A bit of cleanup before putting this in as a filter.

main.go

@@ -6,7 +6,7 @@ import (
   "image"
   "log"
   "image/png"
-  // "image/color"
+  "image/color"
   "github.com/disintegration/gift"
   // "math"
   "flag"
@@ -36,6 +36,26 @@ func SaveImage(filename string, img image.Image) {
   }
 }
 
+func UpscaleImage(bounds image.Rectangle, smaller *image.NRGBA, pixel_width int) *image.NRGBA {
+  upscale := image.NewNRGBA(bounds)
+  var from_x, from_y int
+  var from color.Color
+
+  for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+    for x := bounds.Min.X; x < bounds.Max.X; x++ {
+      if x % pixel_width == 0 {
+        from_x = x / pixel_width
+        from_y = y / pixel_width
+        from = smaller.At(from_x, from_y)
+      }
+
+      upscale.Set(x, y, from)
+    }
+  }
+
+  return upscale
+}
+
 type Opts struct {
   InFile string
   OutFile string
@@ -65,16 +85,7 @@ func main() {
   smaller := image.NewNRGBA(g.Bounds(bounds))
   g.Draw(smaller, src)
 
-  upscale := image.NewNRGBA(bounds)
-
-  for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
-    for x := bounds.Min.X; x < bounds.Max.X; x++ {
-      from_x := x / opts.PixelWidth
-      from_y := y / opts.PixelWidth
-      from := smaller.At(from_x, from_y)
-      upscale.Set(x, y, from)
-    }
-  }
+  upscale := UpscaleImage(bounds, smaller, opts.PixelWidth)
 
   SaveImage(opts.OutFile, upscale)
 }