Implement variable sized image support.

qrcode.go

@@ -23,8 +23,16 @@ Two functions cover most use cases:
 
 	err := qrcode.WriteFile("https://example.org", qrcode.Medium, 256, "qr.png")
 
-Both examples use the qrcode.Medium error Recovery Level and create a 256x256
-pixel, black on white QR Code.
+Both examples use the qrcode.Medium error Recovery Level and create a fixed
+256x256px size, black on white QR Code.
+
+To generate a variable sized image instead, specify a negative size (in place of
+the 256 above), such as -4 or -5. Larger negative numbers create larger images:
+A size of -5 sets each module (QR Code "pixel") to be 5px wide/high.
+
+- Create a PNG image (variable size, with minimum white padding) and write to a file:
+
+	err := qrcode.WriteFile("https://example.org", qrcode.Medium, -5, "qr.png")
 
 The maximum capacity of a QR Code varies according to the content encoded and
 the error recovery level. The maximum capacity is 2,953 bytes, 4,296
@@ -53,7 +61,8 @@ import (
 // Encode a QR Code and return a raw PNG image.
 //
 // size is both the image width and height in pixels. If size is too small then
-// a larger image is silently returned.
+// a larger image is silently returned. Negative values for size cause a
+// variable sized image to be returned: See the documentation for Image().
 //
 // To serve over HTTP, remember to send a Content-Type: image/png header.
 func Encode(content string, level RecoveryLevel, size int) ([]byte, error) {
@@ -70,8 +79,9 @@ func Encode(content string, level RecoveryLevel, size int) ([]byte, error) {
 
 // WriteFile encodes, then writes a QR Code to the given filename in PNG format.
 //
-// size is both the width and height in pixels. If size is too small then a
-// larger image is silently written.
+// size is both the image width and height in pixels. If size is too small then
+// a larger image is silently written. Negative values for size cause a variable
+// sized image to be written: See the documentation for Image().
 func WriteFile(content string, level RecoveryLevel, size int, filename string) error {
 	var q *QRCode
 
@@ -218,11 +228,26 @@ func (q *QRCode) Bitmap() [][]bool {
 
 // Image returns the QR Code as an image.Image.
 //
-// size is both the width and height in pixels.
+// A positive size sets a fixed image width and height (e.g. 256 yields an
+// 256x256px image).
+//
+// Depending on the amount of data encoded, fixed size images can have different
+// amounts of padding (white space around the QR Code). As an alternative, a
+// variable sized image can be generated instead:
+//
+// A negative size causes a variable sized image to be returned. The image
+// returned is the minimum size required for the QR Code. Choose a larger
+// negative number to increase the scale of the image. e.g. a size of -5 causes
+// each module (QR Code "pixel") to be 5px in size.
 func (q *QRCode) Image(size int) image.Image {
 	// Minimum pixels (both width and height) required.
 	realSize := q.symbol.size
 
+	// Variable size support.
+	if size < 0 {
+		size = size * -1 * realSize
+	}
+
 	// Actual pixels available to draw the symbol. Automatically increase the
 	// image size if it's not large enough.
 	if size < realSize {
@@ -268,7 +293,8 @@ func (q *QRCode) Image(size int) image.Image {
 // PNG returns the QR Code as a PNG image.
 //
 // size is both the image width and height in pixels. If size is too small then
-// a larger image is silently returned.
+// a larger image is silently returned. Negative values for size cause a
+// variable sized image to be returned: See the documentation for Image().
 func (q *QRCode) PNG(size int) ([]byte, error) {
 	img := q.Image(size)
 
@@ -287,7 +313,8 @@ func (q *QRCode) PNG(size int) ([]byte, error) {
 // Write writes the QR Code as a PNG image to io.Writer.
 //
 // size is both the image width and height in pixels. If size is too small then
-// a larger image is silently written.
+// a larger image is silently written. Negative values for size cause a
+// variable sized image to be written: See the documentation for Image().
 func (q *QRCode) Write(size int, out io.Writer) error {
 	var png []byte
 
@@ -303,7 +330,8 @@ func (q *QRCode) Write(size int, out io.Writer) error {
 // WriteFile writes the QR Code as a PNG image to the specified file.
 //
 // size is both the image width and height in pixels. If size is too small then
-// a larger image is silently written.
+// a larger image is silently written. Negative values for size cause a
+// variable sized image to be written: See the documentation for Image().
 func (q *QRCode) WriteFile(size int, filename string) error {
 	var png []byte
 

qrcode/main.go

@@ -41,9 +41,6 @@ Usage:
 	}
 	flag.Parse()
 
-	if *size <= 0 {
-		checkError(fmt.Errorf("Error: value of -s should > 0"))
-	}
 	if len(flag.Args()) == 0 {
 		flag.Usage()
 		checkError(fmt.Errorf("Error: no content given"))