Blur or black box: how to hide details in a picture
Why blur and pixelate fall short
Pixelating replaces each small block of a picture with its average colour, so traces of the original remain. Security firm Bishop Fox built a tool called Unredacter that works backwards from pixelated text to recover it, and it advises against using pixelation, blurring or swirling to hide information.
That doesn't mean every blurred picture can be read. It means you can't be sure, and for a card number or an ID, it isn't worth the risk.
A quick comparison
| Method | How safe |
|---|---|
| Solid black box, saved as a new picture | The safest choice. The pixels underneath are replaced. |
| Blur or pixelate | Not safe for text. Traces of the original remain. |
| Highlighter or thin pen | Risky. See-through colour or gaps can leave text readable. |
| Cropping | Fine if the app saves a new picture. Some crop tools can keep the original underneath, so check. |
How to do it well
- Cover generously. Make the box bigger than the text, with no gaps.
- Check for repeats. Details often appear twice, like the long code at the bottom of a passport, or a signature.
- Save a new copy, and send that one. Keep the original safe, or delete it.
- Zoom in and check the copy before you send it.
- Remove the location. Pictures can carry hidden details too. Here's how to remove them.
What about PDFs?
A black box drawn over text in a PDF may not remove the text underneath, so someone could still select and copy it. Use a tool made for redacting PDFs, or turn the page into a picture first.
How FirstHour helps
Black out saves a copy with solid black boxes over what it finds, and lets you add, move, resize and turn your own. Before saving, it reads the copy again and warns you if it can still read a name or number. The copy has no location or camera details, and your original doesn't change. It only covers what it finds and doesn't find faces, so look over the whole picture yourself.
Questions
Can blurred text really be read?
Possibly. Pixelated text has been recovered with a special tool, and the same security firm advises against blur too, though it didn't demonstrate that on blur. You can't tell in advance, so use a solid box for anything important.
Should I blur faces?
Blur may be enough for casual privacy, but a solid box is the safer choice when it really matters.