Can process a directory, force the build, and almost detect if there's a change. Color indexing is wrong in posterization and needs to enforce result width/height.

config.json

@@ -12,7 +12,7 @@
   "Exceptions": {
     "temp/painted_*.png": {
       "PixelWidth": 8,
-      "ColorDepth": 4,
+      "ColorDepth": 16,
       "DitherType": 2,
       "Width": 256,
       "Height": 256

main.go

@@ -24,7 +24,7 @@ func main() {
   opts := ParseOpts()
   config := LoadSettings(opts.Config)
 
-  err := RenderImages(config)
+  err := RenderImages(config, opts.Force)
 
   if err != nil {
     log.Fatalf("failed to walk dir %s: %v", config.Source, err)

processing.go

@@ -2,6 +2,7 @@ package main
 
 import (
   "os"
+  "errors"
   "io/fs"
   "fmt"
   "path/filepath"
@@ -124,7 +125,20 @@ func GetConversion(config Settings, path string) Conversion {
   return config.Base
 }
 
-func RenderImages(config Settings) error {
+func HasChanged(old_info fs.FileInfo, new_path string) bool {
+  new_info, err := os.Stat(new_path)
+
+  if errors.Is(err, fs.ErrNotExist) {
+    return true
+  } else if err != nil {
+    log.Printf("!!!! can't stat target: %s: %v", new_path, err)
+    return false
+  }
+
+   return old_info.ModTime().After(new_info.ModTime())
+}
+
+func RenderImages(config Settings, force bool) error {
   err := filepath.WalkDir(config.Source,
     func(path string, d fs.DirEntry, err error) error {
     if !d.IsDir() && Included(config, path) {
@@ -137,7 +151,15 @@ func RenderImages(config Settings) error {
 
       fmt.Println("FILE: ", path, "TARGET:", target)
       convert := GetConversion(config, path)
-      JankImage(convert, path, target)
+
+      path_info, err := d.Info()
+      if err != nil {
+        log.Fatalf("failed to stat %s: %v", path, err)
+      }
+
+      if force || HasChanged(path_info, target) {
+        JankImage(convert, path, target)
+      }
     } else {
       fmt.Println("SKIP:", path)
     }