Using golang to mark the picture

WaterMark.go

package main

import (
	"bytes"
	"fmt"
	"strings"

	"io/ioutil"
	"os"
	"path"

	"math"
	"math/rand"
	"time"

	"image"
	"image/color"
	"image/draw"
	"image/jpeg"
	"image/png"

	"github.com/gonum/plot/vg"
	"github.com/gonum/plot/vg/vgimg"
)

func init() {
	rand.Seed(time.Now().UnixNano())
}

// WaterMark for adding a watermark on the image
func WaterMark(img image.Image, markText string) (image.Image, error) {
	// image's length to canvas's length
	bounds := img.Bounds()
	w := vg.Length(bounds.Max.X) * vg.Inch / vgimg.DefaultDPI
	h := vg.Length(bounds.Max.Y) * vg.Inch / vgimg.DefaultDPI
	diagonal := vg.Length(math.Sqrt(float64(w*w + h*h)))

	// create a canvas, which width and height are diagonal
	c := vgimg.New(diagonal, diagonal)

	// draw image on the center of canvas
	rect := vg.Rectangle{}
	rect.Min.X = diagonal/2 - w/2
	rect.Min.Y = diagonal/2 - h/2
	rect.Max.X = diagonal/2 + w/2
	rect.Max.Y = diagonal/2 + h/2
	c.DrawImage(rect, img)

	// make a fontStyle, which width is vg.Inch * 0.7
	fontStyle, _ := vg.MakeFont("Courier", vg.Inch*0.7)

	// repeat the markText
	markTextWidth := fontStyle.Width(markText)
	unitText := markText
	for markTextWidth <= diagonal {
		markText += " " + unitText
		markTextWidth = fontStyle.Width(markText)
	}

	// set the color of markText
	c.SetColor(color.RGBA{0, 0, 0, 122})

	// set a random angle between 0 and π/2
	θ := math.Pi * rand.Float64() / 2
	c.Rotate(θ)

	// set the lineHeight and add the markText
	lineHeight := fontStyle.Extents().Height * 1
	for offset := -2 * diagonal; offset < 2*diagonal; offset += lineHeight {
		c.FillString(fontStyle, vg.Point{X: 0, Y: offset}, markText)
	}

	// canvas writeto jpeg
	// canvas.img is private
	// so use a buffer to transfer
	jc := vgimg.PngCanvas{Canvas: c}
	buff := new(bytes.Buffer)
	jc.WriteTo(buff)
	img, _, err := image.Decode(buff)
	if err != nil {
		return nil, err
	}

	// get the center point of the image
	ctp := int(diagonal * vgimg.DefaultDPI / vg.Inch / 2)

	// cutout the marked image
	size := bounds.Size()
	bounds = image.Rect(ctp-size.X/2, ctp-size.Y/2, ctp+size.X/2, ctp+size.Y/2)
	rv := image.NewRGBA(bounds)
	draw.Draw(rv, bounds, img, bounds.Min, draw.Src)
	return rv, nil
}

// MarkingPicture for marking picture with text
func MarkingPicture(filepath, text string) (image.Image, error) {
	f, err := os.Open(filepath)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	img, _, err := image.Decode(f)
	if err != nil {
		return nil, err
	}

	img, err = WaterMark(img, text)
	if err != nil {
		return nil, err
	}
	return img, nil
}

func writeTo(img image.Image, ext string) (rv *bytes.Buffer, err error) {
	ext = strings.ToLower(ext)
	rv = new(bytes.Buffer)
	switch ext {
	case ".jpg", ".jpeg":
		err = jpeg.Encode(rv, img, &jpeg.Options{Quality: 100})
	case ".png":
		err = png.Encode(rv, img)
	}
	return rv, err
}

func main() {
	if len(os.Args) < 3 {
		panic(fmt.Errorf("Please using like this:$ %s [file or directory] [text]", os.Args[0]))
	}

	target := os.Args[1]
	text := os.Args[2]

	if stat, err := os.Stat(target); err == nil && stat.IsDir() {
		files, _ := ioutil.ReadDir(target)
		for _, fn := range files {
			img, err := MarkingPicture(path.Join(target, fn.Name()), text)
			if err != nil {
				continue
			}

			ext := path.Ext(fn.Name())
			base := strings.Split(fn.Name(), ".")[0] + "_marked"
			f, err := os.Create(base + ext)
			if err != nil {
				panic(err)
			}

			buff, err := writeTo(img, ext)
			if err != nil {
				panic(err)
			}
			if _, err = buff.WriteTo(f); err != nil {
				panic(err)
			}
		}
	} else {
		img, err := MarkingPicture(target, text)
		if err != nil {
			panic(err)
		}

		ext := path.Ext(target)
		base := strings.Split(path.Base(target), ".")[0] + "_marked"
		f, err := os.Create(base + ext)
		if err != nil {
			panic(err)
		}

		buff, err := writeTo(img, ext)
		if err != nil {
			panic(err)
		}

		if _, err = buff.WriteTo(f); err != nil {
			panic(err)
		}
	}
}

您好，请问不透明度怎么调