golang png alpha channel issue

transform.go

package hidden_test

import (
	"image"
	"image/color"
	_ "image/jpeg"
	"image/png"
	"net/http"
	"os"
	"testing"

	"github.com/chai2010/webp"
)

func pixCcu(c color.Color) color.Color {
	rr, gg, bb, _ := c.RGBA()

	a := (rr + gg + bb) / 3

	return color.RGBA64{
		R: 65525,
		G: 65525,
		B: 65525,
		A: uint16(a),
	}
}

func Transform(img image.Image) (image.Image, error) {
	XBounds, YBounds := img.Bounds().Max.X, img.Bounds().Max.Y
	outImg := image.NewRGBA64(image.Rect(0, 0, XBounds, YBounds))

	for y := 0; y < YBounds; y++ {
		for x := 0; x < XBounds; x++ {
			outImg.Set(x, y, pixCcu(img.At(x, y)))
		}
	}

	return outImg, nil
}

func TestTransform(t *testing.T) {
	src, err := http.Get("https://key.visualarts.gr.jp/summer_rb/common/image/gallery_cg_02_l.jpg")
	if err != nil {
		t.Error(err)
	}

	img, _, err := image.Decode(src.Body)
	if err != nil {
		t.Error(err)
	}

	outImg, err := Transform(img)
	if err != nil {
		t.Error(err)
	}

	file, err := os.Create("output.webp")
	if err != nil {
		t.Error(err)
	}

	err = webp.Encode(file, outImg, &webp.Options{Lossless: true})
	if err != nil {
		t.Error(err)
	}

	file1, err := os.Create("output.png")
	if err != nil {
		t.Error(err)
	}

	err = png.Encode(file1, outImg)
	if err != nil {
		t.Error(err)
	}
}