Background: human vision basics¶
Video display technology is based on human vision in several ways:
The human eye uses 3 kinds of sensors (in daylight) called cones, each covering some part of the visible spectrum. Their behavior is quite complicated because their response ranges overlap, the count of each sensor differs, the light perception is non-linear, etc. For the human eye, different spectral power distributions can lead to the same perceived color (metamerism).
Long story short, a video display doesn’t have to be able to produce every frequency of the visible spectrum to reproduce colors: it just has to produce 3 of them to trigger each kind of cone at a different level. Typically a video display produces color by combining different amount of red, green and blue lights which roughly correspond to each cone response peak.
Usually video display devices can only produce a subset of the visible color space (or gamut).
If several lights of different colors are emitted very closely to each other, our eyes can’t distinguish them and we don’t perceive the gap between them. Instead we only perceive another color which is composed of the emitted ones.
Screens display an array of lights called pixels (say 1920x1080 pixels) which are very close one to the other so that we perceive a continuous picture without distinguishing the gaps between each pixel individually (at normal viewing distance of the video display).
Screens display pixel themselves as a combination of several (at least 3 but sometimes more) distinct lights (red, green and blue) which are so close that we perceive them as a single color.
Similarly to the spatial integration, our eyes cannot distinguish lights that change too fast over time. Instead we perceive an integration of the light over time.
Screens use this to simulate continuous motion: if different images are displayed very quickly, our eyes don’t distinguish each picture individually. Some video displays even emit a single line of pixels at a time or a single pixel at a time, or alternate between even and odd lines for each frame.
Note that our peripheral vision is better at detecting motion. It explains why virtual reality (VR) headsets have higher refresh rates.
Rendering an image on a video display surface consists in indicating the color of each pixel. Note that pixels are samples of the image we would like to render on the video display surface if it had an infinite resolution, not “small squares”.
We have to render many different images per seconds to simulate moving images.
I recommend watching this video: How a TV works in slow motion