因为java默认的图片像素分辨率DPI=72，分辨率有点低。所以研究了一下如何创建指定DPI的方案。

DPI： 指的是每英尺的像素点(dots per inch)

JPEG图片

JPEG图片的元数据定义参看oracle官网。
https://docs.oracle.com/javase/8/docs/api/javax/imageio/metadata/doc-files/jpeg_metadata.html

参数说明

如下代代码创建一张分辨率为300DPI（dots/inch）的jpg图片。

String path = "res.jpg";
final BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
Graphics2D g2d = image.createGraphics();
//        // fill all the image with white
g2d.setColor(Color.white);
g2d.fillRect(0, 0, w, h);

// Disposes of this graphics context and releases any system resources that it is using.
g2d.dispose();

ImageIO.write(image, "jpg", Files.newOutputStream(Paths.get(path)));

final ImageWriter writer = ImageIO.getImageWritersBySuffix("jpeg").next();
//        writeParams.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
//调整图片质量
//        writeParams.setCompressionQuality(1f);

ImageOutputStream ios = ImageIO.createImageOutputStream(new File(path));
writer.setOutput(ios);

ImageWriteParam param = writer.getDefaultWriteParam();
IIOMetadata metadata = writer.getDefaultImageMetadata(new ImageTypeSpecifier(image.getColorModel(), image.getSampleModel()), param);
Element tree = (Element) metadata.getAsTree("javax_imageio_jpeg_image_1.0");
Element jfif = (Element) tree.getElementsByTagName("app0JFIF").item(0);
jfif.setAttribute("Xdensity", "300");
jfif.setAttribute("Ydensity", "300");
jfif.setAttribute("resUnits", "1"); // density is dots per inch

metadata.mergeTree("javax_imageio_jpeg_image_1.0", tree);
writer.write(null, new IIOImage(image, null, metadata), param);
writer.dispose();

可以在ps上看到分辨率是300.（打开ps，点击“图像”->“图像大小”）

TIFF图片

commons-imaging 对Tiff的支持比较好。

使用commons-imaging 创建一张分辨率为300DPI（dots/inch）的tiff图片。

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-imaging</artifactId>
    <version>1.0-alpha3</version>
</dependency>

String path = "res.tif";
final BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
final TiffImagingParameters params = new TiffImagingParameters();
final PixelDensity pixelDensity = PixelDensity.createFromPixelsPerInch(300, 300);
params.setPixelDensity(pixelDensity);
TiffImageParser tiffImageParser = new TiffImageParser();
tiffImageParser.writeImage(image, new FileOutputStream(path), params);

final ImageInfo imageInfo = Imaging.getImageInfo(new File(path));
if (imageInfo != null) {
   final int xReadDPI = imageInfo.getPhysicalWidthDpi();
   final int yReadDPI = imageInfo.getPhysicalHeightDpi();
   System.out.println(xReadDPI);
   System.out.println(yReadDPI);
}

ps查看下分辨率。确实是300.