Privacy Implications of Transparent Pixels

November 15th, 2019 by Jacob Barkdull

Save color values from transparent pixels?

You may have seen it in your favorite image editing program...

GIMP PNG Export Dialog

What does this option do?

Well, it does what it says. If you leave this option checked when you export your image, any pixels you erased will be saved in the exported image. They will not be truly erased, just made fully transparent. In other words, the data that describes the color of each pixel will be preserved, they will just be made invisible. This option has privacy implications. With it enabled, what you erase from an image may still be present in transparent pixels.

As a demonstration, I provide you with this image...

Empty Image

Empty right? Well, do the following and you will see that it's not...

  1. Download this image. The first thing you may notice is the file size of the image, it is quite big for being completely empty. Normally truly empty images at this size are a couple of kB.
  2. Open the image in GIMP.
  3. Select the eraser tool.
  4. Check the "Anti-erase" option.
  5. Erase the image to reveal my beautiful secret picture.

This can be cool, one could perhaps use this as a basic way of sharing secret pictures and messages. What's more practical, though, is to remember to disable this option. Doing so reduces the file size of your exported images, and can protect your privacy by ensuring what you erased is actually removed from the file.

Thanks for coming to my TED Talk.

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November 15th, 2019
If you leave this option checked when you export your image, any pixels you erased will be saved in the exported image. They will not be truly erased, just made fully transparent. In other words, the data that describes the color of each pixel will be preserved, they will just be made invisible. This option has privacy implications. With it enabled, what you erase from an image may still be present in transparent pixels.
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