Read & Write Image Metadata with Java—Part 1
In Part I we talk about the building blocks of metadata in images and how it may matter to you as developer.
Doing image (photo) metadata processing seems to be some serious voodoo magic. First, there is the alphabet soup to deal with such as IPTC, XMP, EXIF, IIM etc. Then there is the cumbersome support in Java, as the metadata support in Java’s ImageIO is limited. Fortunately, there are several amazing open-source libraries available to patch those gaps. Which library to use also depends on your use-cases: Do you solely need to read metadata or is the writing of metadata essential for your business use-cases?
In order to shed some light into this mess, I am doing a little mini-series. In this first part, I would like to examine the reasons of why image metadata may matter to you as application developer.
Why should I care?
To be aware of the metadata in images is actually quite important, even if you don’t care about the metadata themselves.
Normalizing uploaded image rotation
Do all your app-users see your images with apps/components that handle the Exchangeable image file format (Exif) orientation tag correctly? This is not so much an issue any longer in 2022, but in case you have custom apps that display images, or if you are dealing with legacy browsers etc., chances are that images may be displayed with an incorrect orientation. See the blog post Normalizing uploaded image rotation for further information.
Minimize unnecessary recompression of user images
Your applications may allow users to upload images. In many scenarios you may have to do some form of resizing which involves recompressing the uploaded images. However, sometimes users upload good images; so if you do some analysis, you can minimize that requirement resulting in better image quality overall. E.g. if the dimensions fit, and the compression level is decent, why re-compress? Check out the article Dimensions and Quality For Social Media Photo Sharing for further details.
Privacy concerns
Well, photos can contain A LOT of metadata and some of that data may provide too much identifying information. Every cell phone typically records latitude & longitude information from the Global Navigation Satellite System data (GNSS) in fairly high accuracy down to just a few meters. Detailed information on the used camera settings and the phone device may also be recorded. This may not necessarily be information you want to expose to the world. Thus, this may necessitate you to strip that information out.
Legal Issues
But hold on, depending on the jurisdiction you operate in, stripping certain metadata out of images might violate the law. See the following article on PetaPixel for more details.
Size & Performance
Image "metadata occupies 15.8% of the total image size on an average". That is the result of an analysis (from 2016) outlined in the article Impact of metadata on Image Performance. Not only does this potentially waste precious data but may also negatively impact the user-experience as images may take longer to load and display, as "EXIF metadata appears before other information such as the frame containing the height and width of the image and the scans containing the image data itself".
Search engine optimization (SEO)
Does image metadata help with your search engine results? The answer to this question is a bit murky. Google may or may not use image metadata to assess rankings of images in search results. Check the following article: Does EXIF Metadata in Images Affect Google Search Rankings?. Google does though use copyright and licensing information. See the Quick guide to IPTC Photo Metadata and Google Images for more details. Please also see Google’s own documentation on this matter: Image License in Google Images.
Where is meta-data stored?
This is where the alphabet soup starts. There are multiple locations inside an (JPEG) image where metadata can get stored. Most developers and (hobby-) photographers are probably aware of the first one:
Exif
The Exchangeable image file format (Exif) dates back to 1995 and is mostly used by cameras/phones to store photo and camera settings as well as location data. Exif data can also include copyright information, the name of the creator, and a caption. However, generally the Exif data is populated by the physical device and not by the user (via photo editing applications). The JPEG format defines several segments to store various kinds of data, such as the beginning (SOI) and end of the image (EOI), the actual image data, comment data (COM), but it also defines one or more application (APP) segments for application-specific data. The Exif data (up to 64kb) is stored (mostly) in the APP1 segment. The Exif tag structure is derived from the TIFF image format. Therefore, exif data is literally a TIFF image stored inside a JPEG image. Some additional Exif metadata may also be stored in the comment data (COM) segment and the APP2 segment (by the flashPix extension).
IPTC
As we have learned, Exif is typically populated by camera devices and contains mostly technical information. Then there is IPTC metadata, that describes the image itself and allows user-provided metadata to be added to images, for example via Adobe Photoshop or Adobe Lightroom. IPTC stands for International Press Telecommunications Council. IPTC and the Newspaper Association of America (NAA) (Yes, that old!) co-developed the Information Interchange Model (IIM) in 1990/1991. It “was the first multi-media news exchange format”. IIM metadata is stored in the APP13 segment of JPEG files.
IPTC then created a defined set of metadata specific for images that is stored using IIM. This then also got adopted by Adobe to add metadata in Photoshop to images. For more information, please see the IPTC website.
XMP
Adobe then adopted the IPTC metadata for XML. The eXtensible Metadata Platform (XMP) is basically an XML schema (using the Resource Description Framework (RDF) as storage format) that defines the IPTC attributes in XML. XMP has be standardized and since 2012 is a maintained ISO standard.
Maintain metadata in 2–3 places, oh my!
What this means for you as developer is that image metadata can live in up-to three!! locations in your image. How this is supported across relevant image processing applications and/or operating systems is a bit fuzzy and in case of Windows bordering the bizarre as Carl Seibert describes in his article titled XMP, IPTC/IIM, or Exif; which is preferred? The article provides an impressive summary of how metadata are supported across applications and operating systems in regard to Exif, IIM and XMP.
How can I see the metadata?
Every image application or operating system is using image metadata in some form or fashion. However, it is often confusing how the data relates to where it came from and even how the metadata property relates to what is shown in an image processing application such as Photoshop. Here is an example: what nowadays is often referred to as Job Identifier (Photoshop) is stored under the IPTC property OriginalTransmissionReference. Makes sense, doesn’t it?
The best (only?) comprehensive tool and reference for anything related to metadata is ExifTool. On MacOS, the installation using Homebrew is as simple as:
$ brew install exiftoolThen you can introspect your image metadata. To get all metadata:
$ exiftool your-image.jpgTo get the XMP data:
$ exiftool your-image.jpg -xmp:allTo get the IPTC/IIM data:
$ exiftool your-image.jpg -iptc:allTo get the EXIF data:
$ exiftool your-image.jpg -exif:allFor much more detail information on the background, please see the wonderful blog post by Carl Seibert titled XMP, IPTC/IIM, or Exif; which is preferred?.
What are my options with Java?
Now that we have provided a very abbreviated theoretical overview, it is time to apply the knowledge practically, and to explore our options in Java.
The good news is that you can get it all done using pure Java, meaning reading AND writing of image metadata. The bad news is that it gets a bit messy depending on what your use-cases are. If you’re interested in just reading metadata, you can probably stop reading this blog post right now — all you need is metadata-extractor. It is simple to use, supports every file format under the moon, basically the go-to library for reading metadata. Unfortunately, metadata-extractor cannot write metadata.
If you are interested in writing image metadata things get more messy, still. Writing the rather old-school EXIF and IIM/ITPC data is quite well supported using 3rd-party libraries such as TwelveMonkeys or Apache Commons Imaging.
Strangely enough, the modern option of XMP is rather complex to deal with.
Adobe, originally, did in fact have an XMP Toolkit for Java many moons ago but apparently decided to abandon it. I was eventually able to add XMP data to my images using Apache Commons Imaging in combination with Apache XML Graphics Commons (The name!). It is not perfect and support is a tad limited but at least it is possible.
In the next installment I will explore and provide code example of for the different ways you can skin the metadata cat. Stay tuned!
