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Geraldi Sutanto

Stripper for Beginners

/ 4 min read

how-to , util , imagemagick , metadata

Intro

I was applying for New Zealand visa and my photo got rejected for exceeding the max size AND not meeting the min resolution at the same time 😅. After spending more than 2 seconds trying to form a thought, I decided to just google my problem and hope someone already has.

A photo of a fictious book cover mocking programmer's habit of copy-pasting code from stackoverflow
The engineer’s bible

My first thought was to look for something to strip metadata from the image files. I found most stackoverflow/blogs recommend using exiftool, but I felt reluctant to install a new tool just for this. A couple searches later, I found that imagemagick can do the job. Thats good because I happen to have it installed from a past project.

Anyway here’s how I did it:

Enough talk, show me the code

Okay that should be enough words for SEO, here’s the code:

Setup

First you obviously need to have imagemagick installed, if you’re on a mac you can install it using brew:

Terminal window
brew install imagemagick

What you came here for

Then you can use the mogrify command to strip metadata from the image files.:

Terminal window
mogrify -strip *.png


# Example output:
# input.png PNG 995x956 995x956+0+0 8-bit TrueColorAlpha sRGB 938272B 0.040u 0:00.028
# input.png PNG 995x956 995x956+0+0 8-bit TrueColorAlpha sRGB 938272B 0.160u 0:00.159

Don’t worry about the .png this will work on file format as well. This command will strip any profiles, comments, or these PNG chunks: (bKGD, cHRM, EXIF, gAMA, iCCP, iTXt, sRGB, tEXt, zCCP, zTXt, date) from the image files. You can also use the -verbose flag to see what is being stripped:

Some tips

Terminal window
mogrify -strip -verbose *.png

You can also use the -format flag to convert the images to a different format, for example to convert all the images to webp:

Terminal window
mogrify -strip -format webp *.png

What if I want to keep the original files?

There’s also the convert command to save the output in a different name:

Terminal window
convert input.png -strip output.png

Likewise it has both -verbose and format flags as well:

Terminal window
convert input.png -strip -verbose output.png
convert input.png -strip -format webp output.webp

How do I check if the metadata has been stripped?

You can use the identify command to check if the metadata has been stripped from the image files:

Terminal window
identify -verbose input.png

You should see that the metadata has been stripped from the image files, here’s an example:

Sample output
Terminal window
Image:
  Filename: copas.png
  Permissions: rw-r--r--
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 995x956+0+0
  Units: Undefined
  Colorspace: sRGB
  Type: TrueColorAlpha
  Endianness: Undefined
  Depth: 8-bit
  Channels: 4.0
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
    Alpha: 1-bit
  Channel statistics:
    Pixels: 951220
    Red:
      min: 0  (0)
      max: 255 (1)
      mean: 157.46 (0.617489)
      median: 229 (0.898039)
      standard deviation: 110.821 (0.434593)
      kurtosis: -1.58947
      skewness: -0.493305
      entropy: 0.611703
    Green:
      min: 0  (0)
      max: 255 (1)
      mean: 197.068 (0.772817)
      median: 234 (0.917647)
      standard deviation: 66.679 (0.261486)
      kurtosis: 0.0415249
      skewness: -0.909079
      entropy: 0.604423
    Blue:
      min: 0  (0)
      max: 255 (1)
      mean: 191.779 (0.752075)
      median: 230 (0.901961)
      standard deviation: 70.4805 (0.276394)
      kurtosis: -0.512684
      skewness: -0.729027
      entropy: 0.611983
    Alpha:
      min: 255  (1)
      max: 255 (1)
      mean: 255 (1)
      median: 255 (1)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
      entropy: 0
  Image statistics:
    Overall:
      min: 0  (0)
      max: 255 (1)
      mean: 200.327 (0.785595)
      median: 237 (0.929412)
      standard deviation: 61.9952 (0.243118)
      kurtosis: -0.515157
      skewness: -0.532853
      entropy: 0.457027
  Rendering intent: Perceptual
  Gamma: 0.454545
  Chromaticity:
    red primary: (0.64,0.33,0.03)
    green primary: (0.3,0.6,0.1)
    blue primary: (0.15,0.06,0.79)
    white point: (0.3127,0.329,0.3583)
  Matte color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: black
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 995x956+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: TopLeft
  Properties:
    date:create: 2024-05-01T09:55:25+00:00
    date:modify: 2024-05-01T09:55:25+00:00
    date:timestamp: 2024-05-01T10:11:53+00:00
    png:IHDR.bit-depth-orig: 8
    png:IHDR.bit_depth: 8
    png:IHDR.color-type-orig: 6
    png:IHDR.color_type: 6 (RGBA)
    png:IHDR.interlace_method: 0 (Not interlaced)
    png:IHDR.width,height: 995, 956
    signature: 85e8424ae990ac2d1bfd4d344bd94a7c5962704ac942c6fd7b79e1ae15c50b09
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 980.178KiB
  Number pixels: 951220
  Pixel cache type: Memory
  Pixels per second: 67.3782MP
  User time: 0.020u
  Elapsed time: 0:01.014
  Version: ImageMagick 7.1.1-31 Q16-HDRI aarch64 22148 https://imagemagick.org

Goodbye

That’s all! I hope this helps you strip metadata from image files using imagemagick. Oh and wish me luck on my visa application 🇳🇿.