Persistence of Vision

Look at a bright light for a few seconds and then abruptly close your eyes. The image of the light seems to stay in your eyes a little longer even though your eyes are closed. This phenomenon is termed as Persistence of Vision because the vision seems to persist for a brief moment of time.
When the retina of the eyes are excited by light, they send impulses to the brain which are then interpreted as an image by the visual cortex in the brain. The cells in the retina continue to send impulses even after the incident light is removed. This continues for a few fractions of a second till the retinal cells return back to normal. Until that time, the brain continues to receive impulses from the retina, and hence seems to perceive an image of the source of light, giving rise to the phenomenon called Persistence of Vision.


Principle of Motion Picture

The Principle of Motion Picture is totally based on the phenomenon of Persistence of Vision. Without it, motion picture as we know it simply would'nt exist. Our eyes can retain a picture for a fraction of a second after seeing one. Before this time frame expires, if a another similar picture is shown in its place, the eyes see it as a continuation of the first picture, and don't perceive the gap between the two. If a series of still pictures depicting progressively incrementing action is flashed before the eyes in rapid succession, the eyes see it as a scene depicting smooth, flowing action. All visual media (Movies, TV, Electronic Displays, Laser Light Shows, etc) exploit this phenomenon. Thanks to Persistence of Vision, our entertainment industry could make a transition from perpectual live shows like dance and dramas, to recordable entertainment like movies.


What is Motion

Motion of an object is the continuous displacement of the object in space with reference to another object. In the absence of a reference object, motion ceases to be apparent. What this means is that motion is always measured in relation to another object, which is used as a reference point.

When we drive, the road & the surroundings move past us. Thus we get the sensation of motion. So the road & surroundings are our reference points. When we fly, the earth beneath us is our reference point. But as you can see, the closer the reference point, the more acute the sense of motion. That's why astronauts in orbit seldom sense speed (though they are moving at thousands of miles an hour ) because earth, their only reference point is quite far away.

OK, but what has this got to do with Special Effects ?! A Sfx technique called Compositing totally relies on the way our mind perceives motion. Compositing is one of the most useful tools in a Sfx technician's bag of tricks. Keep these two in mind; the object, and its reference point(s); both of these are necessary to perceive motion in a scene.


Blue Screen

In Compositing, the foreground and background shots are shot seperately and later superimposed one on the other. Certain areas of the foreground have to be transparent for the background detail to show through. (If the foreground detail fills the entire frame, the background detail will not be visible)! For this purpose, the foreground object is usually shot against a plain blue screen. A particular shade of blue called Chroma Blue is used for this purpose. (Human skin does not contain this shade of blue). The blue area is later erased from the frame making the area transparent. The background detail can now show through this blank area.


Story Board

A series of sketches based on the final draft of the script depicting various scenes in the story, which the director, cameraman and the art department use to plan for each scene.