add support for exif orientation tag

if any transformation is requested, first apply any additional
transformation necessary to correct for the EXIF orientation tag, since
it is stripped from the resulting image.

Fixes #63

transform.go

@@ -21,9 +21,11 @@ import (
 	_ "image/gif" // register gif format
 	"image/jpeg"
 	"image/png"
+	"io"
 	"math"
 
 	"github.com/disintegration/imaging"
+	"github.com/rwcarlsen/goexif/exif"
 	"golang.org/x/image/tiff"   // register tiff format
 	_ "golang.org/x/image/webp" // register webp format
 	"willnorris.com/go/gifresize"
@@ -32,6 +34,9 @@ import (
 // default compression quality of resized jpegs
 const defaultQuality = 95
 
+// maximum distance into image to look for EXIF tags
+const maxExifSize = 1 << 20
+
 // resample filter used when resizing images
 var resampleFilter = imaging.Lanczos
 
@@ -50,6 +55,15 @@ func Transform(img []byte, opt Options) ([]byte, error) {
 		return nil, err
 	}
 
+	// apply EXIF orientation for jpeg and tiff source images. Read at most
+	// up to maxExifSize looking for EXIF tags.
+	if format == "jpeg" || format == "tiff" {
+		r := io.LimitReader(bytes.NewReader(img), maxExifSize)
+		if exifOpt := exifOrientation(r); exifOpt.transform() {
+			m = transformImage(m, exifOpt)
+		}
+	}
+
 	// encode webp and tiff as jpeg by default
 	if format == "tiff" || format == "webp" {
 		format = "jpeg"
@@ -187,6 +201,57 @@ func cropParams(m image.Image, opt Options) (x0, y0, x1, y1 int, crop bool) {
 	return x0, y0, x1, y1, true
 }
 
+// read EXIF orientation tag from r and adjust opt to orient image correctly.
+func exifOrientation(r io.Reader) (opt Options) {
+	// Exif Orientation Tag values
+	// http://sylvana.net/jpegcrop/exif_orientation.html
+	const (
+		topLeftSide     = 1
+		topRightSide    = 2
+		bottomRightSide = 3
+		bottomLeftSide  = 4
+		leftSideTop     = 5
+		rightSideTop    = 6
+		rightSideBottom = 7
+		leftSideBottom  = 8
+	)
+
+	ex, err := exif.Decode(r)
+	if err != nil {
+		return opt
+	}
+	tag, err := ex.Get(exif.Orientation)
+	if err != nil {
+		return opt
+	}
+	orient, err := tag.Int(0)
+	if err != nil {
+		return opt
+	}
+
+	switch orient {
+	case topLeftSide:
+		// do nothing
+	case topRightSide:
+		opt.FlipHorizontal = true
+	case bottomRightSide:
+		opt.Rotate = 180
+	case bottomLeftSide:
+		opt.FlipVertical = true
+	case leftSideTop:
+		opt.Rotate = 90
+		opt.FlipVertical = true
+	case rightSideTop:
+		opt.Rotate = -90
+	case rightSideBottom:
+		opt.Rotate = 90
+		opt.FlipHorizontal = true
+	case leftSideBottom:
+		opt.Rotate = 90
+	}
+	return opt
+}
+
 // transformImage modifies the image m based on the transformations specified
 // in opt.
 func transformImage(m image.Image, opt Options) image.Image {