From 63e29adbf57bdc2b6f72eddd9690c2dc4c6e0b52 Mon Sep 17 00:00:00 2001
From: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Date: Fri, 2 Mar 2012 13:41:16 +0100
Subject: [PATCH] h264: fix slice_data_bit_offset calculation.

Unlike what VA-API documentation defines, the slice_data_bit_offset
represents the offset to the first macroblock in the slice data, minus
any emulation prevention bytes in the slice_header().

This fix copes with binary-only VA drivers that won't be fixed any
time soon. Besides, this aligns with the current FFmpeg behaviour
that was based on those proprietary drivers implementing the API
incorrectly.
---
 configure.ac                              | 24 +++++++++++++++++
 gst-libs/gst/vaapi/gstvaapidecoder_h264.c | 44 ++++++++++++++++++++++++++++---
 2 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index f0184df..81583c5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -247,6 +247,30 @@ AC_DEFINE_UNQUOTED(USE_CODEC_PARSERS, $USE_CODEC_PARSERS,
     [Defined to 1 if GStreamer codec parsers are used])
 AM_CONDITIONAL(USE_CODEC_PARSERS, test $USE_CODEC_PARSERS -eq 1)
 
+if test "$enable_codecparsers" = "yes"; then
+AC_CACHE_CHECK([for GstH264SliceHdr::n_emulation_prevention_bytes],
+    ac_cv_have_gst_h264_slice_hdr_epb_count, [
+    saved_CFLAGS="$CFLAGS"
+    CFLAGS="$CFLAGS $GST_CFLAGS $GST_CODEC_PARSERS_CFLAGS"
+    saved_LIBS="$LIBS"
+    LIBS="$LIBS $GST_LIBS $GST_CODEC_PARSERS_LIBS"
+    AC_TRY_COMPILE(
+        [#include <gst/codecparsers/gsth264parser.h>],
+        [GstH264SliceHdr slice_hdr;
+         slice_hdr.n_emulation_prevention_bytes = 0;],
+        [ac_cv_have_gst_h264_slice_hdr_epb_count="yes"],
+        [ac_cv_have_gst_h264_slice_hdr_epb_count="no"]
+    )
+    CFLAGS="$saved_CFLAGS"
+    LIBS="$saved_LIBS"
+])
+fi
+
+if test "$ac_cv_have_gst_h264_slice_hdr_epb_count" = "yes"; then
+    AC_DEFINE_UNQUOTED(HAVE_GST_H264_SLICE_HDR_EPB_COUNT, 1,
+        [Defined to 1 if GstH264SliceHdr::n_emulation_prevention_bytes exists.])
+fi
+
 dnl Check for GStreamer interfaces
 PKG_CHECK_MODULES([GST_INTERFACES],
     [gstreamer-interfaces-$GST_MAJORMINOR >= $GST_PLUGINS_BASE_VERSION_REQUIRED]
diff --git a/gst-libs/gst/vaapi/gstvaapidecoder_h264.c b/gst-libs/gst/vaapi/gstvaapidecoder_h264.c
index e19fa42..9ee3809 100644
--- a/gst-libs/gst/vaapi/gstvaapidecoder_h264.c
+++ b/gst-libs/gst/vaapi/gstvaapidecoder_h264.c
@@ -1965,6 +1965,40 @@ decode_picture_end(GstVaapiDecoderH264 *decoder, GstVaapiPictureH264 *picture)
     return TRUE;
 }
 
+#ifndef HAVE_GST_H264_SLICE_HDR_EPB_COUNT
+static guint
+get_epb_count(const guint8 *buf, guint buf_size, guint header_size)
+{
+    guint i, n = 0;
+
+    if (buf_size > header_size)
+        buf_size = header_size;
+
+    for (i = 2; i < buf_size; i++) {
+        if (!buf[i - 2] && !buf[i - 1] && buf[i] == 0x03)
+            i += 2, n++;
+    }
+    return n;
+}
+#endif
+
+static inline guint
+get_slice_data_bit_offset(GstH264SliceHdr *slice_hdr, GstH264NalUnit *nalu)
+{
+    guint epb_count;
+
+#ifdef HAVE_GST_H264_SLICE_HDR_EPB_COUNT
+    epb_count = slice_hdr->n_emulation_prevention_bytes;
+#else
+    epb_count = get_epb_count(
+        nalu->data + nalu->offset,
+        nalu->size,
+        slice_hdr->header_size / 8
+    );
+#endif
+    return 8 /* nal_unit_type */ + slice_hdr->header_size - epb_count * 8;
+}
+
 static gboolean
 fill_pred_weight_table(GstVaapiDecoderH264 *decoder, GstVaapiSliceH264 *slice)
 {
@@ -2074,13 +2108,17 @@ fill_RefPicList(GstVaapiDecoderH264 *decoder, GstVaapiSliceH264 *slice)
 }
 
 static gboolean
-fill_slice(GstVaapiDecoderH264 *decoder, GstVaapiSliceH264 *slice)
+fill_slice(
+    GstVaapiDecoderH264 *decoder,
+    GstVaapiSliceH264   *slice,
+    GstH264NalUnit      *nalu
+)
 {
     GstH264SliceHdr * const slice_hdr = &slice->slice_hdr;
     VASliceParameterBufferH264 * const slice_param = slice->base.param;
 
     /* Fill in VASliceParameterBufferH264 */
-    slice_param->slice_data_bit_offset          = 8 /* nal_unit_type */ + slice_hdr->header_size;
+    slice_param->slice_data_bit_offset          = get_slice_data_bit_offset(slice_hdr, nalu);
     slice_param->first_mb_in_slice              = slice_hdr->first_mb_in_slice;
     slice_param->slice_type                     = slice_hdr->type % 5;
     slice_param->direct_spatial_mv_pred_flag    = slice_hdr->direct_spatial_mv_pred_flag;
@@ -2137,7 +2175,7 @@ decode_slice(GstVaapiDecoderH264 *decoder, GstH264NalUnit *nalu)
     priv->mb_x = slice_hdr->first_mb_in_slice % priv->mb_width;
     priv->mb_y = slice_hdr->first_mb_in_slice / priv->mb_width; // FIXME: MBAFF or field
 
-    if (!fill_slice(decoder, slice)) {
+    if (!fill_slice(decoder, slice, nalu)) {
         status = GST_VAAPI_DECODER_STATUS_ERROR_UNKNOWN;
         goto error;
     }
-- 
2.7.4