source: branches/TaskRewrite/src/plugins/acegi-0.5.1/src/templates/manager/controllers/_CaptchaController.groovy @ 58

import java.awt.Color
import java.awt.Font
import java.awt.Graphics2D
import java.awt.RenderingHints
import java.awt.geom.Rectangle2D
import java.awt.image.BufferedImage
import javax.imageio.ImageIO

class CaptchaController {

        private static final String SOURCECHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

        def index = {
                response.setContentType('image/png')
                response.setHeader('Cache-control', 'no-cache')

                // Generate and remember the Source Character string (6 characters)
                int l = SOURCECHARS.length()
                StringBuilder b = new StringBuilder()
                6.times {
                    int r = (int)(Math.random() * l)
                    b.append(SOURCECHARS.charAt(r))
                }

                final int height = 200
                final int width = 200
                final int space = 8

                System.setProperty('java.awt.headless', 'true')
                BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB)
                Graphics2D g2d = bufferedImage.createGraphics()
                Font font = new Font('Serif', Font.BOLD, 18)
                g2d.setFont(font)
                g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON)
                Rectangle2D fontRect = font.getStringBounds(b.toString(), g2d.getFontRenderContext())
                // Now, create a graphic 'space' pixels wider and taller than the the font
                bufferedImage = new BufferedImage((int)fontRect.getWidth() + space,
                                (int)fontRect.getHeight() + space,
                                BufferedImage.TYPE_INT_RGB)
                g2d = bufferedImage.createGraphics()
                g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON)
                g2d.setFont(font)

                // Draw the background
                g2d.setColor(Color.WHITE)
                g2d.fillRect(0, 0, width, height)

                // Draw the lines
                g2d.setColor(Color.GRAY)
                int x1
                int y1
                int x2
                int y2
                final int step = 10
                x1 = 0
                y1 = step
                x2 = step
                y2 = 0
                while (x1 < width || x2 < width || y1 < height || y2 < height) {
                    g2d.drawLine(x1, y1, x2, y2)
                    if (y1 < height) {
                                x1 = 0
                                y1 += step
                    }
                    else if (x1 < width) {
                                y1 = height
                                x1 += step
                    }
                    else {
                                x1 = width
                                y1 = height
                    }

                    if (x2 < width) {
                                y2 = 0
                                x2 += step
                    }
                    else if (y2 < height) {
                                x2 = width
                                y2 += step
                    }
                    else {
                                y2 = height
                                x2 = width
                    }
                }

                // Draw the String
                g2d.setColor(Color.BLACK)

                g2d.drawString(b.toString(), (int)(space/2), (int)(space/4) + (int)fontRect.getHeight())

                OutputStream out = response.getOutputStream()
                ImageIO.write(bufferedImage, 'PNG', out)
                out.close()

                session.setAttribute('captcha', b.toString())
        }
}