PC optical illusions

Non-existent objects
Dots in the grid

Hermann’s illusion
Very well-known optical illusion that results in the perception of blur half-transparent dots on cross sections of the elements of the grid. It was “discovered” by Ludimar Hermann in 1870. Look at some examples from the side (it is better seen on the right example). On the sections of lines on the grid you’ll see black opaque dots. But if you look straight into the intersection, you’ll see only the gap (white space) – dots can be seen in the sections that are not looked at directly.

AFTER IMAGES

After image – after watching without blinking you should see a portrait of a human (Jesus, if you like)

After images, or pa-images are optical illusions that use an adaptation of the eye, that after viewing without blinking on a neutral background, you see the negative of that image. Look at the picture on the right fixedly for twenty seconds (the easiest is to “stare” into one of four vertically set points in the middle). Immediately after that, look at a neutral (preferably white) background (blank paper, wall, etc.). On the background you’ll see a human portrait (originally this was a fake made with the intention of showing Jesus), who does not actually exist.

This is, already mentioned, sensory adaptation. Eye is automatically “used” to the current amount of light. In such contrast, some parts of the retina differently get use to the light. When one looks at a neutral background, parts of the retina still remain accustomed to their previous state, or brighten the parts that were too dark (by resulting with brighter areas on a neutral background), and darker the parts that were light (resulting with dark areas). Tentatively we say that the image remains on the retina for some time after.

Different perception of geometric figures

Necker cube and the two possible ways of perception

Schröder stairs – which flat is front: A or B?

With geometric objects that are drawn (the two-dimensional), and can be perceived as three-dimensional, we often can have different possibilities of perception. On the left side there is so called Necker cube, actually an ordinary projection of wire cube model in two dimensions. On the right side of the image there are two possible ways of perception – the first is such that the lower left square is the front flat of the cube, and other is such that the upper right square is front flat pf the cube. It is possible that these two ways exchange while watching the picture. If you perceive the cube in only one way, try blinking. Schröder stairs (on the right) work on the same principle – they can be perceived so that the A flat is in front of B flat and vice versa.
“Trick” is that the three-dimensional objects can not be projected on a two-dimensional surface without severe distortion. If the wire cube model was really ahead of us, we would know which is the front side because of the convergence of the eyes. As the paper (or screen) is two-dimensional, both squares are at the same distance from the eye which allows the brain to “imagine” the situation in both ways. It is interesting that the brain is almost impossible to imagine the figure of the cube as one-sided, or as a set of three squares (two large and one small) and two rectangular triangles. The same thing goes for Schröder stairs.

Misperceptions of size and shape

LENGTH LINE

Muller-Lyer’s illusion – all the horizontal lines are of equal length.

Ponza’s illusion – two horizontal lines are of equal length.
Which of the first two horizontal lines on the left picture is longer? Althought it appears that the second line is longer, they are of equal length. Likewise, on the third line above mark the middle, and after check with a meter (draw on paper). Undoubtedly you will mark the middle closer to the end (tail)of the arrow than it is.
One explanation is that parts of the arrow that “rise“ from the seemingly long line just visually extend the line. The second is that the brain perceives those lines as three-dimensional lines, and for angles assumes that they are real. Easier example to understand this explanation is Ponza’s illusion (picture on the right) – the brain percives a picture as rails (or the equivalent) and adjusts the horizontal lines to them. It is important to note that Muller-Lyer’s illusion does not deceive all  people. Those who live in an environment without a lot of right angles (small number of buildings, circular dwellings, etc.) will not be affected by the illusion, or they will say that the lines are of equal length.

LOCATION AND FORM LINES

The illusion of the wall – all the horizontal lines are straight and parallel.

 
Hering’s illusion – the red lines are straight and parallel.
One of the most famous illusions of this type is called illusion of the wall (Café wall illusion) by Richard Gregory (on the left). Horizontal lines of the wall are parallel. Each “brick” of the wall must be surrounded by a neutral part (in this case in gray color). Another such deception is the Hering’s illusion (Ewald Hering, 1861).
For both illusions our brain is in charge. With Hering’s, it perceives the directions that intersect in the middle (in our case the blue line) as parallel lines that connect seemingly in the distance. Red lines (which are parallel) then adjusts to the blue ones. The illusion of the wall has nothing to do with the perception of depth. Here the problem is in the gray lines (which are apparently sided). If you look to the reduced version (for example, this on the left), you will notice that the gray lines are without any problems seen on the combination of black and black “brick” and on the combination of white and white „brick“. However, on the combination of black and white “brick”, the line can not be seen well, and it looks like part of the black “brick”. However, if this were true, the gray lines would seem intermitten. Therefore, the brain seeks the simplest solution, and that is that there are straight lines between the layers of “brick”, but that they are inclined to one side.

WRONG PERCEPTION OF COLORS

Adelson’s illusion – the fields A and B are the same color. (animation)

Horizontal rectangle is filled with one color.
Fields A and B on the picture on the left are exactly the same shades of gray. If you do not believe, see GIF animation. Ted Adelson owns this illusion, who discovered it in 1995. Likewise, a long rectangle on the image to the right does not exceed the light in a darker shade of gray, but is in the same color on the entire length.
The mechanism is essentially simple – the brain puts colors in the context, which is perfectly logical (we recognize the color of the object and if it is in shadow or in the light). Because of that the brain in Adelson’s illusion takes into account the shadow that covers the field B and therefore determines that the field is in that color. On the right image seems that the color of a rectangle moves from lighter to darker as the brain (again) compares that rectangle with the environment (which goes from dark to light).

Ambiguous images

Duck or rabbit?
There are some pictures that we see in many ways. On the attached picture you can see the duck’s head (the beak is to the left) or rabbit (on the left there are ears, mouth on the right).
Our brain is always trying to interprete the optical stimuli (which are actually a set of colors) as a meaningful whole. It achieves that by observation of well-known forms and reasoning in accordance with them. However, there are forms that can be interpreted as a multiple of known cases. In this case, the left part of the object can remind you of rabbit ears or duck’s beak. The rest of the picture is designed to actually use both versions to perceive a meaningful whole.

Impossible images

M.C. Escher: Waterfall

Explore the green and red parts of the cube.

To actually exist, the cube must be cut.
If you look at the picture of M.S. Escher Waterfall, you’ll see that it falls into itslef, or flows upstream. Although it is not possible in reality, the picture at first glance seems completely normal. A little better focus on the pillars that hold the construction of flowing water. Confused?
Impossible cube uses two ways of perception of Necker’s cube and trys to merge them into one, so that the cube is divided into two parts that are displayed in different ways of perception. The cube is impossible in the real world, except when it is made as illustrated below (it is seen that the green sides are cut), a viewer would shut one eye and stop on the specific location. Penrose’s triangle and impossible “forks” use a similar mechanism.
Impossible images also use distortions while projection of three-dimensional bodys on two-dimensional surface, but in such a way that is impossible to make a drawed image(and not even imagine) in three dimensions.

Penrose’s triangle

Impossible “forks” – imagine them in a three-dimensional world.
Link to download examples of PC optical illusion: http://www.optike.hr/images/zanimljivosti/PC%20Iluzije.pps
Content is taken from the internet encyclopaedia: Wikipedija.com