diff --git a/tools/utils/images/opaque.go b/tools/utils/images/opaque.go
index e1187fdd2..d0e72c605 100644
--- a/tools/utils/images/opaque.go
+++ b/tools/utils/images/opaque.go
@@ -10,33 +10,33 @@ import (
 var _ = fmt.Print
 
 func IsOpaque(img image.Image) bool {
-	switch img.(type) {
+	switch i := img.(type) {
 	case *image.RGBA:
-		return img.(*image.RGBA).Opaque()
+		return i.Opaque()
 	case *image.RGBA64:
-		return img.(*image.RGBA64).Opaque()
+		return i.Opaque()
 	case *image.NRGBA:
-		return img.(*image.NRGBA).Opaque()
+		return i.Opaque()
 	case *image.NRGBA64:
-		return img.(*image.NRGBA).Opaque()
+		return i.Opaque()
 	case *image.Alpha:
-		return img.(*image.Alpha).Opaque()
+		return i.Opaque()
 	case *image.Alpha16:
-		return img.(*image.Alpha16).Opaque()
+		return i.Opaque()
 	case *image.Gray:
-		return img.(*image.Gray).Opaque()
+		return i.Opaque()
 	case *image.Gray16:
-		return img.(*image.Gray16).Opaque()
+		return i.Opaque()
 	case *image.CMYK:
-		return img.(*image.CMYK).Opaque()
+		return i.Opaque()
 	case *image.Paletted:
-		return img.(*image.Paletted).Opaque()
+		return i.Opaque()
 	case *image.Uniform:
-		return img.(*image.Uniform).Opaque()
+		return i.Opaque()
 	case *image.YCbCr:
-		return img.(*image.YCbCr).Opaque()
+		return i.Opaque()
 	case *NRGB:
-		return img.(*NRGB).Opaque()
+		return i.Opaque()
 	}
 	return false
 }
diff --git a/tools/utils/images/to_rgba.go b/tools/utils/images/to_rgba.go
index 5be973603..b312cf359 100644
--- a/tools/utils/images/to_rgba.go
+++ b/tools/utils/images/to_rgba.go
@@ -360,11 +360,11 @@ func (self *Context) paste_nrgba_onto_opaque(background *image.NRGBA, img image.
 
 // Paste pastes the img image to the background image at the specified position. Optionally composing onto the specified opaque color.
 func (self *Context) Paste(background image.Image, img image.Image, pos image.Point, opaque_bg *NRGBColor) {
-	switch background.(type) {
+	switch b := background.(type) {
 	case *image.NRGBA:
-		self.paste_nrgba_onto_opaque(background.(*image.NRGBA), img, pos, opaque_bg)
+		self.paste_nrgba_onto_opaque(b, img, pos, opaque_bg)
 	case *NRGB:
-		self.paste_nrgb_onto_opaque(background.(*NRGB), img, pos, opaque_bg)
+		self.paste_nrgb_onto_opaque(b, img, pos, opaque_bg)
 	default:
 		panic("Unsupported background image type")
 	}