Make zoom levels descend to zero - ie '0' is the most zoomed out, where a single tile encompasses the entire image

main.go

@@ -8,18 +8,25 @@ import "flag"
 import "fmt"
 import "os"
 
+const currentVersion = "0.01"
+
 func main() {
 
-  filenamePtr  := flag.String("filename", "screenshot.png", "filename to open")
-  tileSizePtr := flag.Int   ("tile-size", 512, "tile size, in pixels")
+  filenamePtr    := flag.String("filename", "", "filename to open")
+  tileSizePtr    := flag.Int   ("tile-size", 256, "tile size, in pixels")
   concurrencyPtr := flag.Int ("concurrency", 5, "how many tiles to generate concurrently (threads)")
 
   flag.Parse()
 
+  if (*filenamePtr == "") {
+    fmt.Println("Error: You must specify a filename with --filename");
+    return;
+  }
+
   fmt.Println("opening file:", *filenamePtr)
   src, err := imaging.Open(*filenamePtr)
   if err != nil {
-    fmt.Println("could not open file:", err)
+    fmt.Println("Error: Could not open file:", err)
     return;
   }
 
@@ -28,7 +35,20 @@ func main() {
   tile_size_x := *tileSizePtr
   tile_size_y := *tileSizePtr
 
-  z := 0
+  // work out maximum zoom
+  var max_zoom int
+  zoom_test_size_x := size.X
+  zoom_test_size_y := size.Y
+  for max_zoom = 0 ; ; max_zoom++ {
+    if zoom_test_size_x < tile_size_x && zoom_test_size_y < tile_size_y {
+      break
+    }
+    zoom_test_size_x = zoom_test_size_x >> 1
+    zoom_test_size_y = zoom_test_size_y >> 1
+  }
+
+  z := max_zoom
+  fmt.Println("maximum zoom level is", max_zoom)
 
   concurrency := *concurrencyPtr
   sem := make(chan bool, concurrency)
@@ -37,7 +57,7 @@ func main() {
 
   // outer loop for zoom
   for {
-    if (z == 0) {
+    if (z == max_zoom) {
       // do nothing
     } else {
       // halve image size
@@ -45,10 +65,6 @@ func main() {
       runtime.GC()
       // recalculate size
       size = src.Bounds().Max
-      // we are done if we are now smaller then the tile
-      if (size.X < tile_size_x || size.Y < tile_size_y) {
-        break;
-      }
     }
 
     fmt.Print(fmt.Sprintf("zoom level: %d (%d x %d)\n", z, size.X, size.Y))
@@ -61,7 +77,10 @@ func main() {
 
     }
 
-    z++
+    z--
+    if (z < 0) {
+      break
+    }
   }
 
   // drain at the end of each zoom level