src/org/nutz/img/Images.java
public static BufferedImage cutoutByChannel(Object srcIm, int channel) {
        BufferedImage srcImage = read(srcIm);
        BufferedImage resultImage = new BufferedImage(srcImage.getWidth(),
                                                      srcImage.getHeight(),
                                                      BufferedImage.TYPE_4BYTE_ABGR);
        // 开始绘制
        for (int i = 0; i < srcImage.getWidth(); i++) {
            for (int j = 0; j < srcImage.getHeight(); j++) {
                int pixel = srcImage.getRGB(i, j);
                int alpha = 0;
                switch (channel) {
                case CHANNEL_RED:
                    alpha = Colors.getRGB(pixel)[0];
                    break;
                case CHANNEL_GREEN:
                    alpha = Colors.getRGB(pixel)[1];
                    break;
                case CHANNEL_BLUE:
                    alpha = Colors.getRGB(pixel)[2];
                    break;
                default:
                    alpha = Colors.getLuminance(srcImage, i, j);
                    break;
                }
                pixel = (alpha << 24) & 0xff000000 | (pixel & 0x00ffffff);
                resultImage.setRGB(i, j, pixel);
            }
        }

        return resultImage;
    }
public static BufferedImage cutoutByChannel(Object srcIm, int channel) {
        BufferedImage srcImage = read(srcIm);
        BufferedImage resultImage = new BufferedImage(srcImage.getWidth(),
                                                      srcImage.getHeight(),
                                                      BufferedImage.TYPE_4BYTE_ABGR);
        // 开始绘制
        for (int i = 0; i < srcImage.getWidth(); i++) {
            for (int j = 0; j < srcImage.getHeight(); j++) {
                int pixel = srcImage.getRGB(i, j);
                int alpha = 0;
                switch (channel) {
                case CHANNEL_RED:
                    alpha = Colors.getRGB(pixel)[0];
                    break;
                case CHANNEL_GREEN:
                    alpha = Colors.getRGB(pixel)[1];
                    break;
                case CHANNEL_BLUE:
                    alpha = Colors.getRGB(pixel)[2];
                    break;
                default:
                    alpha = Colors.getLuminance(srcImage, i, j);
                    break;
                }
                pixel = (alpha << 24) & 0xff000000 | (pixel & 0x00ffffff);
                resultImage.setRGB(i, j, pixel);
            }
        }

        return resultImage;
    }
根据指定通道的亮度值（灰度值）来自动计算哪些像素需要扣掉。

@param srcIm
源图片
@param channel
通道编号，0:red 1:green 2:blue 其他:亮度
@return 抠图后图片对象