Transparancy check

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"image"
 	"image/gif"
 	"image/jpeg"
 	"image/png"
@@ -112,6 +113,28 @@ func main() {
 
 }
 
+// Check image for transparency
+func Opaque(im image.Image) bool {
+    // Check if image has Opaque() method:
+    if oim, ok := im.(interface {
+        Opaque() bool
+    }); ok {
+        return oim.Opaque() // It does, call it and return its result!
+    }
+
+    // No Opaque() method, we need to loop through all pixels and check manually:
+    rect := im.Bounds()
+    for y := rect.Min.Y; y < rect.Max.Y; y++ {
+        for x := rect.Min.X; x < rect.Max.X; x++ {
+            if _, _, _, a := im.At(x, y).RGBA(); a != 0xffff {
+                return false // Found a non-opaque pixel: image is non-opaque
+            }
+        }
+
+    }
+    return true // All pixels are opaque, so is the image
+}
+
 // doSingleImage processes a single file and updateds the run stats as it goes
 func doSingleImage(f string, c config, s stats) stats {
 	st, err := os.Stat(f)
@@ -143,6 +166,12 @@ func doSingleImage(f string, c config, s stats) stats {
 		}
 	}
 	in.Close()
+	if !Opaque(orig) {
+		if !c.Quiet {
+			fmt.Printf("Skipping %s as it has transparancy", f)
+		}
+	return s
+	}
 	// Check bounds and resize if necessary
 	ores:=orig.Bounds()
 	if (c.MaxWidth !=0 && ores.Dx() > int(c.MaxWidth) ) || (c.MaxHeight!=0 && ores.Dy()>int(c.MaxHeight)) {