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I've seen this sentiment mentioned quite a bit, but is it still true with the level of blur being shown in their example images? the blur level is extremely hig

I've seen this sentiment mentioned quite a bit, but is it still true with the level of blur being shown in their example images? the blur level is extremely high to the point that it has essentially left behind a smooth gradient. Even with the algorithm known is there enough reversible information left?

steerablesafe 6y ago
Well, if you want to leave behind a smooth gradient, then leave behind a smooth gradient. Suggestion for an algorithm: * start with the blur * sample the four colors at the four corners of the blurred region * quantize them * fill in the region with bilinear interpolation. Then your whole region can only reveal these four quantized color values. If you only blur then you will have a harder time proving the leaked information content.
- bigiain 6y ago
  Or perhaps * detect (or get the user to select) faces * replace the pixels in the face bounding box with a generic face * blur the bounding box edges * do whatever blur you think ends up "looking nice"
  - numlock86 6y ago
    It's missing the step of detecting and applying detected skin tone to the generic face. Right now I can't imagine people being to happy about "generic white face blur", implying the generic face would be white.
    - bigiain 6y ago
      I reckon all faces should de-blur into George Floyd's face... (From the examples in the Signal blog post, I don't think that grey gradient box is gonna be able to specifically imply white or POC faces...)
samstave 6y ago
Why dont they just literally cut-out/replace whatever would be blurred with just black pixels? Why blur anything anyway?
- hutzlibu 6y ago
  Aesthetic. A blurred face looks better on a picture than a fat black box. But like others have pointed out, you can achieve (allmost) the same effect, if you remove enough information before blurring, or just drawing a smooth gradient, but this alone is harder to make it look as nice, as blurring the actual image.
rainforest 6y ago
I suspect there's not much information in the individual blurred face, but I wonder if given enough examples you'd be able to determine if an unblurred face is the one in a sample of images with any level of confidence? You can do that with text (http://dheera.net/projects/blur http://dheera.net/projects/blur).
- KingOfCoders 6y ago
  The number of different characters is quite limited. Does this work for Chinese or only for Latin type languages?
- Arnt 6y ago
  The face and its surroundings are blurred almost to a single colour. The average RGB value of my face might be unique-ish, but if you mix in some variable background, photographed on a camera whose lens has been smeared against the pocket of someone's jeans, the result should be human, not individual. A side comment: AFAICT what the Signal developers have done is take code that was developed so that the phone camera could autofocus on faces, and and used that code to defocus faces. What a sweet hack.
  - TheCraiggers 6y ago
    > "The face and its surroundings are blurred almost to a single colour." To your eyes, maybe. To a machine, you have an array of pixels, each with different values which, using an algorithm, could be adjusted into something your eyes can resolve into a unique face.
    - Arnt 6y ago
      Seriously? Look at https://www.androidpolice.com/wp-content/uploads/2020/06/04/signal-face-blur-b.png https://www.androidpolice.com/wp-content/uploads/2020/06/04/... — do you really think there's enough information in those two rectangles to reconstruct the faces even approximately?
      - 6y ago
StavrosK 6y ago
Probably not. You can remove Gaussian blur by performing the inverse convolution (it's tricky because you need to find the actual parameters that caused the initial blur), you can remove motion blur the same way, etc. This looks like there isn't nearly enough information there to do any of this, though.
IshKebab 6y ago
No chance. That level of blur is essentially impossible to reverse. I think lots of people here are a bit confused because they know that all gaussian blurs are theoretically reversible. But they aren't thinking about how ill-conditioned the inverse gets as the blur gets larger and larger.
- laughinghan 6y ago
  There's another concern—even if I can't usably invert the convolution, if I have photos of a thousand people's faces and one of them is that blurred face, can I figure out which one with high confidence?
  - IshKebab 6y ago
    No, not with this level of blur.

steerablesafe 6y ago

Well, if you want to leave behind a smooth gradient, then leave behind a smooth gradient. Suggestion for an algorithm: * start with the blur * sample the four colors at the four corners of the blurred region * quantize them * fill in the region with bilinear interpolation. Then your whole region can only reveal these four quantized color values. If you only blur then you will have a harder time proving the leaked information content.

bigiain 6y ago
Or perhaps * detect (or get the user to select) faces * replace the pixels in the face bounding box with a generic face * blur the bounding box edges * do whatever blur you think ends up "looking nice"
- numlock86 6y ago
  It's missing the step of detecting and applying detected skin tone to the generic face. Right now I can't imagine people being to happy about "generic white face blur", implying the generic face would be white.
  - bigiain 6y ago
    I reckon all faces should de-blur into George Floyd's face... (From the examples in the Signal blog post, I don't think that grey gradient box is gonna be able to specifically imply white or POC faces...)

samstave 6y ago

Why dont they just literally cut-out/replace whatever would be blurred with just black pixels? Why blur anything anyway?

hutzlibu 6y ago
Aesthetic. A blurred face looks better on a picture than a fat black box. But like others have pointed out, you can achieve (allmost) the same effect, if you remove enough information before blurring, or just drawing a smooth gradient, but this alone is harder to make it look as nice, as blurring the actual image.

rainforest 6y ago

I suspect there's not much information in the individual blurred face, but I wonder if given enough examples you'd be able to determine if an unblurred face is the one in a sample of images with any level of confidence? You can do that with text (http://dheera.net/projects/blur http://dheera.net/projects/blur).

KingOfCoders 6y ago
The number of different characters is quite limited. Does this work for Chinese or only for Latin type languages?
Arnt 6y ago
The face and its surroundings are blurred almost to a single colour. The average RGB value of my face might be unique-ish, but if you mix in some variable background, photographed on a camera whose lens has been smeared against the pocket of someone's jeans, the result should be human, not individual. A side comment: AFAICT what the Signal developers have done is take code that was developed so that the phone camera could autofocus on faces, and and used that code to defocus faces. What a sweet hack.
- TheCraiggers 6y ago
  > "The face and its surroundings are blurred almost to a single colour." To your eyes, maybe. To a machine, you have an array of pixels, each with different values which, using an algorithm, could be adjusted into something your eyes can resolve into a unique face.
  - Arnt 6y ago
    Seriously? Look at https://www.androidpolice.com/wp-content/uploads/2020/06/04/signal-face-blur-b.png https://www.androidpolice.com/wp-content/uploads/2020/06/04/... — do you really think there's enough information in those two rectangles to reconstruct the faces even approximately?
    - 6y ago

StavrosK 6y ago

Probably not. You can remove Gaussian blur by performing the inverse convolution (it's tricky because you need to find the actual parameters that caused the initial blur), you can remove motion blur the same way, etc. This looks like there isn't nearly enough information there to do any of this, though.

IshKebab 6y ago

No chance. That level of blur is essentially impossible to reverse. I think lots of people here are a bit confused because they know that all gaussian blurs are theoretically reversible. But they aren't thinking about how ill-conditioned the inverse gets as the blur gets larger and larger.

laughinghan 6y ago
There's another concern—even if I can't usably invert the convolution, if I have photos of a thousand people's faces and one of them is that blurred face, can I figure out which one with high confidence?
- IshKebab 6y ago
  No, not with this level of blur.