Auto-width and height calculations now round instead of floor.

resize.go

@@ -403,11 +403,11 @@ func imageCalculations(o *Options, inWidth, inHeight int) float64 {
 	// Fixed width, auto height
 	case o.Width > 0:
 		factor = xfactor
-		o.Height = int(math.Floor(float64(inHeight) / factor))
+		o.Height = roundFloat(float64(inHeight) / factor)
 	// Fixed height, auto width
 	case o.Height > 0:
 		factor = yfactor
-		o.Width = int(math.Floor(float64(inWidth) / factor))
+		o.Width = roundFloat(float64(inWidth) / factor)
 	// Identity transform
 	default:
 		o.Width = inWidth
@@ -418,6 +418,14 @@ func imageCalculations(o *Options, inWidth, inHeight int) float64 {
 	return factor
 }
 
+func roundFloat(f float64) int {
+	if f < 0 {
+		return int(math.Ceil(f - 0.5))
+	} else {
+		return int(math.Floor(f + 0.5))
+	}
+}
+
 func calculateCrop(inWidth, inHeight, outWidth, outHeight int, gravity Gravity) (int, int) {
 	left, top := 0, 0
 

resize_test.go

@@ -1,6 +1,9 @@
 package bimg
 
 import (
+	"bytes"
+	"image"
+	"image/jpeg"
 	"io/ioutil"
 	"os"
 	"path"
@@ -112,6 +115,24 @@ func TestResizeCustomSizes(t *testing.T) {
 	}
 }
 
+func TestResizePrecision(t *testing.T) {
+	// see https://github.com/h2non/bimg/issues/99
+	img := image.NewGray16(image.Rect(0, 0, 1920, 1080))
+	input := &bytes.Buffer{}
+	jpeg.Encode(input, img, nil)
+
+	opts := Options{Width: 300}
+	newImg, err := Resize(input.Bytes(), opts)
+	if err != nil {
+		t.Fatalf("Resize(imgData, %#v) error: %#v", opts, err)
+	}
+
+	size, _ := Size(newImg)
+	if size.Width != opts.Width {
+		t.Fatalf("Invalid width: %d", size.Width)
+	}
+}
+
 func TestRotate(t *testing.T) {
 	options := Options{Width: 800, Height: 600, Rotate: 270, Crop: true}
 	buf, _ := Read("fixtures/test.jpg")