264 *f1;
+
+    f1 = gst_vaapi_picture_h264_new_clone(f0);
+    if (!f1)
+        goto error_allocate_field;
+
+    gst_vaapi_picture_replace(&priv->missing_picture, f1);
+    gst_vaapi_picture_unref(f1);
+
+    GST_VAAPI_PICTURE_FLAG_SET(f1,
+        (GST_VAAPI_PICTURE_FLAG_ONEFIELD |
+         GST_VAAPI_PICTURE_FLAG_SKIPPED |
+         GST_VAAPI_PICTURE_FLAG_GHOST));
+
+    gst_vaapi_picture_h264_set_reference(f1, 0, FALSE);
+    return f1;
+
+    /* ERRORS */
+error_allocate_field:
+    GST_ERROR("failed to allocate missing field for current frame store");
+    return NULL;
+}
+
+static gboolean
+fill_picture_first_field_gap_done(GstVaapiDecoderH264 *decoder,
+    GstH264SliceHdr *slice_hdr)
+{
+    GstVaapiDecoderH264Private * const priv = &decoder->priv;
+    GstVaapiPictureH264 * const lost_field = priv->missing_picture;
+    GstH264SliceHdr lost_slice_hdr;
+    gboolean success = FALSE;
+
+    g_return_val_if_fail(priv->current_picture != NULL, FALSE);
+
+    if (!lost_field)
+        return TRUE;
+
+    lost_field->frame_num       = slice_hdr->frame_num;
+    lost_field->frame_num_wrap  = slice_hdr->frame_num;
+
+    gst_vaapi_picture_h264_set_reference(lost_field,
+        (GST_VAAPI_PICTURE_FLAGS(priv->current_picture) &
+         GST_VAAPI_PICTURE_FLAGS_REFERENCE), FALSE);
+
+    lost_slice_hdr = *slice_hdr;
+    lost_slice_hdr.bottom_field_flag = !lost_slice_hdr.bottom_field_flag;
+
+    init_picture_poc(decoder, lost_field, &lost_slice_hdr);
+    init_picture_ref_lists(decoder, lost_field);
+    init_picture_refs_pic_num(decoder, lost_field, &lost_slice_hdr);
+    if (!exec_ref_pic_marking_sliding_window(decoder))
+        goto error_exec_ref_pic_marking;
+    if (!dpb_add(decoder, lost_field))
+        goto error_dpb_add;
+    success = TRUE;
+
+cleanup:
+    gst_vaapi_picture_replace(&priv->missing_picture, NULL);
+    return success;
+
+    /* ERRORS */
+error_exec_ref_pic_marking:
+    GST_ERROR("failed to execute reference picture marking process");
+    goto cleanup;
+error_dpb_add:
+    GST_ERROR("failed to store lost picture into the DPB");
+    goto cleanup;
+}
+
 static GstVaapiPictureH264 *
 fill_picture_other_field_gap(GstVaapiDecoderH264 *decoder,
     GstVaapiPictureH264 *f0)
@@ -3089,6 +3176,7 @@ init_picture(
                 GST_VAAPI_PICTURE_FLAG_SHORT_TERM_REFERENCE);
     }
 
+    fill_picture_first_field_gap_done(decoder, slice_hdr);
     init_picture_poc(decoder, picture, slice_hdr);
     return TRUE;
 }
@@ -3651,6 +3739,8 @@ find_first_field(GstVaapiDecoderH264 *decoder, GstVaapiParserInfoH264 *pi,
     /* At this point, the current frame is known to be interlaced */
     if (gst_vaapi_frame_store_has_frame(fs)) {
         f1 = NULL;
+        if (fill_gaps && !same_field_parity(f0, slice_hdr))
+            f1 = fill_picture_first_field_gap(decoder, f0);
         return f1;
     }
 
@@ -3668,6 +3758,8 @@ find_first_field(GstVaapiDecoderH264 *decoder, GstVaapiParserInfoH264 *pi,
     f1 = NULL;
     if (fill_gaps) {
         fill_picture_other_field_gap(decoder, f0);
+        if (!same_field_parity(f0, slice_hdr))
+            f1 = fill_picture_first_field_gap(decoder, f0);
     }
     return f1;
 }
@@ -3698,6 +3790,7 @@ decode_picture(GstVaapiDecoderH264 *decoder, GstVaapiDecoderUnit *unit)
         return status;
 
     priv->decoder_state = 0;
+    gst_vaapi_picture_replace(&priv->missing_picture, NULL);
 
     first_field = find_first_field(decoder, pi, TRUE);
     if (first_field) {