shithub: libvpx

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Refine vp9_kmeans()

Reduce the number of group_idx initialization.
Initialize the center to the median of the data group.

Change-Id: Ie16150610480bf54a6b5e2bc048ba1e940bef10f

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

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

@@ -5841,7 +5841,7 @@

   // initialize the center points

   for (j = 0; j < k; ++j) {

-    ctr_ls[j] = arr[(size * j) / k].value;

+    ctr_ls[j] = arr[(size * (2 * j + 1)) / (2 * k)].value;

   }

   for (itr = 0; itr < 10; ++itr) {

@@ -5851,10 +5851,13 @@

       count[i] = 0;

     }

+    // Both the data and centers are sorted in ascending order.

+    // As each data point is processed in order, its corresponding group index

+    // can only increase. So we only need to reset the group index to zero here.

+    group_idx = 0;

     for (i = 0; i < size; ++i) {

-      // place samples into clusters

-      group_idx = 0;

       while (arr[i].value >= boundary_ls[group_idx]) {

+        // place samples into clusters

         ++group_idx;

         if (group_idx == k - 1) {

           break;