Added the start of a simple posterize effect.

filters/posterize.go

@@ -0,0 +1,43 @@
+package filters
+
+import (
+  "image"
+  "image/draw"
+  "image/color"
+  "github.com/disintegration/gift"
+)
+
+type PosterizeFilter struct {
+  Depth uint8
+}
+
+func Posterize(depth uint8) gift.Filter {
+  return PosterizeFilter{depth}
+}
+
+func (filter PosterizeFilter) Bounds(src image.Rectangle) (dst_bounds image.Rectangle) {
+  return src
+}
+
+func bin(c uint8) uint8 {
+  if c <= 64 {
+    return 0
+  } else {
+    return 255
+  }
+}
+
+func (filter PosterizeFilter) Draw(dst draw.Image, src image.Image, _ *gift.Options) {
+  bounds := src.Bounds()
+
+  for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+    for x := bounds.Min.X; x < bounds.Max.X; x++ {
+      from := src.At(x, y)
+      c := from.(color.NRGBA)
+      c.R = bin(c.R)
+      c.G = bin(c.G)
+      c.B = bin(c.B)
+      dst.Set(x, y, c)
+    }
+  }
+}

filters/upscale.go

@@ -12,7 +12,7 @@ type UpscaleFilter struct {
   PixelWidth int
 }
 
-func UpscaleImage(size image.Rectangle, width int) gift.Filter {
+func Upscale(size image.Rectangle, width int) gift.Filter {
   return UpscaleFilter{size, width}
 }
 

main.go

@@ -58,9 +58,10 @@ func main() {
   bounds := src.Bounds()
 
   resize := gift.Resize(bounds.Max.X / opts.PixelWidth, 0, gift.NearestNeighborResampling)
-  upscale := filters.UpscaleImage(bounds, opts.PixelWidth)
+  posterize := filters.Posterize(16)
+  upscale := filters.Upscale(bounds, opts.PixelWidth)
 
-  g := gift.New(resize, upscale)
+  g := gift.New(posterize, resize, upscale)
   smaller := image.NewNRGBA(g.Bounds(bounds))
   g.Draw(smaller, src)