No need to use map, just use an array. Duh.

filters/posterize.go

@@ -17,13 +17,13 @@ const (
 
 type PosterizeFilter struct {
   Depth uint16
-  Bins map[uint16]uint16
+  Bins [math.MaxUint16]uint16
   DitherType int
 }
 
 func Posterize(depth uint16, dither_type int) gift.Filter {
   var i uint16
-  bins := make(map[uint16]uint16)
+  var bins [math.MaxUint16]uint16
   chunk := uint16((math.MaxUint16 + 1) / int(depth))
 
   // BUG: this was fine with uint8, now it's dumb as hell