[FFmpeg-devel] [PATCH 01/11] lavc/hevcdec: move handling of byte alignment at the end of slice header

[Anton Khirnov]

Do it in hls_slice_header() rather than cabac_init_decoder() - the
former is a more logical place as according the spec the byte alignment
is a part of the slice header, not slice data. Avoids a second instance
of alignment handling in vaapi_hevc.

Also, check that alignment_bit_equal_to_one is, in fact, equal to one.
---
 libavcodec/hevc_cabac.c | 2 --
 libavcodec/hevcdec.c    | 7 +++++++
 libavcodec/hevcdec.h    | 1 +
 libavcodec/vaapi_hevc.c | 4 +---
 4 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/libavcodec/hevc_cabac.c b/libavcodec/hevc_cabac.c
index 2e639a7e41..3f95c9ca05 100644
--- a/libavcodec/hevc_cabac.c
+++ b/libavcodec/hevc_cabac.c
@@ -430,8 +430,6 @@ static int cabac_reinit(HEVCLocalContext *lc)
 static int cabac_init_decoder(HEVCLocalContext *lc)
 {
     GetBitContext *gb = &lc->gb;
-    skip_bits(gb, 1);
-    align_get_bits(gb);
     return ff_init_cabac_decoder(&lc->cc,
                           gb->buffer + get_bits_count(gb) / 8,
                           (get_bits_left(gb) + 7) / 8);
diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
index ff9a418926..ad2cbd7ece 100644
--- a/libavcodec/hevcdec.c
+++ b/libavcodec/hevcdec.c
@@ -1005,6 +1005,13 @@ static int hls_slice_header(HEVCContext *s)
             skip_bits(gb, 8);  // slice_header_extension_data_byte
     }
 
+    ret = get_bits1(gb);
+    if (!ret) {
+        av_log(s->avctx, AV_LOG_ERROR, "alignment_bit_equal_to_one=0\n");
+        return AVERROR_INVALIDDATA;
+    }
+    sh->data_offset = align_get_bits(gb) - gb->buffer;
+
     // Inferred parameters
     sh->slice_qp = 26U + s->ps.pps->pic_init_qp_minus26 + sh->slice_qp_delta;
     if (sh->slice_qp > 51 ||
diff --git a/libavcodec/hevcdec.h b/libavcodec/hevcdec.h
index 5aa3d40450..3824bf621b 100644
--- a/libavcodec/hevcdec.h
+++ b/libavcodec/hevcdec.h
@@ -277,6 +277,7 @@ typedef struct SliceHeader {
     int16_t chroma_offset_l1[16][2];
 
     int slice_ctb_addr_rs;
+    unsigned data_offset;
 } SliceHeader;
 
 typedef struct CodingUnit {
diff --git a/libavcodec/vaapi_hevc.c b/libavcodec/vaapi_hevc.c
index 0f5dd50351..f0a0f295d9 100644
--- a/libavcodec/vaapi_hevc.c
+++ b/libavcodec/vaapi_hevc.c
@@ -485,9 +485,7 @@ static int vaapi_hevc_decode_slice(AVCodecContext *avctx,
         .slice_data_size               = size,
         .slice_data_offset             = 0,
         .slice_data_flag               = VA_SLICE_DATA_FLAG_ALL,
-        /* Add 1 to the bits count here to account for the byte_alignment bit, which
-         * always is at least one bit and not accounted for otherwise. */
-        .slice_data_byte_offset        = (get_bits_count(&h->HEVClc->gb) + 1 + 7) / 8,
+        .slice_data_byte_offset        = sh->data_offset,
         .slice_segment_address         = sh->slice_segment_addr,
         .slice_qp_delta                = sh->slice_qp_delta,
         .slice_cb_qp_offset            = sh->slice_cb_qp_offset,
-- 
2.43.0