Javaでサムネイル画像のような縮小画像を作る

意外にもメジャーなライブラリはないっぽい。

JDKのImageIOを使う。

import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

import javax.imageio.ImageIO;

public class ImageReducer {

	private int width;

	private int height;

	public ImageReducer() {
		// nop
	}

	public ImageReducer(int width, int height) {
		this.width = width;
		this.height = height;
	}

	public int getWidth() {
		return width;
	}

	public void setWidth(int width) {
		this.width = width;
	}

	public int getHeight() {
		return height;
	}

	public void setHeight(int height) {
		this.height = height;
	}

	public InputStream reduce(InputStream original) {

		BufferedImage image = null;
		try {
			image = ImageIO.read(original);
		} catch (IOException e) {
			// handle exception
		}
		BufferedImage shrinkImage = new BufferedImage(this.width, this.height, image.getType());
		Graphics2D g2d = shrinkImage.createGraphics();
		g2d.setRenderingHint(RenderingHints.KEY_ALPHA_INTERPOLATION, RenderingHints.VALUE_ALPHA_INTERPOLATION_QUALITY);
		g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
		g2d.setRenderingHint(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
		g2d.setRenderingHint(RenderingHints.KEY_DITHERING, RenderingHints.VALUE_DITHER_ENABLE);
		g2d.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
		g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
		g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
		g2d.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
		g2d.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_NORMALIZE);
		g2d.drawImage(image, 0, 0, this.width, this.height, null);

		ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
		try {
			ImageIO.write(shrinkImage, "jpeg", outputStream);
			outputStream.flush();
			ByteArrayInputStream inputStream = new ByteArrayInputStream(outputStream.toByteArray());
			outputStream.close();
			return inputStream;
		} catch (IOException e) {
			// handle exception
		}
	}

}

サムネイル作成のクオリティ向上 - 侍ズム #samuraismを参考にさせていただきました。