diff --git a/transform.go b/transform.go
index f199eb3..79c56db 100644
--- a/transform.go
+++ b/transform.go
@@ -79,13 +79,10 @@ func Transform(img []byte, opt Options) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
-// transformImage modifies the image m based on the transformations specified
-// in opt.
-func transformImage(m image.Image, opt Options) image.Image {
+func resizeParams(m image.Image, opt *Options) (w, h int) {
 	// convert percentage width and height values to absolute values
 	imgW := m.Bounds().Max.X - m.Bounds().Min.X
 	imgH := m.Bounds().Max.Y - m.Bounds().Min.Y
-	var w, h int
 	if 0 < opt.Width && opt.Width < 1 {
 		w = int(float64(imgW) * opt.Width)
 	} else if opt.Width < 0 {
@@ -110,9 +107,14 @@ func transformImage(m image.Image, opt Options) image.Image {
 			h = imgH
 		}
 	}
+	return
+}
 
-	// resize
-	if w != 0 || h != 0 {
+// transformImage modifies the image m based on the transformations specified
+// in opt.
+func transformImage(m image.Image, opt Options) image.Image {
+	// resize if needed
+	if w, h := resizeParams(m, &opt); w != 0 || h != 0 {
 		if opt.Fit {
 			m = imaging.Fit(m, w, h, resampleFilter)
 		} else {
diff --git a/transform_test.go b/transform_test.go
index 3c4b465..c4c0bf0 100644
--- a/transform_test.go
+++ b/transform_test.go
@@ -37,6 +37,27 @@ func newImage(w, h int, pixels ...color.NRGBA) image.Image {
 	return m
 }
 
+func TestResizeParams(t *testing.T) {
+	src := image.NewNRGBA(image.Rect(0, 0, 64, 128))
+	tests := []struct {
+		opt Options
+		w,h int
+	}{
+		{Options{Width:0.5}, 32, 0},
+		{Options{Height:0.5}, 0, 64},
+		{Options{Width:0.5, Height:0.5}, 32, 64},
+		{Options{Width:100, Height: 200}, 64, 128},
+		{Options{Width:64}, 64, 0},
+		{Options{Height:128}, 0, 128},
+	}
+	for ti,tt := range tests {
+		w,h := resizeParams(src, &tt.opt)
+		if w != tt.w || h!= tt.h {
+			t.Errorf("test %d problem, want: %d,%d got: %d,%d", ti, tt.w, tt.h, w, h)
+		}
+	}
+}
+
 func TestTransform(t *testing.T) {
 	src := newImage(2, 2, red, green, blue, yellow)