Re: How Do You Specify DPI in .png Image File

From:

Steve Yates <stevegoogle@REELpinball.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 9 Aug 2009 21:11:15 -0700 (PDT)

Message-ID:

<d067741a-0ae1-40ff-a7e1-2674767dd014@o35g2000vbi.googlegroups.com>

On Aug 9, 7:12 pm, markspace <nos...@nowhere.com> wrote:

Steve Yates wrote:

contains this information I will see if I can browse the metadata of
this file in Java and determine where this value is stored.

That's what I would do, just dump all the metadata and see what's in
there. Note that some devices or applications assume a default value i=

none is present, so if you don't see anything likely in your dump, you
might have to edit the DPI value to something else to make PSP actually
store a value.

This is what I came up with. It seems that there must be a more
direct way to get at some of these objects, but this does give me what
I was looking for. I am seeing an unwanted change in the metadata
though that concerns me.

    static private void writePngFile(RenderedImage image, String
filename, int dotsPerInch) {

        String dotsPerMeter = String.valueOf((int) (dotsPerInch / 0.0254));

        // retrieve list of ImageWriters for png images (most likely only
one but who knows)
        Iterator<ImageWriter> imageWriters =
ImageIO.getImageWritersByFormatName("png");

        // loop through available ImageWriters until one succeeds
        while (imageWriters.hasNext()) {
            ImageWriter iw = imageWriters.next();

            // get default metadata for png files
            ImageWriteParam iwp = iw.getDefaultWriteParam();
            IIOMetadata metadata = iw.getDefaultImageMetadata(new
ImageTypeSpecifier(image), iwp);

            // get png specific metatdata tree
            String pngFormatName = metadata.getNativeMetadataFormatName();
            IIOMetadataNode pngNode =
                (IIOMetadataNode) metadata.getAsTree(pngFormatName);

            // find pHYs node, or create it if it doesn't exist
            IIOMetadataNode physNode = null;
            NodeList childNodes = pngNode.getElementsByTagName("pHYs");
            if (childNodes.getLength() == 0) {
                physNode = new IIOMetadataNode("pHYs");
                pngNode.appendChild(physNode);
            } else if (childNodes.getLength() == 1) {
                physNode = (IIOMetadataNode) childNodes.item(0);
            } else {
                throw new IllegalStateException("Don't know what to do with
multiple pHYs nodes");
            }

            physNode.setAttribute("pixelsPerUnitXAxis", dotsPerMeter);
            physNode.setAttribute("pixelsPerUnitYAxis", dotsPerMeter);
            physNode.setAttribute("unitSpecifier", "meter");

            try {
                metadata.setFromTree(pngFormatName, pngNode);
                IIOImage iioImage = new IIOImage(image, null, metadata);
                File file = new File(filename);
                ImageOutputStream ios = ImageIO.createImageOutputStream(file);
                iw.setOutput(ios);
                iw.write(iioImage);
                ios.flush();
                ios.close();
            } catch (Exception e) {
                e.printStackTrace();
                continue;
            }

            break;
        }

    }

I copied a dump of the image metadata tree after the first line in the
try block blow. The pHYs block in the javax_imageio_png_1.0 is what
my code added, and the HorizontalPixelSize and VerticalPixelSize nodes
in the javax_imageio_1.0/Dimension block were added as a side effect.

That is fine, but the bitDepth in javax_imageio_png_1.0/IHDR changed
from 8 to 3, and the BitsPerSample in javax_imageio_1.0/Data changed
from "8 8 8" to "3 3 3". From the documentation I have seen, 3 is not
a valid value for bitDepth, so this concerns me. Everything else is
unchanged from the dump of the defaults retrieved from the writer.

When I read the file with an ImageReader and dump the metadata I see 8
for the bitDepth though, and when I open the document in Paint Shop
Pro it shows me the resolution is 300 dots per inch as I wanted. It
still shows the color depth as 24 bits/16 million colors, which is a
good sign, because I would expect only 256 colors with 3 bits. So
while I don't understand why the values are changing in the metadata
tree, they are somehow getting changed back before the image file is
actually written. So I guess this solution will work.

  javax_imageio_png_1.0:|null|
    IHDR:|null|
      Attributes
        width:|0|
        height:|0|
        bitDepth:|3|
        colorType:|RGB|
        compressionMethod:|deflate|
        filterMethod:|adaptive|
        interlaceMethod:|none|
    pHYs:|null|
      Attributes
        pixelsPerUnitXAxis:|11811|
        pixelsPerUnitYAxis:|11811|
        unitSpecifier:|meter|

  javax_imageio_1.0:|null|
    Chroma:|null|
      ColorSpaceType:|null|
        Attributes
          name:|RGB|
      NumChannels:|null|
        Attributes
          value:|3|
      BlackIsZero:|null|
        Attributes
          value:|true|
    Compression:|null|
      CompressionTypeName:|null|
        Attributes
          value:|deflate|
      Lossless:|null|
        Attributes
          value:|true|
      NumProgressiveScans:|null|
        Attributes
          value:|1|
    Data:|null|
      PlanarConfiguration:|null|
        Attributes
          value:|PixelInterleaved|
      SampleFormat:|null|
        Attributes
          value:|UnsignedIntegral|
      BitsPerSample:|null|
        Attributes
          value:|3 3 3|
    Dimension:|null|
      PixelAspectRatio:|null|
        Attributes
          value:|1.0|
      ImageOrientation:|null|
        Attributes
          value:|Normal|
      HorizontalPixelSize:|null|
        Attributes
          value:|0.08466683|
      VerticalPixelSize:|null|
        Attributes
          value:|0.08466683|
    Transparency:|null|
      Alpha:|null|
        Attributes
          value:|none|