Exploring Photographic EXIF data (using PowerShell, of course)

My PowerShell project for the last few days has been to get into the EXIF data which is embedded in Pictures. It turns out to be a bit easier than I imagined.

The first thing I found is that the .NET framework has a "BITMAP" object-class which gives access to these properties. Just get a new instance of the object passing it the file name: this works for JPEGs and TIFFs. It doesn't work for RAW files from my camera (presumably because Windows doesn't see those as bit maps)

The Second thing I found is Powershell doesn't load the required assembly by default - which spawned the earlier post about VB complementing PowerShell. So before I can get the object I need to do this 

 [reflection.assembly]::loadfile( "C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Drawing.dll")

Then I can do this

 $foo=New-Object -TypeName system.drawing.bitmap -ArgumentList "C:\Dump\Unsorted Pictures\Tour\RAW\TdF23760.JPG"

The object has an array of propertyItems . Each PropertyItem object has an ID, a type, a size, and a value. This is pretty flexible but it is also a darn nuisance for anyone wanting to code for the first time, because you have to know, for example that field ID 271 (or 0x010F in hex) means "Maker name", and type 7 means string.  or ID 41996 (0xA40C hex) means "Subject distance" and type 3 means 16 bit integer; but the value isn't in feet or meters but  a value of 1 Means "Macro", 2 means "Close" and 3 means "Distant". Worst of all 37380 (0x9204) means "Exposure compensation" and type 10 means "Expressed as the Ratio of 2 32 bit signed integers". 

$foo.GetPropertyItem(271) will get the Maker name property (if it exists - it will return an error if the property hasn't been set), but before we can get properties we're interested in we need to find a list of IDs. I've already got EXIF UTILS from HUGSAN and it's documentation covers ID, so does the competing EXIFTOOL and there are others. Here are the main fields - not an exhaustive list.

 For completeness here's a table of type codes.

All these types might make you want to scream but we can cheat. Since all the integers seem to be padded with zeros, we can treat them as long integers. In practice no long integers are used to represent numbers greater than 2^31 so we can treat unsigned Long integers (and ordinary integers) as signed. Finally, Rational numbers are sometimes written as 0,0 to mean 0, but this will cause a divide by zero error. To avoid this we can change the second number from 0 to 1. Since the single number like 123 is  padded with zeros to take the same space as a long, if we treat it like a rational well get 123 / 0 which we convert to 123/1. So we can treat ALL numbers as ratios of signed longs.  I'll add one more refinement. If the value is 1/123 then it's probably representing a shutter speed which I want to see in that form, not 0.00813. So I just need to write [fanfare] my first PowerShell function. I can type this in at the prompt (deja Vu for commodore and Applesoft basics which I mentioned recently)

 function MakeNumber {
$First =$args[0].value[0] + 256 * $args[0].value[1] + 65536 * $args[0].value[2] + 16777216 * $args[0].value[3] ;
$Second=$args[0].value[4] + 256 * $args[0].value[5] + 65536 * $args[0].value[6] + 16777216 * $args[0].value[7] ; 
if ($first -gt 2147483648) {$first = $first  - 4294967296} ;
if ($Second -gt 2147483648) {$Second= $Second - 4294967296} ; 
if ($Second -eq 0) {$Second= 1} ; 
if (($first –eq 1) -and ($Second -ne 1)) {write-output ("1/" + $Second)} 
else {write-output ($first / $second)}
}

The first two lines, smash the two sets of 4 bytes together,  the next two lines convert large-enough numbers to two's compliment negative ones, the 5th line ensures we don't divide by zero but divide by one, and the 6th makes sure if the result is 1/ something it comes back as a fraction.   

 PS C:\Users\Jamesone> "Shutter speed= " + (makenumber $foo.GetPropertyItem(33434))
Shutter speed= 1/160PS C:\Users\Jamesone> "Apperture= f/" + (makenumber $foo.GetPropertyItem(33437))
Apperture= f/3.5 
PS C:\Users\Jamesone> "ISO= " + (makenumber $foo.GetPropertyItem(34855))
ISO= 100 
PS C:\Users\Jamesone> "Width= " + (makenumber $foo.GetPropertyItem(40962))
Width= 3872 

Now, it might be my naivety with Powershell but I couldn't find away to turn an array of bytes into a string, so I had to write my Second powershell function

 function MakeString { $s="" ; for ($i=0 ; $i -le $args[0].value.length; $i ++) {$s = $s+ [char]$args[0].value[$i] }; Write-Output $s}

Which is just a for next loop to convert the array to a string; here's the result  

 PS C:\Users\Jamesone> "Model= " + (makestring $foo.GetPropertyItem(272))
Model= PENTAX K10D

Technorati tags: Microsoft, Powershell, EXIF, Photography

• Anonymous
  January 01, 2003
  I nearly answered the first one before I answered the second. There's a list of fields that Windows desktop search understands here http://www.microsoft.com/technet/scriptcenter/topics/desktop/wdsprops.mspx Exiftool is very cool - Phil seems to be the definitive compiler of vendor specific data. I ended up with Exifutils from http://www.hugsan.com which also works well; but if I had got into Perl I would have go for Exiftool instead.

