Now that Twitter has changed how it handles uploaded images, this unexpected behavior is perhaps more important now than before.
Your challenge: Tell me what I've redacted from this image.
(Anybody I've talked to about this so far is ineligible to play)
It can be done w/o tools.
Your challenge: Tell me what I've redacted from this image.
(Anybody I've talked to about this so far is ineligible to play)
It can be done w/o tools.
Answer:
Several apps (e.g. @GIMP_Official, @Apple Preview) do not actually delete content from images with an alpha channel. They simply create an alpha-channel tunnel through the content you think that you're removing.
You may think you've removed content, but it's just hidden.
Several apps (e.g. @GIMP_Official, @Apple Preview) do not actually delete content from images with an alpha channel. They simply create an alpha-channel tunnel through the content you think that you're removing.
You may think you've removed content, but it's just hidden.
If you remove the alpha channel, you now can see what's behind it. You can do this with ImageMagick, e.g.
convert input.png -alpha off output.png
You now have an image that doesn't have the alpha channel, so therefore is unredacted.
But it's actually even easier than this!
convert input.png -alpha off output.png
You now have an image that doesn't have the alpha channel, so therefore is unredacted.
But it's actually even easier than this!
When you open an image in Twitter, there are URI arguments. For example, for the image for this thread, it's:
pbs.twimg.com/media/EN8zPZeX…
What if you change "format=png" to "format=jpg" ?
pbs.twimg.com/media/EN8zPZeX…
You how get the image, but Twitter removes the alpha channel for you!
pbs.twimg.com/media/EN8zPZeX…
What if you change "format=png" to "format=jpg" ?
pbs.twimg.com/media/EN8zPZeX…
You how get the image, but Twitter removes the alpha channel for you!
Similarly, even if you export directly to something that doesn't support alpha channels in Gimp (e.g. JPEG), by default you'll get a thumbnail in the EXIF data. Any guesses as to what that thumbnail contains?
I've found Apple Preview and GIMP to not remove image data properly. If you wish to test your own app, the easiest way is to:
1) Open pic with an alpha channel
2) Cut out content
3) Save as PNG
4) Send it to compress-or-die.com/analyze
1) Open pic with an alpha channel
2) Cut out content
3) Save as PNG
4) Send it to compress-or-die.com/analyze
And actually, as I verify this further, I believe that I was mistaken to say that Apple Preview is affected. I cannot seem to reproduce what I thought I originally saw with Preview.
So perhaps just GIMP for now.
I'm curious if anybody can confirm another misbehaving application.
So perhaps just GIMP for now.
I'm curious if anybody can confirm another misbehaving application.
And apparently this is "expected behavior" for GIMP users?
That is, when a user deletes pixels (e.g. Some are selected and the "Delete" key is pressed on the keyboard), the user should expect that the pixels aren't deleted. They are just made transparent.
Neat.
That is, when a user deletes pixels (e.g. Some are selected and the "Delete" key is pressed on the keyboard), the user should expect that the pixels aren't deleted. They are just made transparent.
Neat.
I can't help but think that this is the developer equivalent of "I reject your reality and substitute my own."
