Question

我正在使用以下代码来设置[ng文件的dpi。此代码仅显示在此站点的一个主题中。但是我无法理解此代码的第一行创建异常：

static BufferedImage gridImage;
....

private static 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 static void setDPI(IIOMetadata metadata) throws IIOInvalidTreeException {

// for PMG, it's dots per millimeter
double dotsPerMilli = 1.0 * 300 / 10 / 2.54;

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);
 }

例外是：

java.lang.IllegalArgumentException: image == null!

请帮我弄明白，因为我是java的新手..

Answer 1

显然从未分配

gridImage字段，当new IIOImage(gridImage, null, metadata)被调用时，它是空的。

在java中设置png图像的DPI

1 个答案: