[Java] 製作縮圖筆記

package test_image;

import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;

import javax.imageio.ImageIO;

public class TestImageResize {

    private static int targetWidth = 120;
    private static int targetHeight = 80;
    private static double targetRate = (double) targetWidth / targetHeight;

    public static void main(String[] args) throws Exception {

        System.out.printf("Target w:%s, h:%s, r:%s\n", 
            targetWidth, targetHeight, targetRate);


        BufferedImage image = ImageIO.read(new File("input.jpg"));

        int type = image.getType();
        if(type == 0) { type = BufferedImage.TYPE_INT_ARGB; }

        int width = image.getWidth();
        int height = image.getHeight();
        double rate = (double) width / height;

        System.out.printf("Source w:%s, h:%s, r:%s\n", width, height, rate);



        /* 等比例縮小至指定大小內 */
        int rWidth = targetWidth;
        int rHeight = targetHeight;

        if(width < targetWidth && height < targetHeight) {
            rWidth = width;
            rHeight = height;
        } else if(rate > targetRate) {
            rHeight = (int) (targetWidth / rate);
        } else {
            rWidth = (int) (targetHeight * rate);
        }
        System.out.printf("Resize w:%s, h:%s\n", rWidth, rHeight);

        BufferedImage resize1 = new BufferedImage(rWidth, rHeight, type);
        Graphics g1 = resize1.getGraphics();
        g1.drawImage(image, 0, 0, rWidth, rHeight, null);
        g1.dispose();

        ImageIO.write(resize1, "jpg", new File("output_1.jpg"));




        /* 等比例縮小填滿指定大小 */
        BufferedImage resize2 = new BufferedImage(targetWidth,targetHeight,type);
        Graphics g2 = resize2.getGraphics();

        int startX = 0;
        int startY = 0;
        int size = 0;

        if(rate > targetRate) {
            startX = (int) (width - height * targetRate) / 2;
            size = height;
        } else {
            startY = (int) (height - width / targetRate) / 2;
            size = width;
        }
        System.out.printf("x:%s, y:%s, size:%s\n", startX, startY, size);

        g2.drawImage(
            image,
            0, 0, targetWidth, targetHeight,
            startX, startY, (size + startX), (size + startY),
            null
        );

        g2.dispose();

        ImageIO.write(resize2, "jpg", new File("output_2.jpg"));
    }
}

參考文件：
Graphics (Java 2 Platform SE 6)