diff --git a/src/ps_image.cxx b/src/ps_image.cxx
index 67d46b070449d83b3388e2e467a939553901d40c..61534238c29f276db6e70ee3cc7ff05b1a28f7f7 100644
--- a/src/ps_image.cxx
+++ b/src/ps_image.cxx
@@ -182,22 +182,13 @@ int Fl_PSfile_Device::alpha_mask(const uchar * data, int w, int h, int D, int LD
   return 0;
 }
 
+// bitwise inversion of all 4-bit quantities
+static const unsigned char swapped[16] = {0,8,4,12,2,10,6,14,1,9,5,13,3,11,7,15};
 
-
-// TODO: anybody has more efficient algoritm?
-static inline uchar swap_byte(const uchar i){
-  uchar b =0;
-  if(i & 1) b |= 128;
-  if(i & 2) b |= 64;
-  if(i & 4) b |= 32;
-  if(i & 8) b |= 16;
-  if(i & 16) b |= 8;
-  if(i & 32) b |= 4;
-  if(i & 64) b |= 2;
-  if(i & 128) b |= 1;
-  return b;
-}
-
+// bitwise inversion of a byte
+static inline uchar swap_byte(const uchar b){
+ return (swapped[b & 0xF] << 4) | swapped[b >> 4];
+ }
 
 extern uchar **fl_mask_bitmap;