The following java applet illustrates a physical property of the retina. The retina is the surface of the eye where the images of the world we see are projected. We may consider the retina as being separated into two regions: the fovea and the periphery. The fovea is a small region located at the centre of the retina, whereas the periphery is the region surrounding the fovea. The main difference between these two regions is the resolution power: image parts projected on the fovea are clear and on the periphery are blurred. This change in the resolution power is however progressive: the resolution power is roughly proportional to the inverse of the distance from the centre of the fovea. The best way to understand what all this means is to use the java applet, but first we should describe it.

The input square (the one at the left) is where you will have the opportunity to artistically express yourself by drawing lines and points of your favourite colours. The resolution of the input square is like the one of a TV screen: uniform. The magenta grid at the centre of the square corresponds to the location of the fovea on the output square.

Now, suppose that you stare with one eye at the centre of the input square where you drew your artistic creation. If you reduce the distance between your eye and the centre of the input square to about 15 cm
(don't do it!), then you will see how your artistic creation looks like when retinally transformed onto the output square. In order not to damage your eye, please, press instead the compute bottom to obtain the retinal transformation of your input creation. As you have understood, the cyan non-uniform grid in the output square corresponds to the periphery.

Before letting you play, please, read also the recommendations below:

1. Press the compute button once and be patient. On, the PC where this applet was developed it takes less than 10 sec to compute the output. On other machines, it may take up to 1 min to compute the output.
2. When you want to clean the squares, press reset In-Out one or more times until both squares are clean. Ignore the exception-handling messages.
3. Have fun!

What do the computed output results suggest? As you can observe, the parts of your creation drawn close to the centre of the input square are preserved, whereas those drawn around are non-uniformly smoothed. Thus, the results suggest that:

• parts of the visible world projected outside of the fovea provide little details about the world: shapes are strongly distorted, colours are partially preserved.
• parts of the visible world projected inside of the fovea are the only ones clearly visible to us.

To clarify all these definitions, we shall now have a look to the following animation.

The brain behind the retinal transformation algorithm used here is my supervisor. Note that in the literature, the proper name for such algorithm is "foveation algorithm".