Segmentation map reference logic

The previous code raised an error if !segmentation.update_map but the
reference frame didn't yield any segmentation data. (The first "goto
error" that the patch removes happens if the reference frame was the
right size but had no segmentation data; the second happens if the
reference frame was the wrong size).

This doesn't match the logic in the description of
load_previous_segment_ids in section 6.8.2 of the spec.

This patch allows such streams, allocating and zeroing cur_segmap in
this case. It is still an error for a stream to signal a temporal
update but not to have valid segmentation data from the ref frame -
that's the error case that the patch puts back in.

1 files changed, 27 insertions, 13 deletions

diff --git a/src/decode.c b/src/decode.c
index ca8ea63..7438998 100644
--- a/src/decode.c
+++ b/src/decode.c
@@ -3090,6 +3090,13 @@ int dav1d_submit_frame(Dav1dContext *const c) {
 
     // segmap
     if (f->frame_hdr.segmentation.enabled) {
+
+        // By default, the previous segmentation map is not initialised.
+        f->prev_segmap_ref = NULL;
+        f->prev_segmap = NULL;
+
+        // We might need a previous frame's segmentation map. This
+        // happens if there is either no update or a temporal update.
         if (f->frame_hdr.segmentation.temporal || !f->frame_hdr.segmentation.update_map) {
             const int pri_ref = f->frame_hdr.primary_ref_frame;
             assert(pri_ref != PRIMARY_REF_NONE);
@@ -3097,30 +3104,37 @@ int dav1d_submit_frame(Dav1dContext *const c) {
             const int ref_h = ((f->refp[pri_ref].p.p.h + 7) >> 3) << 1;
             if (ref_w == f->bw && ref_h == f->bh) {
                 f->prev_segmap_ref = c->refs[f->frame_hdr.refidx[pri_ref]].segmap;
-                if (f->prev_segmap_ref == NULL) {
-                    res = -EINVAL;
-                    goto error;
+                if (f->prev_segmap_ref) {
+                    dav1d_ref_inc(f->prev_segmap_ref);
+                    f->prev_segmap = f->prev_segmap_ref->data;
                 }
-                dav1d_ref_inc(f->prev_segmap_ref);
-                f->prev_segmap = f->prev_segmap_ref->data;
-            } else {
-                f->prev_segmap_ref = NULL;
-                f->prev_segmap = NULL;
             }
-        } else {
-            f->prev_segmap_ref = NULL;
-            f->prev_segmap = NULL;
+            // It is an error to signal a temporal update if the
+            // previous frame was the wrong size or had no
+            // segmentation data.
+            if (f->frame_hdr.segmentation.temporal && !f->prev_segmap_ref) {
+                res = -EINVAL;
+                goto error;
+            }
         }
+
         if (f->frame_hdr.segmentation.update_map) {
+            // We're updating an existing map, but need somewhere to
+            // put the new values. Allocate them here (the data
+            // actually gets set elsewhere)
             f->cur_segmap_ref = dav1d_ref_create(f->b4_stride * 32 * f->sb128h);
             f->cur_segmap = f->cur_segmap_ref->data;
         } else if (f->prev_segmap_ref) {
+            // We're not updating an existing map, and we have a valid
+            // reference. Use that.
             f->cur_segmap_ref = f->prev_segmap_ref;
             dav1d_ref_inc(f->cur_segmap_ref);
             f->cur_segmap = f->prev_segmap_ref->data;
         } else {
-            res = -EINVAL;
-            goto error;
+            // We need to make a new map. Allocate one here and zero it out.
+            f->cur_segmap_ref = dav1d_ref_create(f->b4_stride * 32 * f->sb128h);
+            f->cur_segmap = f->cur_segmap_ref->data;
+            memset(f->cur_segmap_ref->data, 0, f->b4_stride * 32 * f->sb128h);
         }
     } else {
         f->cur_segmap = NULL;