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vp9-SVC: Fix to choose_partition when LAST ref is NULL.

This causes assert to trigger in choose_partitioning().
This can happen in some cases when enhancement layers
are enabled midway during the stream.

Change-Id: I69c3c8b4b1e3f1c7d8d7294d633ca5ddca148e8b

--- a/vp9/encoder/vp9_encodeframe.c

+++ b/vp9/encoder/vp9_encodeframe.c

@@ -1244,7 +1244,7 @@

   // For the variance computation under SVC mode, we treat the frame as key if

   // the reference (base layer frame) is key frame (i.e., is_key_frame == 1).

-  const int is_key_frame =

+  int is_key_frame =

       (cm->frame_type == KEY_FRAME ||

        (is_one_pass_cbr_svc(cpi) &&

         cpi->svc.layer_context[cpi->svc.temporal_layer_id].is_key_frame));

@@ -1254,6 +1254,15 @@

   int variance4x4downsample[16];

   int segment_id;

   int sb_offset = (cm->mi_stride >> 3) * (mi_row >> 3) + (mi_col >> 3);

+

+  // For SVC: check if LAST frame is NULL and if so treat this frame as a key

+  // frame, for the purpose of the superblock partitioning. This can happen

+  // (LAST is NULL) in some cases where enhancement spatial layers are enabled

+  // dyanmically in the stream and the only reference is the spatial

+  // reference (GOLDEN).

+  if (cpi->use_svc) {

+    if (get_ref_frame_buffer(cpi, LAST_FRAME) == NULL) is_key_frame = 1;

+  }

   set_offsets(cpi, tile, x, mi_row, mi_col, BLOCK_64X64);

   segment_id = xd->mi[0]->segment_id;