update to latest

escpos_printer.go

@@ -91,7 +91,8 @@ func toMonochrome(img image.Image) *image.Gray {
 	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
 		for x := bounds.Min.X; x < bounds.Max.X; x++ {
 			r, g, b, _ := img.At(x, y).RGBA()
-			grayValue := uint8((r + g + b) / 3 >> 8)
+			// grayValue := uint8((r + g + b) / 3 >> 8)
+			grayValue := uint8((0.299*float64(r) + 0.587*float64(g) + 0.114*float64(b)) / 256.0)
 			if grayValue > 128 {
 				gray.Set(x, y, color.White)
 			} else {
@@ -138,7 +139,6 @@ func escposRaster(img *image.Gray) []byte {
 			data = append(data, b)
 		}
 	}
-
 	return data
 }