diff --git a/detect.cc b/detect.cc
index e04f1bf..7d05fc7 100644
--- a/detect.cc
+++ b/detect.cc
@@ -210,10 +210,10 @@ void Detect::InputDetect(int *posx, int *posy) {
 		}
 }
 
-/* returns approx. gray value */
-inline uint16_t calc_gray(uint8_t r, uint8_t g, uint8_t b) {
-  /* Gray ~ 0.3*R + 0.59*G + 0.11*B -> approx. 0.3125*R + 0.625*G + 0.125*B */
-  return (r >> 2) + (r >> 3) + (g >> 1) + (g >> 3) + (b >> 3);
+/* returns approx. 8-bit gray value */
+inline uint8_t calc_gray(uint8_t r, uint8_t g, uint8_t b) {
+  /* Gray ~ 0.3*R + 0.59*G + 0.11*B -> approx. 0.28125*R + 0.625*G + 0.09375*B */
+  return (r >> 2) + (r >> 5) + (g >> 1) + (g >> 4) + (b >> 4) + (b >> 5);
 }
 
 /* returns 4-vector product */