--- src/lib/LRFCollector.cpp
+++ src/lib/LRFCollector.cpp
@@ -50,7 +50,7 @@
 {
   assert(0 == bg.a);
 
-  const double a = fg.a;
+  const double a = fg.a / 255.0;
 
   const double r = (1 - a) * bg.r + a * fg.r;
   const double g = (1 - a) * bg.g + a * fg.g;