commit 5eeefbb1c5c1f294999c8fe518997686b4fba9ed
author Ruiling Song <ruiling.song@intel.com> Thu Aug 15 14:46:36 2019 +0800
committer Cheng Chen <chengchen@google.com> Fri Aug 16 16:47:46 2019 +0000
tree aaa386e3d32769a0ee1d3661118a2fe32c842e18
parent 0cce44c63391aedd4436ec0c940f8b84d6b1301f

Avoid underflow in exp()

Too small negative values will cause underflow in exp().
Here we only care the value that can be accumulated.
So the clamp has no effect on final result.
exp(-15)*1000*255 = 0.07
exp(-20)*1000*65535 = 0.13

BUG=aomedia:2433

Change-Id: I00d171822cbcc45c30e5c38987c900fb5cfba7d8

av1/encoder/temporal_filter.c

diff --git a/av1/encoder/temporal_filter.c b/av1/encoder/temporal_filter.c
index 8941760..e755595 100644
--- a/av1/encoder/temporal_filter.c
+++ b/av1/encoder/temporal_filter.c
@@ -693,7 +693,10 @@
       }
       diff_sse /= WINDOW_SIZE;
 
-      double w = exp(-diff_sse / (2 * beta * h * h));
+      double scaled_diff = -diff_sse / (2 * beta * h * h);
+      // clamp the value to avoid underflow in exp()
+      if (scaled_diff < -15) scaled_diff = -15;
+      double w = exp(scaled_diff);
       const int weight = (int)(w * SCALE);
 
       count[k] += weight;
@@ -736,7 +739,10 @@
       }
       diff_sse /= WINDOW_SIZE;
 
-      double w = exp(-diff_sse / (2 * beta * h * h));
+      double scaled_diff = -diff_sse / (2 * beta * h * h);
+      // clamp the value to avoid underflow in exp()
+      if (scaled_diff < -20) scaled_diff = -20;
+      double w = exp(scaled_diff);
       const int weight = (int)(w * SCALE);
 
       count[k] += weight;