如何在图像中设置 DPI 信息?

问题描述

我有一个应用程序，我想导出高分辨率(或更确切地说，高像素密度?)图像进行打印 - 例如，我想要以每英寸 250 点 (DPI) 打印的图像，而不是默认值，我理解为 72 DPI.

I have an application that I want to export high-resolution (or rather, high pixel density?) images for printing - for example, I want images that print at 250 dots per inch (DPI), instead of the default, which I understand to be 72 DPI.

我使用带有 Graphics2D 对象的 BufferedImage 来绘制图像，然后使用 ImageIO.write() 来保存图像.

I'm using a BufferedImage with a Graphics2D object to draw the image, then ImageIO.write() to save the image.

知道如何设置 DPI 吗?

Any idea how I can set the DPI?

推荐答案

Kurt 的回答指明了方向，但我还是花了很长时间才让它运行，所以这里是保存 PNG 时设置 DPI 的代码.要找到合适的作家，还有很多工作要做……

Kurt's answer showed the way, still it took me quite some time to get it run, so here is the code that sets DPI when saving a PNG. There is a lot to do to get the proper writers and such...

 private BufferedImage gridImage;
 ...

 private void saveGridImage(File output) throws IOException {
    output.delete();

    final String formatName = "png";

    for (Iterator<ImageWriter> iw = ImageIO.getImageWritersByFormatName(formatName); iw.hasNext();) {
       ImageWriter writer = iw.next();
       ImageWriteParam writeParam = writer.getDefaultWriteParam();
       ImageTypeSpecifier typeSpecifier = ImageTypeSpecifier.createFromBufferedImageType(BufferedImage.TYPE_INT_RGB);
       IIOMetadata metadata = writer.getDefaultImageMetadata(typeSpecifier, writeParam);
       if (metadata.isReadOnly() || !metadata.isStandardMetadataFormatSupported()) {
          continue;
       }

       setDPI(metadata);

       final ImageOutputStream stream = ImageIO.createImageOutputStream(output);
       try {
          writer.setOutput(stream);
          writer.write(metadata, new IIOImage(gridImage, null, metadata), writeParam);
       } finally {
          stream.close();
       }
       break;
    }
 }

 private void setDPI(IIOMetadata metadata) throws IIOInvalidTreeException {

    // for PMG, it's dots per millimeter
    double dotsPerMilli = 1.0 * DPI / 10 / INCH_2_CM;

    IIOMetadataNode horiz = new IIOMetadataNode("HorizontalPixelSize");
    horiz.setAttribute("value", Double.toString(dotsPerMilli));

    IIOMetadataNode vert = new IIOMetadataNode("VerticalPixelSize");
    vert.setAttribute("value", Double.toString(dotsPerMilli));

    IIOMetadataNode dim = new IIOMetadataNode("Dimension");
    dim.appendChild(horiz);
    dim.appendChild(vert);

    IIOMetadataNode root = new IIOMetadataNode("javax_imageio_1.0");
    root.appendChild(dim);

    metadata.mergeTree("javax_imageio_1.0", root);
 }

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