1package captcha
  2
  3import (
  4	"image"
  5	"image/png"
  6	"io"
  7	"os"
  8	"rand"
  9	"time"
 10)
 11
 12const (
 13	// Standard width and height of a captcha image.
 14	StdWidth  = 300
 15	StdHeight = 80
 16
 17	maxSkew = 2
 18)
 19
 20type Image struct {
 21	*image.NRGBA
 22	primaryColor image.NRGBAColor
 23	numWidth     int
 24	numHeight    int
 25	dotSize      int
 26}
 27
 28func init() {
 29	rand.Seed(time.Seconds())
 30}
 31
 32// NewImage returns a new captcha image of the given width and height with the
 33// given digits, where each digit must be in range 0-9.
 34func NewImage(digits []byte, width, height int) *Image {
 35	img := new(Image)
 36	img.NRGBA = image.NewNRGBA(width, height)
 37	img.primaryColor = image.NRGBAColor{
 38		uint8(rand.Intn(129)),
 39		uint8(rand.Intn(129)),
 40		uint8(rand.Intn(129)),
 41		0xFF,
 42	}
 43	img.calculateSizes(width, height, len(digits))
 44	// Draw background (10 random circles of random brightness).
 45	img.fillWithCircles(10, img.dotSize)
 46	// Randomly position captcha inside the image.
 47	maxx := width - (img.numWidth+img.dotSize)*len(digits) - img.dotSize
 48	maxy := height - img.numHeight - img.dotSize*2
 49	x := rnd(img.dotSize*2, maxx)
 50	y := rnd(img.dotSize*2, maxy)
 51	// Draw digits.
 52	for _, n := range digits {
 53		img.drawDigit(font[n], x, y)
 54		x += img.numWidth + img.dotSize
 55	}
 56	// Draw strike-through line.
 57	img.strikeThrough()
 58	return img
 59}
 60
 61// WriteTo writes captcha image in PNG format into the given writer.
 62//
 63// Bug: while Image conforms to io.WriterTo interface, this function returns 0
 64// instead of the actual bytes written because png.Encode doesn't report this.
 65func (img *Image) WriteTo(w io.Writer) (int64, os.Error) {
 66	return 0, png.Encode(w, img)
 67}
 68
 69func (img *Image) calculateSizes(width, height, ncount int) {
 70	// Goal: fit all digits inside the image.
 71	var border int
 72	if width > height {
 73		border = height / 5
 74	} else {
 75		border = width / 5
 76	}
 77	// Convert everything to floats for calculations.
 78	w := float64(width - border*2)
 79	h := float64(height - border*2)
 80	// fw takes into account 1-dot spacing between digits.
 81	fw := float64(fontWidth) + 1
 82	fh := float64(fontHeight)
 83	nc := float64(ncount)
 84	// Calculate the width of a single digit taking into account only the
 85	// width of the image.
 86	nw := w / nc
 87	// Calculate the height of a digit from this width.
 88	nh := nw * fh / fw
 89	// Digit too high?
 90	if nh > h {
 91		// Fit digits based on height.
 92		nh = h
 93		nw = fw / fh * nh
 94	}
 95	// Calculate dot size.
 96	img.dotSize = int(nh / fh)
 97	// Save everything, making the actual width smaller by 1 dot to account
 98	// for spacing between digits.
 99	img.numWidth = int(nw)
100	img.numHeight = int(nh) - img.dotSize
101}
102
103func (img *Image) drawHorizLine(color image.Color, fromX, toX, y int) {
104	for x := fromX; x <= toX; x++ {
105		img.Set(x, y, color)
106	}
107}
108
109func (img *Image) drawCircle(color image.Color, x, y, radius int) {
110	f := 1 - radius
111	dfx := 1
112	dfy := -2 * radius
113	xx := 0
114	yy := radius
115
116	img.Set(x, y+radius, color)
117	img.Set(x, y-radius, color)
118	img.drawHorizLine(color, x-radius, x+radius, y)
119
120	for xx < yy {
121		if f >= 0 {
122			yy--
123			dfy += 2
124			f += dfy
125		}
126		xx++
127		dfx += 2
128		f += dfx
129		img.drawHorizLine(color, x-xx, x+xx, y+yy)
130		img.drawHorizLine(color, x-xx, x+xx, y-yy)
131		img.drawHorizLine(color, x-yy, x+yy, y+xx)
132		img.drawHorizLine(color, x-yy, x+yy, y-xx)
133	}
134}
135
136func (img *Image) fillWithCircles(n, maxradius int) {
137	color := img.primaryColor
138	maxx := img.Bounds().Max.X
139	maxy := img.Bounds().Max.Y
140	for i := 0; i < n; i++ {
141		setRandomBrightness(&color, 255)
142		r := rnd(1, maxradius)
143		img.drawCircle(color, rnd(r, maxx-r), rnd(r, maxy-r), r)
144	}
145}
146
147func (img *Image) strikeThrough() {
148	r := 0
149	maxx := img.Bounds().Max.X
150	maxy := img.Bounds().Max.Y
151	y := rnd(maxy/3, maxy-maxy/3)
152	for x := 0; x < maxx; x += r {
153		r = rnd(1, img.dotSize/2-1)
154		y += rnd(-img.dotSize/2, img.dotSize/2)
155		if y <= 0 || y >= maxy {
156			y = rnd(maxy/3, maxy-maxy/3)
157		}
158		img.drawCircle(img.primaryColor, x, y, r)
159	}
160}
161
162func (img *Image) drawDigit(digit []byte, x, y int) {
163	skf := rand.Float64() * float64(rnd(-maxSkew, maxSkew))
164	xs := float64(x)
165	minr := img.dotSize / 2               // minumum radius
166	maxr := img.dotSize/2 + img.dotSize/4 // maximum radius
167	y += rnd(-minr, minr)
168	for yy := 0; yy < fontHeight; yy++ {
169		for xx := 0; xx < fontWidth; xx++ {
170			if digit[yy*fontWidth+xx] != blackChar {
171				continue
172			}
173			// Introduce random variations.
174			or := rnd(minr, maxr)
175			ox := x + (xx * img.dotSize) + rnd(0, or/2)
176			oy := y + (yy * img.dotSize) + rnd(0, or/2)
177
178			img.drawCircle(img.primaryColor, ox, oy, or)
179		}
180		xs += skf
181		x = int(xs)
182	}
183}
184
185func setRandomBrightness(c *image.NRGBAColor, max uint8) {
186	minc := min3(c.R, c.G, c.B)
187	maxc := max3(c.R, c.G, c.B)
188	if maxc > max {
189		return
190	}
191	n := rand.Intn(int(max-maxc)) - int(minc)
192	c.R = uint8(int(c.R) + n)
193	c.G = uint8(int(c.G) + n)
194	c.B = uint8(int(c.B) + n)
195}
196
197func min3(x, y, z uint8) (o uint8) {
198	o = x
199	if y < o {
200		o = y
201	}
202	if z < o {
203		o = z
204	}
205	return
206}
207
208func max3(x, y, z uint8) (o uint8) {
209	o = x
210	if y > o {
211		o = y
212	}
213	if z > o {
214		o = z
215	}
216	return
217}
218
219// rnd returns a random number in range [from, to].
220func rnd(from, to int) int {
221	return rand.Intn(to+1-from) + from
222}