implement web handler for QR code generation

qrcodegenerator.go

@@ -0,0 +1,90 @@
+package qrbill
+
+import (
+	"image"
+	"image/draw"
+	"os"
+
+	"github.com/boombuler/barcode"
+	"github.com/boombuler/barcode/qr"
+)
+
+// This is a port of the Java 1.7 reference example from paymentstandards.ch:
+// https://www.paymentstandards.ch/dam/downloads/qrcodegenerator.java
+//
+// The priority was to write idiomatic Go code first, and match the reference
+// example as good as possible second.
+
+const (
+	swissCrossEdgeSidePx = 166
+
+	swissCrossEdgeSideMm = 7
+
+	// The edge length of the qrcode inclusive its white border.
+	qrCodeEdgeSideMm = 42 + 13
+
+	qrCodeEdgeSidePx = swissCrossEdgeSidePx / swissCrossEdgeSideMm * qrCodeEdgeSideMm
+)
+
+func generateSwissQrCode(payload string) (image.Image, error) {
+	// generate the qr code from the payload
+	qrCodeImage, err := generateQrCodeImage(payload)
+	if err != nil {
+		return nil, err
+	}
+
+	// overlay the qr code with a Swiss Cross
+	return overlayWithSwissCross(qrCodeImage)
+}
+
+func generateQrCodeImage(payload string) (image.Image, error) {
+	code, err := qr.Encode(payload, qr.M, qr.Unicode)
+	if err != nil {
+		return nil, err
+	}
+	qrcode, err := barcode.Scale(code, qrCodeEdgeSidePx, qrCodeEdgeSidePx)
+	if err != nil {
+		return nil, err
+	}
+	return qrcode, nil
+}
+
+func overlayWithSwissCross(qrCodeImage image.Image) (image.Image, error) {
+	// TODO: bundle the swiss cross image instead of reading it
+	const swissCrossPath = "/home/michael/go/src/github.com/stapelberg/qrbill/third_party/swiss-cross/CH-Kreuz_7mm/CH-Kreuz_7mm.png"
+
+	f, err := os.Open(swissCrossPath)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	swissCrossImage, _, err := image.Decode(f)
+	if err != nil {
+		return nil, err
+	}
+
+	combinedQrCodeImage := image.NewRGBA(qrCodeImage.Bounds())
+
+	{
+		sr := qrCodeImage.Bounds() // source rect
+		destRect := image.Rectangle{image.Point{0, 0}, sr.Size()}
+		draw.Draw(combinedQrCodeImage, destRect, qrCodeImage, sr.Min, draw.Src)
+	}
+
+	{
+		sr := swissCrossImage.Bounds() // source rect
+		const swissCrossPosition = (qrCodeEdgeSidePx / 2) - (swissCrossEdgeSidePx / 2)
+		destPoint := image.Point{
+			X: swissCrossPosition,
+			Y: swissCrossPosition,
+		}
+		// Convert the source image bounds into the destination image’s coordinate
+		// space:
+		destRect := image.Rectangle{
+			destPoint,
+			destPoint.Add(sr.Size()),
+		}
+		draw.Draw(combinedQrCodeImage, destRect, swissCrossImage, sr.Min, draw.Src)
+	}
+	return combinedQrCodeImage, nil
+}