Hackney Anarchists

Resizing images

Images often need to be resized for the web, partly to save storage space.

Cross-platform tools include: GIMP, ImageMagick.

Removing metadata

Images and other documents may contain meta data with identifying information which should be removed.

From the options listed below, using ExifTool seems best and Signal most convenient for images. PDFs can be cleaned with Linux command line tools. Note the caveats below.

Images

• ExifTool is a leading program for managing image meta data. It is used on the ‘command line’ rather than having a GUI (‘Graphical User Interface’).
  
  For example: exiftool -all= image.jpg removes meta data from image.jpg.
  
• Another option is mat2 (for Linux or Mac).:
  • For example: mat2 file.png
  • Note: mat2 does not clean files in-place, instead, it will produce a new file with the word “cleaned” in the filename
    
• Privacy friendly apps such as Signal remove meta data when images are added, so you could send an image to yourself and then download it. Note this is for images only.
  
• GIMP can remove meta data when re-exporting images from the File menu.
  
• There are in-built facilities on Windows and some mobile systems to remove image meta data, and third-party apps can be installed on Mac.
  • To view image meta data On Mac, you can use Preview: Tools: Inspect or command line mdls
  • Windows has options to remove meta data in the Properties window that can be opened via the right-click menu in File Explorer
    
• Check your phone settings to disable storing location and other meta data with pictures in the first place

PDFs

• PDFs are more problematic as removed meta data may be recoverable. Embedded images should also be checked.
  
• Some useful tools (mostly Linux-based):
  • ExifTool can be used to view and remove PDF meta data (noting warning about recoverability)
    • After running ExifTool the old information may be removed permanently using the qpdf utility, for example:
      qpdf --linearize in.pdf out.pdf
  • Using mat2 e.g. mat2 file.pdf
    • This can be installed e.g. on Linux with sudo apt install mat2
  • Images can be extracted from PDFs with the Linux pdfimages command
    

Caveats

• Some basic but unproblematic metadata is usually retained. Some reported meta data (such as file location and dates) describe the file on the current computer, rather than meta data in the file itself
• Also consider the storage and (secure) deletion of any original copies of files
• Other files not described here may also store meta data, for example Word processor documents.
• Also consider content, for example revealing details in images
• This guidance does not address general computing security, and wider concerns with proprietary operating systems (such as Mac or Windows) – Linux is preferable
• Using apps such as Signal assumes a degree of trust in the provider

See also

• https://everestpipkin.github.io/image-scrubber/