Try this: Reversing perspective

You will need

• A printed copy of this file on separate pages
• Coloured pencils
• Scissors
• Glue

What to do

1. Colour in the house picture.
2. Cut out the picture along the thick black lines.
3. Fold along the dotted lines so the picture sticks up in the middle like a flat-topped pyramid.
4. Put glue on the tabs marked ‘a’, and stick them down to keep the pyramid shape.
5. Put glue on the ‘b’ tabs, and stick the pyramid onto the rectangle on the other sheet of paper.

To view the illusion

1. Put your house picture up on the wall in a big room.
2. Go to the other side of the room and close one eye.
3. Look at your house picture – does it look like it goes in instead of sticking out?
4. Move around a bit and keep looking at the picture – does it move strangely?

What’s happening?

Your body has several different ways of seeing in 3D. You have two eyes for a good reason – they see the world from two slightly different angles. By comparing these two images, your brain can work out how close objects are. This type of 3D vision works best with objects that are close to you. When you are looking at objects far away, the images from your eyes are too similar and your brain can’t work out the distance. If you have one eye closed, then your brain doesn’t have a comparison at all – everything looks flat!

Your brain can also pick up clues from experience. You’re quite used to seeing rooms, so if you see something that looks like one, then your brain will try to match what you see with what you know. Rooms are hollow, so you imagine the house picture in this activity is also hollow, instead of sticking out as it does. As you move around looking at the picture, it doesn’t change the way you’d expect, leading to a very strange effect.

Applications

To create the illusion, this picture uses perspective drawing. This technique was originally developed hundreds of years ago to draw accurate pictures of objects in the real world. In perspective drawing, parallel lines such as straight train tracks are drawn so they would meet at a ‘vanishing point’. It also gives mathematical rules for drawing far away objects smaller.

A picture that follows these rules closely will look realistic and 3D. The house picture you made uses these rules to trick you – the walls get smaller as they move towards the centre, making them look further away, and the lines along the edges of the rug, pictures, skirting and cornices appear to meet at a ‘vanishing point’ in the centre of the image.

By David Shaw

More information

• Reverse perspective illusions
• The maths of perspective

Figure 1: Colour in the printout of the house.

Figure 2: Cut out the house along the thick black lines.

Figure 3: When you're viewing the illusion,try to get rid of big shadows by lighting from the front instead of above.