[PATCH 05/18] avcodec/h2645_parse: parse the H.264 MVC NAL unit header extension

NAL types 14 (prefix), 20 (coded slice extension) and 21 (depth coded slice
extension) carry a three byte nal_unit_header_mvc_extension() (H.7.3.1.1) ahead
of their payload. Parse it here so that view_id and the anchor/inter-view flags
are available alongside the rest of the NAL header, and so that nal->gb is left
positioned at the payload proper -- for a coded slice extension that is
slice_header(), which means the existing slice header parser can be reused
unchanged.

The SVC variant of the extension is the same size, so consume and ignore it
rather than misparsing the payload that follows.

Signed-off-by: Dom Cobley <[email protected]>
---
 libavcodec/h2645_parse.c | 38 ++++++++++++++++++++++++++++++++++++++
 libavcodec/h2645_parse.h | 13 +++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/libavcodec/h2645_parse.c b/libavcodec/h2645_parse.c
index 3c2b3aa889..6420d621e7 100644
--- a/libavcodec/h2645_parse.c
+++ b/libavcodec/h2645_parse.c
@@ -455,6 +455,44 @@ static int h264_parse_nal_header(H2645NAL *nal, void *logctx)
     nal->ref_idc = get_bits(gb, 2);
     nal->type    = get_bits(gb, 5);
 
+    /* NALs are reused between packets, so these must always be initialised. */
+    nal->view_id         = -1;
+    nal->anchor_pic_flag = 0;
+    nal->inter_view_flag = 0;
+    nal->non_idr_flag    = 0;
+
+    if (nal->type == H264_NAL_PREFIX || nal->type == H264_NAL_EXTEN_SLICE ||
+        nal->type == H264_NAL_DEPTH_EXTEN_SLICE) {
+        /* nal_unit_header_svc_extension() and nal_unit_header_mvc_extension()
+         * are both three bytes wide, including the svc_extension_flag. Consuming
+         * them here leaves nal->gb at the start of the payload proper, i.e. at
+         * slice_header() for a coded slice extension. */
+        if (get_bits_left(gb) < 24)
+            return AVERROR_INVALIDDATA;
+
+        if (get_bits1(gb)) {            // svc_extension_flag
+            /* SVC (Annex G) is not supported; skip the header so that the NAL is
+             * merely ignored rather than misparsed. */
+            skip_bits(gb, 23);
+        } else {
+            nal->non_idr_flag    = get_bits1(gb);
+            skip_bits(gb, 6);           // priority_id
+            nal->view_id         = get_bits(gb, 10);
+            nal->temporal_id     = get_bits(gb, 3);
+            nal->anchor_pic_flag = get_bits1(gb);
+            nal->inter_view_flag = get_bits1(gb);
+            skip_bits1(gb);             // reserved_one_bit
+
+            av_log(logctx, AV_LOG_DEBUG,
+                   "nal_unit_type: %d(%s), nal_ref_idc: %d, view_id: %d, "
+                   "temporal_id: %d, anchor: %d, inter_view: %d\n",
+                   nal->type, h264_nal_unit_name(nal->type), nal->ref_idc,
+                   nal->view_id, nal->temporal_id, nal->anchor_pic_flag,
+                   nal->inter_view_flag);
+            return 0;
+        }
+    }
+
     av_log(logctx, AV_LOG_DEBUG,
            "nal_unit_type: %d(%s), nal_ref_idc: %d\n",
            nal->type, h264_nal_unit_name(nal->type), nal->ref_idc);
diff --git a/libavcodec/h2645_parse.h b/libavcodec/h2645_parse.h
index 0e1e950294..1cc2e079ed 100644
--- a/libavcodec/h2645_parse.h
+++ b/libavcodec/h2645_parse.h
@@ -66,6 +66,19 @@ typedef struct H2645NAL {
      */
     int nuh_layer_id;
 
+    /**
+     * H.264 MVC only, from nal_unit_header_mvc_extension() (H.7.3.1.1), which is
+     * present on NAL types 14, 20 and 21 only.
+     *
+     * view_id is -1 when this NAL has no MVC extension header, either because
+     * its type does not carry one or because it carries an SVC one instead; the
+     * other fields are then meaningless.
+     */
+    int view_id;
+    uint8_t anchor_pic_flag;
+    uint8_t inter_view_flag;
+    uint8_t non_idr_flag;
+
     int skipped_bytes;
     int skipped_bytes_pos_size;
     int *skipped_bytes_pos;
-- 
2.53.0

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