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How to read XML metadata from a TIFF/JPEG/DNG file with IDL

Atle Borsholm

This is an example showing how to extract various image metadata contained in an XML packet inside of a DNG file. The use of XML packets to store metadata is especially popular with UAV captured image data. The XML packets are not specific to the raw DNG image format, but can also occur in JPEG and TIFF image files. The code example below will work for JPEG, TIFF as well as DNG that contain an XML packet. In this case, I implemented the parsing code using an object class and static methods. This approach is nice if you have multiple functions that you want contained in the same .pro file, but that you can still easily call from the command line. Other methods could be added to this parser. Here is the content of my “dng__define.pro” source file:

; Extracts XML metadata from DNG
function dng::XmlMetadata, filename
  compile_opt idl2,logical_predicate,static
  info = OrderedHash()
  exif = read_exif(filename)
  xml = IDLffXmlDomDocument()
  xml->Load, STRING=string(exif['Exif_Image_XMLPacket'])
  iter = xml->CreateNodeIterator(obj_new(), what_to_show=5)
  node = iter->NextNode()
  while obj_valid(node) do begin
    if isa(node,'IDLffXmlDomText') then begin
      key = (node->GetParentNode())->GetTagName()
      info[key] = node->GetData()
    node = iter->NextNode()
  return, info
; Class definition
pro dng__define
  s = { dng, inherits IDL_Object }

I tested this code on a random sample DNG file that I found here:
http://www.phantompilots.com/threads/hands-on-experience-phantom-3-4k-plus-inside-the-hood-pictures.38694/ But, the code example is not specific to this file. It shows how to parse any XML packet embedded in a TIFF, JPEG, or DNG file.
The listing is rather lengthy, so, I am only including the first few lines to show how the code can be called:

IDL> dng.XmlMetadata(filename)
    "x:xmpmeta": "\n",
    "rdf:RDF": "\n\n",
    "rdf:Description": "\n ",
    "crs:Version": "3.7",
    "crs:RawFileName": "L1004220.DNG",
    "crs:WhiteBalance": "As Shot",
    "crs:Temperature": "6700",
    "crs:Tint": "+27",
    "crs:Exposure": "+0.20",
    "crs:Shadows": "0",
    "crs:Brightness": "+50",
    "crs:Contrast": "+32",

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