# The Naked Scientists Forum

### Author Topic: Pixilated images  (Read 4345 times)

#### paul.fr

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##### Pixilated images
« on: 07/05/2007 06:49:31 »
Sometimes on the news they will pixilate a persons face. But if i squint whilst looking at the image i can make the face out. Why is that?

#### Batroost

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##### Pixilated images
« Reply #1 on: 07/05/2007 13:09:19 »
Ok, I think this is to do with spatial filtering...

One of the things that's important in optics is the concept of spatial frequency. This isn't the same as the light's frequency - which determines its colour - but is instead a way of describing an image. Every image can be treated as a mixture of simple repeating patterns. This is similar to the idea that any sound wave can be built-up from a variety of simpler sine waves of different frequencies. An image composed of a regular grid might only have a single spatial frequency, a more complicated image will have lots.

Now here's the odd bit. When light passes through a lens an image is transformed into a 'map' of its own spatial frequencies. This is known as a Fourier Trransform. Those components of an image that have a low spatial frequency will be closest to the focal point, those with a higher frequency will be further out from the centre of the beam. So, one of the tricks in optics is to put a tight mechanical iris around a beam, just at the focal point of a lens, and thereby remove all of the higher frequency components of an image. This will, for example, remove a 'pixelated' effect from an image.

When you squint you restrict the size of the aperture through which light must pass to get into your eye. This isn't ideal spatial filtering as it is the wrong side of the lens, and the cornea (which does a lot of the light bending) complicates things still fuirther. However, what I believe you are doing is a crude form of filtering that removes the regular pixelated effect from the image and smears out the data in a way that the brain more readily processes as a face.

Sound reasonable?

#### daveshorts

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##### Pixilated images
« Reply #2 on: 07/05/2007 18:56:40 »
This could be because your brain tends to split the world into objects- if there are lots of hard edges it sees the pixels as individual objects, but if you blur the picture it softens the edges so you see the groups of the pixels better.

#### JimBob

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##### Pixilated images
« Reply #3 on: 08/05/2007 03:08:09 »
Like me, your eyes are bad in just the right way - or your brain is mis-processing the image. It could be the time we spend staring at our computer screens. Heck, I don't really know. I just wanted to bug you!!

Heh, heh,heh, (chokes on spittle from his drool)

#### RD

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##### Pixilated images
« Reply #4 on: 08/05/2007 16:31:46 »
It is vitally important for an individual to differentiate friend from foe: there is strong pressure to evolve an acute ability to recognise faces.

The brain is "hard wired" to recognise faces: demonstrated by the fact that injury to a specific part of the brain causes "face blindness" (Prosopagnosia)
http://en.wikipedia.org/wiki/Prosopagnosia

#### neilep

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##### Pixilated images
« Reply #5 on: 08/05/2007 17:08:59 »
Sometimes on the news they will pixilate a persons face. But if i squint whilst looking at the image i can make the face out. Why is that?

This is cool..I will try this next time I see it on TV.

#### The Naked Scientists Forum

##### Pixilated images
« Reply #5 on: 08/05/2007 17:08:59 »