Started the config file format and can process one image with it.

config.json

@@ -0,0 +1,20 @@
+{
+  "Base": {
+    "PixelWidth": 4,
+    "ColorDepth": 16,
+    "DitherType": 2,
+    "Width": 0,
+    "Height": 0
+  },
+  "Source": "tests/images/bash_test.png",
+  "Target": "atkinson.png",
+  "Exceptions": {
+    "temp/painted_*.png": {
+      "PixelWidth": 8,
+      "ColorDepth": 4,
+      "DitherType": 2,
+      "Width": 256,
+      "Height": 256
+    }
+  }
+}

main.go

@@ -3,11 +3,11 @@ package main
 import (
   "os"
   "image"
+  "encoding/json"
   "log"
   "image/png"
   "github.com/disintegration/gift"
   "flag"
-  "math"
   "lcthw.dev/go/jankifier/filters"
 )
 
@@ -36,44 +36,69 @@ func SaveImage(filename string, img image.Image) {
 }
 
 type Opts struct {
-  InFile string
-  OutFile string
+  Config string
+  Force bool
+}
+
+type Conversion struct {
   PixelWidth int
   ColorDepth int
   DitherType int
+  Width int
+  Height int
+}
+
+type Settings struct {
+  Source string
+  Target string
+  Base Conversion
+  Exceptions map[string]Conversion
+}
+
+func LoadSettings(path string) Settings {
+  var settings Settings
+
+  config, err := os.ReadFile(path)
+  if err != nil {
+      log.Fatalf("invalid config path: %s", path)
+  }
+
+  err = json.Unmarshal(config, &settings)
+  if err != nil {
+      log.Fatalf("json format error:", err)
+  }
+
+  return settings
 }
 
 func ParseOpts() Opts {
   var opts Opts
 
-  flag.StringVar(&opts.InFile, "input", "", "input file.png")
-  flag.StringVar(&opts.OutFile, "output", "", "output file.png")
-  flag.IntVar(&opts.PixelWidth, "pixel-width", 4, "pixel width")
-  flag.IntVar(&opts.ColorDepth, "color-depth", 16, "number of colors in the palette")
-  flag.IntVar(&opts.DitherType, "dither", 0, "0=none, 1=floyd, 2=atkinson")
+  flag.StringVar(&opts.Config, "config", "config.json", "config file")
+  flag.BoolVar(&opts.Force, "force", false, "force a full convert")
   flag.Parse()
 
-  if opts.ColorDepth > math.MaxUint8 {
-    log.Fatalf("color-depth can't be greater than %d", math.MaxUint8);
-  }
-
   return opts
 }
 
-func main() {
-  opts := ParseOpts()
-
-  src := LoadImage(opts.InFile)
+func JankImage(settings Conversion, in_file string, out_file string) {
+  src := LoadImage(in_file)
   bounds := src.Bounds()
 
-  resize := gift.Resize(bounds.Max.X / opts.PixelWidth, 0, gift.NearestNeighborResampling)
-  posterize := filters.Posterize(uint16(opts.ColorDepth), opts.DitherType)
-  upscale := filters.Upscale(bounds, opts.PixelWidth)
+  resize := gift.Resize(bounds.Max.X / settings.PixelWidth, 0, gift.NearestNeighborResampling)
+  posterize := filters.Posterize(uint16(settings.ColorDepth), settings.DitherType)
+  upscale := filters.Upscale(bounds, settings.PixelWidth)
   sharpen := gift.UnsharpMask(1, 1, 0)
 
   g := gift.New(resize, posterize, upscale, sharpen)
-  smaller := image.NewNRGBA(g.Bounds(bounds))
-  g.Draw(smaller, src)
+  out_img := image.NewNRGBA(g.Bounds(bounds))
+  g.Draw(out_img, src)
+
+  SaveImage(out_file, out_img)
+}
 
-  SaveImage(opts.OutFile, smaller)
+func main() {
+  opts := ParseOpts()
+  settings := LoadSettings(opts.Config)
+  JankImage(settings.Base, settings.Source, settings.Target)
 }