Fix parsing segmentation data in parse_frame_hdr

The first memset is dead code: if primary_ref_frame is
PRIMARY_REF_NONE then segmentation.update_data is always true. The
patch removes this memset and explains why the copy in the other
branch is correct.

The second memset should always fire: if segmentation is not enabled
for this frame, the seg_data structure should be set to zero rather
than copied from a reference frame (see section 5.9.14 of the AV1
spec).

1 files changed, 4 insertions, 8 deletions

diff --git a/src/obu.c b/src/obu.c
index 5e901db..a62f34f 100644
--- a/src/obu.c
+++ b/src/obu.c
@@ -616,21 +616,17 @@ static int parse_frame_hdr(Dav1dContext *const c, GetBits *const gb,
                     hdr->segmentation.seg_data.preskip = 1;
                 }
             }
-        } else if (hdr->primary_ref_frame == PRIMARY_REF_NONE) {
-            memset(&hdr->segmentation.seg_data, 0, sizeof(Av1SegmentationDataSet));
-            for (int i = 0; i < NUM_SEGMENTS; i++)
-                hdr->segmentation.seg_data.d[i].ref = -1;
         } else {
+            // segmentation.update_data was false so we should copy
+            // segmentation data from the reference frame.
+            assert(hdr->primary_ref_frame != PRIMARY_REF_NONE);
             const int pri_ref = hdr->refidx[hdr->primary_ref_frame];
             hdr->segmentation.seg_data = c->refs[pri_ref].seg_data;
         }
-    } else if (hdr->primary_ref_frame == PRIMARY_REF_NONE) {
+    } else {
         memset(&hdr->segmentation.seg_data, 0, sizeof(Av1SegmentationDataSet));
         for (int i = 0; i < NUM_SEGMENTS; i++)
             hdr->segmentation.seg_data.d[i].ref = -1;
-    } else {
-        const int pri_ref = hdr->refidx[hdr->primary_ref_frame];
-        hdr->segmentation.seg_data = c->refs[pri_ref].seg_data;
     }
 #if DEBUG_FRAME_HDR
     printf("HDR: post-segmentation: off=%ld\n",