Fix ransac random generator seeding

Ransac's get_rand_indices originally used rand_r seeded with the
same value every time, producing the same random sequence at every
iteration. This causes the global motion parameters to be slightly
less accurate because ransac cannot improve the model fit after
the first attempt.

Change-Id: Idca2f88468ea21d19ba41ab66e5a2744ee33aade

av1/encoder/ransac.c

diff --git a/av1/encoder/ransac.c b/av1/encoder/ransac.c
index 0c8ad67..714d567 100644
--- a/av1/encoder/ransac.c
+++ b/av1/encoder/ransac.c
@@ -92,16 +92,16 @@
   }
 }
 
-static int get_rand_indices(int npoints, int minpts, int *indices) {
+static int get_rand_indices(int npoints, int minpts, int *indices,
+                            unsigned int *seed) {
   int i, j;
-  unsigned int seed = (unsigned int)npoints;
-  int ptr = rand_r(&seed) % npoints;
+  int ptr = rand_r(seed) % npoints;
   if (minpts > npoints) return 0;
   indices[0] = ptr;
   ptr = (ptr == npoints - 1 ? 0 : ptr + 1);
   i = 1;
   while (i < minpts) {
-    int index = rand_r(&seed) % npoints;
+    int index = rand_r(seed) % npoints;
     while (index) {
       ptr = (ptr == npoints - 1 ? 0 : ptr + 1);
       for (j = 0; j < i; ++j) {
@@ -132,6 +132,7 @@
   int N = 10000, trial_count = 0;
   int i;
   int ret_val = 0;
+  unsigned int seed = (unsigned int)npoints;
 
   int max_inliers = 0;
   double best_variance = 0.0;
@@ -139,7 +140,7 @@
   WarpedMotionParams wm;
   double points1[2 * MAX_MINPTS];
   double points2[2 * MAX_MINPTS];
-  int indices[MAX_MINPTS];
+  int indices[MAX_MINPTS] = { 0 };
 
   double *best_inlier_set1;
   double *best_inlier_set2;
@@ -153,10 +154,6 @@
   double *cnp1, *cnp2;
   double T1[9], T2[9];
 
-  // srand((unsigned)time(NULL)) ;
-  // better to make this deterministic for a given sequence for ease of testing
-  srand(npoints);
-
   *number_of_inliers = 0;
   if (npoints < minpts * MINPTS_MULTIPLIER) {
     printf("Cannot find motion with %d matches\n", npoints);
@@ -203,7 +200,7 @@
     int num_degenerate_iter = 0;
     while (degenerate) {
       num_degenerate_iter++;
-      if (!get_rand_indices(npoints, minpts, indices)) {
+      if (!get_rand_indices(npoints, minpts, indices, &seed)) {
         ret_val = 1;
         goto finish_ransac;
       }