PNG grayscale alpha does not work

[pgundlach]

I can't include grayscale png images with alpha, they appear solid.

See this file:

package main

import (
	"github.com/signintech/gopdf"
)

func main() {
	pdf := gopdf.GoPdf{}
	pdf.Start(gopdf.Config{PageSize: gopdf.Rect{W: 595.28, H: 841.89}}) //595.28, 841.89 = A4
	pdf.AddPage()
	pdf.Image("gradient.png", 0, 0, nil)
	pdf.WritePdf("image.pdf")
}

and the following gradient.png:

The following diff solved it for me, but I don't know if this is correct or good.

diff --git a/image_obj_parse.go b/image_obj_parse.go
index 4304689..90375f5 100644
--- a/image_obj_parse.go
+++ b/image_obj_parse.go
@@ -427,17 +427,16 @@ func parsePng(f *bytes.Reader, info *imgInfo, imgConfig image.Config) error {
 
 				i++
 			}
-			info.smask, err = compress(alpha)
-			if err != nil {
-				return err
-			}
-
-			info.data, err = compress(color)
-			if err != nil {
-				return err
-			}
+		}
+		info.smask, err = compress(alpha)
+		if err != nil {
+			return err
 		}
 
+		info.data, err = compress(color)
+		if err != nil {
+			return err
+		}
 	} else {
 		info.data = data
 	}

[oneplus1000]

Yes, I think you got it right. I have solved as you suggested and pushed up already. thank you very much