Do you see that symmetrical reason a little floral in the center of the cover image? Here is that reason actually in that image is not there. Try to zommare the image and you will see it disappear! It is a aliasing effect, what in some contexts we could call effect glitch. Visually, it is a Picture of the images Due to the difficulty in some images of having the return an intricate pattern, which generates a series of bizarre visual artifacts, both in the photos and in the videos, where we see turn the wheels on the contrary.
This phenomenon that does not only concern static images, but also on moving sounds and images: everyone will have seen a car pass and have the impression that the wheels turn around on the contrary, that is also an aliasing effect.
What is the aliasing effect: the example of the sound
To understand well what it is convenient to start from the easiest case from the point of view mathematical or that of sound: In fact, it may happen that in a recording, or in an audio transmission, noises or sounds extraneous to the original audio are felt. By simplifying a lot, a sound can be mathematically represented as one sinusoid (or more combined sinusoids), or a curve with many waves (such as the one in the figure): different sounds correspond to different waves.
This problem was born when a sound, To be reproduced, it comes digitized. The acquisition devices, in fact, do not capture all the sound in one fell swoop, but take samples at regular intervals of time. It is conly some parts were recordedas long as they are quite dense to seem all in all an excellent reproduction of the sound as a whole.
In our example, therefore, a device would not see the red wave of the figure as a continuous line but like many dots, as in the figure below.
In this case, the form suggested by the dots of the figure is very reminiscent of that of the original wave, and the corresponding digitized sound will be quite faithful to the original. But things can change drastically If the sampling intervals are too large: in the example below the recorded sound curve, which indulges the trend of the samples taken – the black points – it is very different from that of the original sound.
In this case, listening to the recorded sound we would hear a different sound from the original: a real aliasing effect!
How to avoid the aliasing effect in the images
Forimage As a cover, the situation is analogous, the image was made to be clearly visible on a very large screen, at least 4 times larger than a normal laptop monitor. It therefore contains a lot of details they go lost When we look at it via the screen of a mobile phone, or through a normal monitor of a computer: if the screen is too small compared to the original image, in fact, it is shown to be a replica made based on a small number of samples. Here, therefore, that, due to the effect, graphic elements cannot appear in the original image, just as happened with the wave seen above.
To this one wonders if there is a way of Avoid the aliasing effect. The answer is yes, the Shannon-Nyquist sampling theorem who, without going to too much detail, tells us how many samples you have to takeand how close they must be, so that the digitized content can be reproduced in the most faithful way possible to the original based on the use we have to do.
Why do the wheels or propellers seem to turn on the contrary?
But what does all this have to do with what happens when we look at one rotate turn And It seems to us on the contrary? We cannot perceive the whole movement of the wheel, it is a bit like if Our eyes sampled the one they see instant by instant: If from an instant to the next, the wheel has traveled almost a complete rotation, but it did not complete it, it seems to us that it turned a little backwards, instead of forward, giving the impression of a rotation on the contrary, as explained in the image below.
It can be seen in the example in video Below in which a wheel turns first slowly and then always faster but at a certain point it seems to us that it turns on the contrary or that oscillates back and forth. This particular example of an alias effect takes the name of Wagon Wheel Effecttranslatable into Italian with the effect of the wheel of the chariot!
