BBC HomeExplore the BBC
This page has been archived and is no longer updated. Find out more about page archiving.

24 September 2014
NottinghamNottingham

BBC Homepage
»BBC Local
Nottingham
Things to do
People & Places
Nature
History
Religion & Ethics
Arts and Culture
BBC Introducing
TV & Radio

Sites near Nottingham

Derby
Humberside
Leicester
Lincolnshire
South Yorkshire

Related BBC Sites

England
 

Contact Us


SIRDS : An optical illusion
A lot of dots - Can you spot part of a Triceratops
A lot of dots or part of a Triceratops?

Remember those 3-D images that pop out of a jumble of dots? We revisit the optical illusion phenomena and ask an expert how they work.

- What is a SIRDS
- How do they work
- How do you see them
- Have a go

SEE ALSO
Peter's SIRDS:
- Triceratops
- X-Wing Fighter
- A face
- Starship Enterprise
- A glass

Just for fun:

Robin Hood name generator

Nottingham E-cards
WEB LINKS

Peter's SIRDS site
The BBC is not responsible for the content of external websites.

FACTS

It is generally easier to see stereograms under bright light.

To see stereo images, you need to have passable use of both eyes. If you wear glasses try with and without them on. Some short-sighted people can see them easier without their glasses on (if they get closer to the picture).

PRINT THIS PAGE
View a printable version of this page.
get in contact

Peter Chang is a reseach fellow in the School of Electrical and Electronic Engineering at the University of Nottingham.

He works in the field of applied optics such as using polarized light scattering to measure suspensions of very small particles (with diameters less than 200th of the typical thickness of human hair) and thermal emissions to characterise surface properties.

In his spare time he has produced a programme that creates SIRDS.

Have a go
Have a look at some of Peter's SIRDS and see if you can see them:

Triceratops
X-Wing Fighter
A face
The Starship Enterprise
A glass

We spoke to Peter to find out more about SIRDS...

Interview with Peter Chang - SIRDS expert
What is a SIRDS?
SIRDS stands for Single Image Random Dot Stereogram.

It's a type of optical illusion that exploits the way our brains work to figure out spatial information from stereographic vision.

Cognitive psychologists have been interested in how we see and assimilate information about our environment.

One such scientist, Bela Julesz, invented RDS to demonstrate that depth perception can be obtained from pictures of random dots if the image seen in each eye are subtly different (visual disparity).
What sparked your interest in SIRDS?
I first came across SIRDS in the spring of 1991 when a friend's friend in the computer department showed us them.

My interest in SIRDS was rewoken by a friend, Gareth Richards, who found some on the net and printed them out in May 1993.

From there, we took it upon ourselves to write a SIRDS program using proper 3D datasets in C with Xlib graphics.

By 15th July 1993, we released a free and simple SIRDS animation package called xps. Soon after getting hold of the Thimbleby paper, we modified our program to implement his algorithms and released xpgs, version 1.5 (the change of name was due to the existence of another free X package with the same name) on 26th October 1993.

Gareth also reworked the code to work on a PC and called the subsequent program 'SIRDSAni'.
How do they work?
To understand the mechanism which allows you to get this peculiar effect, we should take a look at the process of vision.

The feeling of depth that you get by looking at a statue instead of looking at a photo of the same statue is due to the fact that the human body has two eyes.

You only need one eye to get the general shape of the statue. It is the second eye that provides some extra information. This extra information is the depth of the various parts of the statue. In fact a photo gives just a two-dimensional representation, to get the third dimension you need some extra information.

By having two pictures of the same object, taken by two different positions, which is the case of the human eyes, you can get the third dimension.

You then transform this information to a SIRDS - My program (xpgs) constructs the two perspectives of an object and combines them to make a stereogram that can be viewed when cross-eyed or wide-eyed.

Does it say anything about you if you cannot manage to see the 3-D image?
Given 'normal', corrected binocular vision, there's no reason you cannot see the embedded image.

It can take a degree of practice before you can decouple your focusing mechanism from the convergence/divergence of your eyes. That is, you need to be able to go cross-eyed or wide-eyed whilst retaining your focus on the SIRDS.

Have you made money out of the program?
No, the program (xpgs) is free as in open source. We do have a shareware program (SIRDSANI) that runs in DOS but it's unlikely to run on modern versions of Windows. I think we may have had a couple of takers for that.

I've been commissioned to do a few things in the past and been paid either in kind or a token sum.

Apart from 'just for fun' can SIRDS be used for anything else?
Its novelty value is used as a gimmick in commercial art. I've had requests for information from a greeting cards manufacturer, the BBC itself, an advertising agency, a school teacher wanting to design a cover for their yearbook...


How do you see a SIRDS?
Information compiled by Stuart Inglis, Department of Computer Science, University of Waikato

Most stereogram pictures are usually generated so that if you look at (converge your eyes on) a position twice as far away as the picture, and focus on the picture, generally after a few minutes you see a surprising 3D image!

Most people find this extremely difficult for the first time. You have to focus on a point which is different from where you are looking. This is known as "de-coupling" your vision process.

Instinctively people focus at the same point they are looking at, and this is the main obstacle in seeing images of this type.

This is why most posters come with a reflective surface such as glass or plastic covering them - if you try to look at your reflection you will be looking at a point twice as far away as the actual poster.

There are many ways to teach this de-coupling to either yourself or to others, including (in almost no particular order):

The pull-back
Hold the picture (or move your face) so your nose is touching the picture. Most people can not possibly focus with something this close to their eyes, and they will be content with their inability to focus. With the picture up close, pretend that you are looking straight ahead, right through it. Now slowly pull the picture (or your face) away while keeping your eyes pointed straight ahead. If you do this slow enough, an image usually appears when the picture is at the correct distance.

The reflection
As mentioned above, with a reflective surface it is sometimes a lot easier to converge your eyes in the correct position. You simply focus on your nose or some central reflection in the picture, and wait until you focus on the image.

The drunk-eyes
This method is used to describe the feeling of the process of deconverging your eyes. It is very much like being drunk or having "staring-eyes". Your eyes don't look at the object, but rather through it. This state is common to some in the morning before the coffee caffine fix.

The wall, or the finger
Hold the picture so that it is half between you and a wall. Look over the top of the picture towards the wall, and focus on something such as a picture hook or mark. While keeping this gaze either slowly lift the picture or lower your eyes while keeping them converged on the wall.

A similar approach (but for cross-eyed viewing) is to stand arms length away from the picture and put your finger on the picture. While slowly pulling your finger towards your face, keep looking at your finger, you will notice the picture becoming blurry, and at an intermediate position you will (eventually) see the 3D image.

The see-through
Photocopy the picture onto a transparency. Then focus through the transparency onto something twice as far away. This is similar to 'the wall' except now you don't need to change the position of your gaze.

Wide-Eyes
This method involves building a device to widen your interocular distance, as well as allowing the adjustment of the convergence of your eyes. It's so simple, you almost don't have to be there!

To find out more about SIRDS visit Peter's website.

Top | Features Index | Home
Also in this section
Features
Wicked summer out gallery

Xylophone Man memorial

Jamcams Weather forecast - today and tomorrow News in brief
Meet the team - the webmasters Contact Us
BBC Nottingham website
London Road
Nottingham, NG2 4UU
(+44) 0115 955 0500
nottingham@bbc.co.uk



About the BBC | Help | Terms of Use | Privacy & Cookies Policy