以目标图片尺寸按原比例缩放切割图片
来源：http://my.oschina.net/u/873047/blog/106551
需求：原始图片缩放为目标尺寸后，要求缩放后的图片保持原图片的样式不改变，但是缩放后图片的四边可以裁减。
原理：将待裁剪图片宽高相除值和目标尺寸宽高相除值进行比较，如果前者较大，说明待裁剪图片相对于目标尺寸来说要宽出一块，现在已高为基准进行裁剪，即将原始图片的高度缩放到目标尺寸的高度。计算出原始图片高度和目标尺寸高度的比例，和原始图片宽度相乘，得到缩放后的宽度。此时的缩缩放后的高度就是目标尺寸的高度，但是宽度*肯定*会比目标尺寸宽度宽。将缩放后的多余的宽度分为两份右边和左边各裁去一份。
以宽为基准原理类似。
代码：
public class ImgUtil {
/**
*
* @param srcImgPath 待切割图片路径
* @param destImgPath 切割后图片路径
* @param destImgW 所需宽度
* @param destImgH 所需高度
*/
public static void createThumb (String srcImgPath , String destImgPath, int destImgW , int destImgH){
//原图片等比例缩小或放大之后的图片
int narrowImgW ;
int narrowImgH ;
//原图片大小
int srcImgW ;
int srcImgH ;
try {
BufferedImage bi = ImageIO. read(new File(srcImgPath));
srcImgW = bi.getWidth();
srcImgH = bi.getHeight();
// 转换图片尺寸与目标尺寸比较 ， 如果转换图片较小，说明转换图片相对于目标图片来说高较小，需要以高为基准进行缩放。
if((float )srcImgW /srcImgH > (float)destImgW / destImgH){
narrowImgW = ( int)(((float )destImgH / (float)srcImgH)*srcImgW);
narrowImgH = destImgH;
//按照原图以高为基准等比例缩放、或放大。这一步高为所需图片的高度，宽度肯定会比目标宽度宽。
int cutNarrowImgSize = (narrowImgW - destImgW)/2;
BufferedImage narrowImg = new BufferedImage(narrowImgW, narrowImgH,BufferedImage.TYPE_INT_RGB);
narrowImg.getGraphics().drawImage(bi.getScaledInstance(narrowImgW, narrowImgH, Image.SCALE_SMOOTH ), 0, 0, null);
//等比例缩放完成后宽度与目标尺寸宽度相比较 ， 将多余宽的部分分为两份 ，左边删除一部分
Image image = narrowImg.getScaledInstance(narrowImgW, narrowImgH, Image.SCALE_DEFAULT );
CropImageFilter cropFilter = new CropImageFilter(cutNarrowImgSize, 0, narrowImgW-cutNarrowImgSize, narrowImgH);
Image img = Toolkit.getDefaultToolkit().createImage( new FilteredImageSource(image.getSource(), cropFilter));
BufferedImage cutLiftNarrowImg = new BufferedImage( narrowImgW-cutNarrowImgSize, narrowImgH,BufferedImage.TYPE_INT_RGB );
cutLiftNarrowImg.getGraphics().drawImage(img, 0, 0, null);
//右边删除一部分
image = cutLiftNarrowImg.getScaledInstance(narrowImgW-cutNarrowImgSize, narrowImgH, Image.SCALE_DEFAULT );
cropFilter = new CropImageFilter(0, 0, narrowImgW-cutNarrowImgSize*2, narrowImgH);
img = Toolkit.getDefaultToolkit().createImage( new FilteredImageSource(image.getSource(), cropFilter));
BufferedImage cutRightNarrowImg = new BufferedImage( narrowImgW-cutNarrowImgSize*2, narrowImgH,BufferedImage.TYPE_INT_RGB );
Graphics g = cutRightNarrowImg.getGraphics();
g.drawImage(img, 0, 0, null); // 绘制截取后的图
g.dispose();
//输出为文件 最终为所需要的格式
ImageIO. write(cutRightNarrowImg, "JPEG", new File(destImgPath));
}
else{ //以宽度为基准
narrowImgW = destImgW;
narrowImgH = ( int) (((float )destImgW / (float)srcImgW)*srcImgH);
int cutNarrowImgSize = (narrowImgH - destImgH)/2;
BufferedImage narrowImg = new BufferedImage(narrowImgW, narrowImgH,BufferedImage.TYPE_INT_RGB);
narrowImg.getGraphics().drawImage(bi.getScaledInstance(narrowImgW, narrowImgH, Image.SCALE_SMOOTH ), 0, 0, null);
Image image = narrowImg.getScaledInstance(narrowImgW, narrowImgH, Image.SCALE_DEFAULT );
CropImageFilter cropFilter = new CropImageFilter(0, cutNarrowImgSize, narrowImgW, narrowImgH-cutNarrowImgSize);
Image img = Toolkit.getDefaultToolkit().createImage( new FilteredImageSource(image.getSource(), cropFilter));
BufferedImage cutTopNarrowImg = new BufferedImage( narrowImgW, narrowImgH-cutNarrowImgSize,BufferedImage. TYPE_INT_RGB);
cutTopNarrowImg.getGraphics().drawImage(img, 0, 0, null);
image = cutTopNarrowImg.getScaledInstance(narrowImgW, narrowImgH-cutNarrowImgSize, Image. SCALE_DEFAULT);
cropFilter = new CropImageFilter(0, 0, narrowImgW, narrowImgH-cutNarrowImgSize*2);
img = Toolkit.getDefaultToolkit().createImage( new FilteredImageSource(image.getSource(), cropFilter));
BufferedImage cutBottomNarrowImg = new BufferedImage( narrowImgW, narrowImgH-cutNarrowImgSize*2,BufferedImage. TYPE_INT_RGB);
Graphics g = cutBottomNarrowImg.getGraphics();
g.drawImage(img, 0, 0, null);
g.dispose();
ImageIO. write(cutBottomNarrowImg, "JPEG", new File(destImgPath));
}
} catch (IOException e) {
e.printStackTrace();
}
}
public static void main(String []args){
createThumb("G://3.png", "G://223.jpg", 320, 212);
}