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Creating a blue screen composite image starts with a subject that has been photographed in front of an evenly lit, bright, pure blue background. The compositing process, whether photographic or electronic, replaces all the blue in the picture with another image, known as the background plate.

Blue screen composites can be made optically for still photos or movies, electronically for live video, and digitally to computer images. Until very recently all blue screen compositing for films was done optically and all television composites were done using analog real time circuits.

In addition to blue, other colors can be used, green is the most common, although sometimes red has been used for special purposes.

In addition to blue, other colors can be used, green is the most common, although sometimes red has been used for special purposes.

Sources : Internet

Difference between Non Linear Editing System & After Effects :-

• After Effects and NLEs is that After Effects is layer-oriented, and NLEs are generally track-oriented.
  
• After Effects, each individual media object (video clip, audio clip, still image, etc.) occupies its own track. However, NLEs use a system where individual media objects can occupy the same track as long as they do not overlap in time.
  
• Track-oriented system is more suited for editing and can keep project files much more concise. The layer-oriented system that After Effects adopts is suited for extensive effects work and keyframing

After Effects uses a system of layers organized on a timeline to create composites from still images and motion footage, such as video files. Properties such as position and opacity can be controlled independently for each layer, and each layer can have effects applied. After Effects is often described as the "Photoshop of video", because its flexibility allows compositors to alter video in any way they see fit, as Photoshop does for images.

Although After Effects can create images of its own, it is generally used to composite material from other sources to make moving graphics (also known as motion graphics). For example, with a picture of a space ship and a picture of a star background, After Effects could be used to place the ship in front of the background and animate it to move across the stars.

The main interface consists of several panels (windows in versions prior to After Effects 7.0). Three of the most commonly used panels are the Project panel, the Composition panel, and the Timeline panel. The Project panel acts as a bin to import stills, video, and audio footage items. Footage items in the Project panel are used in the Timeline panel, where layer order and timing can be adjusted. The items visible at the current time marker are displayed in the Composition panel.

• Color information channels are created automatically when you open a new image.
  
• The image’s color mode determines the number of color channels created. For example, an RGB image has a channel for each color (red, green, and blue) plus a composite channel used for editing the image.
  
• Alpha channels store selections as grayscale images. You can add alpha channels to create and store masks, which let you manipulate or protect parts of an image.
  
• Spot color channels specify additional plates for printing with spot color inks.
  
• An image can have up to 56 channels. All new channels have the same dimensions and number of pixels as the original image.
  
• As long as you save a file in a format supporting the image’s color mode, the color channels are preserved.
  
• Alpha channels are preserved only when you save a file in Photoshop, PDF, PICT, Pixar, TIFF, PSB, or raw formats. DCS 2.0 format preserves only spot channels. Saving in other formats may cause channel information to be discarded.
  
  

• When you select part of an image, the area that is not selected is “masked” or protected from editing. So, when you create a mask, you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.
  
• Masks are stored in alpha channels. Masks and channels are grayscale images, so you can edit them like any other image with painting tools, editing tools and filters. Areas painted black on a mask are protected, and areas painted white are editable.
  
• To save a selection more permanently, you can store it as an alpha channel. The alpha channel stores the selection as an editable grayscale mask in the Channels palette. Once stored as an alpha channel, you can reload the selection at any time or even load it into another image.
  

There is hardly a movie that is made in Hollywood these days that does not have extensive special effects (SFX) work. Bollywood has also started embracing the SFX bandwagon with some gusto. A large part of SFX is animation. But SFX goes way beyond animation. In this issue, we go behind the scenes to see how some of the most spectacular and some of the most realistic SFX seen in recent times have been achieved. How various effects are executed is explained towards the end of this section. You may want to read that first.

Creating Spiderman
If you have seen the recent blockbuster Spiderman, you would recall the last scene of Tobey Maguire swinging away, amidst the high rises of Manhattan. Surely, you would have wondered how a human being, even an especially adept and agile stuntman, could have pulled one off. If you have wondered and would like to know, welcome to the world of computer-generated SFX.

Let us take the Spiderman scene described above as an example. The bulk of the scene was computer-generated imagery (CGI). There was no real Manhattan, and no real Maguire, most of the time. Almost everything was generated by software.

This software-generated imagery was interspersed with live shots of stunt doubles for street-level shots, and the occasional shot of Maguire himself, to create the footage you and I saw.

What software was used for this? Like most other SFX projects of this scale, Spiderman used standard effects packages like Maya, extensions specifically written for the Spiderman project, and completely new packages written just to create Spiderman-specific effects like spider webs.

Spiderman and the Green Goblin were animated for their many stunts in Maya. So was the genetically-altered spider that bit Peter Parker, turning him into the superhero. Spider webs, web-slinging effects and the Manhattan buildings were digitally created in Houdini, software from Side Effects. Renderman from Pixar was also used. Animation created in Houdini and render paths for Renderman were coordinated using PERL scripts.

Houdini is available for NT, Linux, IRIX and Solaris. Maya from Alias WaveFront is available for NT, IRIX, Linux and MacOS X.Rendering all this is a demanding task in itself, and SFX and animation studios build render farms for the purpose. Initially, most of the work was done on heavy-duty RISC machines from SGI and Sun. Recently there’s been a shift towards Linux and render farms built of commodity Intel-based machines. A render farm is a collection of machines, networked together by a high bandwidth connection, and dedicated to running the rendering. Specially written tools divide the rendering work amongst the machines in the render farm and also keep track of what is going on.