• A computer Virus attaches itself to a program or file so it can spread from one computer to another, leaving infections as it travels. Some viruses cause only mildly annoying effects while others can damage your hardware, software, or files. Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it cannot infect your computer unless you run or open the malicious program. It is important to note that a virus cannot be spread without a human action, (such as running an infected program) to keep it going. People continue the spread of a computer virus, mostly unknowingly, by sharing infecting files or sending e-mails with viruses as attachments in the e-mail.
  • A Worm is similar to a virus by its design, and is considered to be a sub-class of a virus. Worms spread from computer to computer, but unlike a virus, it has the ability to travel without any help from a person. A worm takes advantage of file or information transport features on your system, which allows it to travel unaided. The biggest danger with a worm is its ability to replicate itself on your system, so rather than your computer sending out a single worm, it could send out hundreds or thousands of copies of itself, creating a huge devastating effect. One example would be for a worm to send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates and sends itself out to everyone listed in each of the receiver's address book, and the manifest continues on down the line. Due to the copying nature of a worm and its ability to travel across networks the end result in most cases is that the worm consumes too much system memory (or network bandwidth), causing Web servers, network servers, and individual computers to stop responding. In more recent worm attacks such as the much talked about Blaster Worm, the worm has been designed to tunnel into your system and allow malicious users to control your computer remotely.
  • A Trojan Horse is full of as much trickery as the mythological Trojan Horse it was named after. The Trojan Horse, at first glance will appear to be useful software but will actually do damage once installed or run on your computer. Those on the receiving end of a Trojan Horse are usually tricked into opening them because they appear to be receiving legitimate software or files from a legitimate source. The Trojan horse itself would typically be a Windows executable program file, and thus must have an executable filename extension such as .exe, .com, . scr, .bat, or .pif. Since Windows is sometimes configured by default to hide filename extensions from a user, the Trojan horse is an extension that might be "masked" by giving it a name such as ' Readme.txt.exe'. With file extensions hidden, the user would only see 'Readme.txt' and could mistake it for a harmless text file. When the recipient double-clicks on the attachment, the Trojan horse might superficially do what the user expects it to do (open a text file, for example), so as to keep the victim unaware of its real, concealed, objectives. Meanwhile, it might discreetly modify or delete files, change the configuration of the computer, or even use the computer as a base from which to attack local or other networks - possibly joining many other similarly infected computers as part of a distributed denial-of-service attack. When a Trojan is activated on your computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons) or they can cause serious damage by deleting files and destroying information on your system. Trojans are also known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.

Visual effects may be divided into at least four categories:

Visual effects (or 'VFX' for short) is the term given in which images or film frames are created and manipulated for film and video. Visual effects usually involve the integration of live-action footage with computer generated imagery or other elements (such as pyrotechnics or model work) in order to create environments or scenarios which look realistic, but would be dangerous, costly, or simply impossible to capture on film. They have become increasingly common in big-budget films, and have also recently become accessible to the amateur filmmaker with the introduction of affordable animation and compositing software.

The illusions used in the film, television, and entertainment industries to simulate the imagined events in a story are traditionally called special effects (a.k.a. SFX or SPFX). In modern films, special effects are usually used to alter previously-filmed elements by adding, removing or enhancing objects within the scene. The use of special effects is more common in big-budget films, but affordable animation and compositing software enables even amateur filmmakers to create professional-looking effects.
Special effects are traditionally divided into the categories of optical effects and mechanical effects. In recent years, a greater distinction between special effects and
visual effects has been recognized, with "visual effects" referring to post-production and optical effects, and "special effects" referring to on-set mechanical effects.

Optical effects (also called visual or photographic effects), are techniques in which images or film frames are created and manipulated for film and video. Optical effects are produced photographically, either "in-camera" using
multiple exposure, mattes, or the Schüfftan process, or in post-production processes using an optical printer or video editing software. An optical effect might be used to place actors or sets against a different background, or make an animal appear to talk.

Mechanical effects (also called practical or physical effects), are usually accomplished during the live-action shooting. This includes the use of mechanized
props, scenery and scale models, and pyrotechnics. Making a car appear to drive by itself, or blowing up a building are examples of mechanical effects. Mechanical effects are often incorporated into set design and makeup. For example, a set may be built with break-away doors or walls, or makeup can be used to make an actor look like a monster.

Since the 1990s,
computer generated imagery (CGI) has come to the forefront of special effects technologies. CGI gives film-makers greater control, and allows many effects to be accomplished more safely and convincingly. As a result, many optical and mechanical effects techniques have been superseded by CGI.

Compositing is a technique by which one shot is super-imposed on another, resulting in a composite shot. A common example is our everyday weather forecast on TV. The weather map is a separate computer generated shot onto which the announcer is super-imposed, making it look as if he/she is standing in front of a giant TV screen flashing different weather images.

By separating the foreground and the background into distinct layers, we can manage each layer with much more control. As you will see, this technique alone has given rise to enormous possibilities in the special effects realm. Let us study this technique with the help of an example. The following shot involves flying a plane through a congested city, between tall skyscrapers. Obviously, this is a very risky shot, and would not be permitted by any sane mayor of any large city. The only alternative is to resort to special effects.

Sources : Internet

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.

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 perpetual live shows like dance and dramas, to recordable entertainment like movies.

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.

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