Funding for this site is provided by readers like you.
The Senses

Help Deux ancêtres méconnus du cinéma PERSISTENCE OF VISION Why do we see a series of still photos as moving pictures?
Frame-Rate Technique Delivers Flicker-Free Motion-Picture Performance The Search For the Great Science Fiction Movie (or, I Am Doooomed)
Motion Picture Perception and Change Blindness
Movement Perception
History of Motion Pictures The brain determines how you see cinema

Link : Super Size Me : a film of epic portions


From their earliest origins, human communities have always felt the need to tell stories, whether about themselves or about other groups of people. In our societies today, going to watch a film together in a darkened theatre is a well established natural extension of this ritual. Motion pictures derive their power from their ability to evoke reality, and in particular from the illusion of motion that they create.

The best way to understand the mechanisms that let us portray reality by running film through a projector is to think of them as having solved two distinct problems. The first was to keep the audience from noticing the intervals of darkness between the images, as well as the flicker (fluctuating light intensity) produced by the alternation between images and darkness. The second problem was to create the central illusion of the cinema—how to take a series of still images that differ slightly from one another and make the audience perceive them as a single image that is moving. Retinal persistence was long thought to play a central role in solving both of these problems, but its role is now considered negligible.

Because of the constraints imposed by your computer screen, this animation can unfortunately give only an approximation of the real effect.

The first problem was solved by increasing the number of images projected per second onto the screen compared with the number shot per second by the movie camera. For older films, shot at 16 frames per second, the number was tripled; for today's films, shot at 24 frames per second, it is doubled. The viewer thus sees close to 50 images per second, the threshold at which flicker becomes too rapid to perceive (see box below). So what happens to the intervals of darkness, which account for almost half the projection time? Apparently, retinal persistence has nothing to do with it. We just ignore these intervals, because for the brain they simply constitute an absence of information.

As regards the second problem, it is now generally agreed that what makes us perceive motion in place of a rapid succession of still images is a psychological effect, known as the beta effect, that has nothing to do with retinal persistence either. The simplest example of the beta effect occurs when two closely spaced points of light go on and off in succession. Though there is no actual movement, our perceptual processes subjectively link the two points into a single one that is moving. The same principle is at work, in more complex form, in the moving signs used on billboards and in sports arenas, where hundreds of tiny lights going on and off in succession produce highly realistic motion effects.

The beta effect can also create the illusion of motion toward or away from the viewer. For example, when you show people a series of progressively smaller images of the same object, they will generally experience it as moving away from them. And conversely, if the images are growing larger, then people will experience the object as moving toward them. Similarly, if the series of images begins with bright colours that gradually grow duller and start to fade into the background, people will usually say that the object has moved away from them. Thus the beta effect is the basis not only for the illusion of motion in films in general, but also for numerous graphic subterfuges in animated films in particular.

The frequency at which the flicker caused by a succession of images becomes imperceptible to the human visual system is called the flicker fusion threshold. This threshold is not absolute. It depends on the degree of illumination of the images, and is higher for brighter ones. It also depends on what part of the retina is capturing the image. The rods respond to light more rapidly than the cones. Hence we may sometimes perceive flicker in our peripheral field of vision when we cannot see it in our central field, where the image is captured by the fovea, which consists of cones.

Lien : Flicker fusion threshold

On television, as in the movies, the illusion of motion is created by a rapid succession of still images. But instead of being projected from a film, they are produced by a varying-intensity electronic beam that scans the inner surface of the television's cathode-ray screen at high speed. This surface is coated with phosphorus, so the variations in the beam's intensity leave behind traces of light of varying intensity that last for a few moments.

On a conventional television receiver, the electron beam reconstructs each image by drawing horizontal lines across the screen, starting at the top and working down. The number of lines per screen depends on which standard is used in your country: 625 for the PAL/SECAM standard, or 525 for the NTSC standard. PAL/SECAM television cameras record 25 images per second, while NTSC cameras record 30, so a television displays a new image either 25 or 30 times per second.

In television, it is not possible to double the number of images per second by means of a shutter, so another strategy is used to eliminate flicker. Each image is drawn twice. The first time, the electron beam draws the odd lines, and the second time, it draws the even ones. Thus each image actually consists of two fields, which means that 50 or 60 fields are displayed per second, thus eliminating the problem of flicker.

Link : Conventional Analog Television - An Introduction Link : How television works ? Link : Television Link : How the TV Process Works Link : PAL - NTSC - SECAM


  Presentations | Credits | Contact | Copyleft