• Anonymous
  January 01, 2003
  The Equivalent for Windows is the Windows desktop search iFilter for JPEGs, and that puts all the properties into the Windows index (see http://blogs.technet.com/jamesone/archive/2007/07/10/vista-s-desktop-index-and-powershell.aspx for an example about how I get the properties out from there). However the are some properties which are there in the exif that are not there in the index (e.g spotlight's filter doesn't put in 35mm equivalent focal length - the windows ifilter doesn't put in ... [memory fails me] some fields. Which you'd use ... well it depends on the circumstances : and I haven't quite finished with this one :-)

• Anonymous
  July 14, 2007
  I'm enjoying your series on Powershell (it's cool that Windows finally has a good shell), but wow, that seems like a lot of work. On OS X, the EXIF data is imported by the Spotlight importer for JPEGs.  I can list all the metadata (see example below) and get at individual parts pretty trivially. But, if I wanted to fiddle with EXIF data, I'd probably use something like jhead (http://www.sentex.net/~mwandel/jhead/) or exiftags (http://johnst.org/sw/exiftags/).  They're both free and even have Windows binaries. Here's the output from mdls ~/Pictures/TestPic.jpg /Users/melissa/Pictures/TestPic.jpg ------------- kMDItemAcquisitionMake         = "Canon" kMDItemAcquisitionModel        = "Canon PowerShot SD700 IS" kMDItemAperture                = 4.65625 kMDItemAttributeChangeDate     = 2007-07-14 09:31:24 -0700 kMDItemBitsPerSample           = 32 kMDItemColorSpace              = "RGB" kMDItemContentCreationDate     = 2007-07-14 09:31:23 -0700 kMDItemContentModificationDate = 2007-07-14 09:31:23 -0700 kMDItemContentType             = "public.jpeg" kMDItemContentTypeTree         = ("public.jpeg", "public.image", "public.data", "public.item", "public.content") kMDItemDisplayName             = "TestPic.jpg" kMDItemEXIFVersion             = "2.2" kMDItemExposureMode            = 0 kMDItemExposureTimeSeconds     = 0.0025 kMDItemFlashOnOff              = 0 kMDItemFocalLength             = 18.633 kMDItemFSContentChangeDate     = 2007-07-14 09:31:23 -0700 kMDItemFSCreationDate          = 2007-07-14 09:31:23 -0700 kMDItemFSCreatorCode           = 0 kMDItemFSFinderFlags           = 1024 kMDItemFSInvisible             = 0 kMDItemFSIsExtensionHidden     = 0 kMDItemFSLabel                 = 0 kMDItemFSName                  = "TestPic.jpg" kMDItemFSNodeCount             = 0 kMDItemFSOwnerGroupID          = 20 kMDItemFSOwnerUserID           = 501 kMDItemFSSize                  = 1409646 kMDItemFSTypeCode              = 0 kMDItemHasAlphaChannel         = 0 kMDItemID                      = 2799798 kMDItemKind                    = "JPEG Image" kMDItemLastUsedDate            = 2007-07-14 09:31:23 -0700 kMDItemOrientation             = 0 kMDItemPixelHeight             = 2112 kMDItemPixelWidth              = 2816 kMDItemProfileName             = "sRGB Profile" kMDItemRedEyeOnOff             = 0 kMDItemResolutionHeightDPI     = 180 kMDItemResolutionWidthDPI      = 180 kMDItemUsedDates               = (2007-07-14 09:31:23 -0700) kMDItemWhiteBalance            = 0

• Anonymous
  July 19, 2007
  Actually, the focal length is in there:    kMDItemFocalLength = 18.633 although admittedly, it's hard to see it in with all the other metadata.  I don't know whether there are obscure EXIF tags that the Spotlight importer ignores, but it's certainly possible.  I can tell you all the metadata tags listed for images in Apple's documentation, which presumably get pulled from EXIF when possible, namely: kMDItemAcquisitionMake, kMDItemAcquisitionModel, kMDItemAlbum, kMDItemAperture, kMDItemBitsPerSample, kMDItemColorSpace, kMDItemEXIFVersion, kMDItemExposureMode, kMDItemExposureProgram, kMDItemExposureTimeSeconds, kMDItemExposureTimeString, kMDItemFNumber, kMDItemFlashOnOff, kMDItemFocalLength, kMDItemHasAlphaChannel, kMDItemISOSpeed, kMDItemLayerNames, kMDItemMaxAperture, kMDItemMeteringMode, kMDItemOrientation, kMDItemPixelHeight, kMDItemPixelWidth, kMDItemProfileName, kMDItemRedEyeOnOff, kMDItemResolutionHeightDPI, kMDItemResolutionWidthDPI and kMDItemWhiteBalance (taken from the Spotlight Metadata Attributes reference, at http://developer.apple.com/documentation/Carbon/Reference/MetadataAttributesRef/index.html). Thanks for the pointer to your earlier article.  It's cool that Windows now has something like Spotlight's Query Expression Syntax (see http://developer.apple.com/documentation/Carbon/Conceptual/SpotlightQuery/Concepts/QueryFormat.html).   BTW, in that earlier article, was that all the metadata Windows keeps, or did you cut it down to a subset to reduce clutter?

• Anonymous
  July 19, 2007
  I just played with exiftool (http://www.sno.phy.queensu.ca/~phil/exiftool/), and it pulled out more than 150 separate pieces of information out of the EXIF data for that photo, so clearly there is plenty of stuff that both Spotlight and Windows Desktop Search miss.  I also get the difference between focal length and 35mm focal length now (duh!). It's a cool tool. Written in Perl for easy programmatic access, cross-platform too, and so it runs happily in Windows.  It seems like it groks everything you could imagine, and makes it easy to do modifications like fixing times for those occasions when you forgot to adjust the clock on your camera.