Fix scaling of QR codes in fixed sized images.

Previously each QR code module was the same number of pixels in width/height; this leads to some unfavourably small QR codes generated, with lots of white space.

Now, we map each image pixel to the nearest "pixel" in the QR code instead. QR code modules can now vary width/height by a pixel or two.

qrcode.go

@@ -282,12 +282,7 @@ func (q *QRCode) Image(size int) image.Image {
 		size = realSize
 	}
 
-	// Size of each module drawn.
-	pixelsPerModule := size / realSize
-
-	// Center the symbol within the image.
-	offset := (size - realSize*pixelsPerModule) / 2
-
+	// Output image.
 	rect := image.Rectangle{Min: image.Point{0, 0}, Max: image.Point{size, size}}
 
 	// Saves a few bytes to have them in this order
@@ -295,18 +290,20 @@ func (q *QRCode) Image(size int) image.Image {
 	img := image.NewPaletted(rect, p)
 	fgClr := uint8(img.Palette.Index(q.ForegroundColor))
 
+	// QR code bitmap.
 	bitmap := q.symbol.bitmap()
-	for y, row := range bitmap {
-		for x, v := range row {
+
+	// Map each image pixel to the nearest QR code module.
+	modulesPerPixel := float64(realSize) / float64(size)
+	for y := 0; y < size; y++ {
+		for x := 0; x < size; x++ {
+			y2 := int(float64(y) * modulesPerPixel)
+			x2 := int(float64(x) * modulesPerPixel)
+
+			v := bitmap[y2][x2]
 			if v {
-				startX := x*pixelsPerModule + offset
-				startY := y*pixelsPerModule + offset
-				for i := startX; i < startX+pixelsPerModule; i++ {
-					for j := startY; j < startY+pixelsPerModule; j++ {
-						pos := img.PixOffset(i, j)
-						img.Pix[pos] = fgClr
-					}
-				}
+				pos := img.PixOffset(x, y)
+				img.Pix[pos] = fgClr
 			}
 		}
 	}