Resizing Image (Jpeg) for golang

resize_jpg.go

package library

import (
	"bytes"
	"image"
	"image/jpeg"
	"io"

	"github.com/disintegration/imaging"
)

const (
	maxImageByteSize = 200 * 1024 // 200KB
	maxImagePx       = 1024       // px
	jpegQuality      = 90         // %

)

// 画像をリサイズする.
func ResizeImage(b []byte) ([]byte, error) {
	if len(b) < maxImageByteSize {
		return b, nil
	}

	r := bytes.NewReader(b)
	img, err := imaging.Decode(r, imaging.AutoOrientation(true))
	if err != nil {
		return nil, err
	}

	resized := scaleDown(img)

	// TODO: exifの書き戻し
	var buf bytes.Buffer
	if err := imaging.Encode(&buf, resized, imaging.JPEG, imaging.JPEGQuality(jpegQuality)); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil

}

// 画像を小さくリサイズする.
func scaleDown(src image.Image) image.Image {
	rect := src.Bounds()

	x := rect.Dx()
	y := rect.Dy()

	switch {
	case maxImagePx > x && maxImagePx > y:
		// サイズが小さい場合は何もしない
		return src
	case x > y:
		return imaging.Resize(src, maxImagePx, 0, imaging.MitchellNetravali)
	default:
		return imaging.Resize(src, 0, maxImagePx, imaging.MitchellNetravali)
	}
}

// IsJpegImage checks if the given bytes is jpeg file or not.
func IsJpegImage(b []byte) error {
	return isJpegImage(bytes.NewReader(b))
}

func isJpegImage(r io.Reader) error {
	_, err := jpeg.DecodeConfig(r)
	return err
}