vfx workflow- blue green matte compositing

BLUE/GREEN MATTE :

Hello guys now i am going to tell you about another major technique in visual effcts industry. that is chroma key .you might have seen many movie making scenes with green or blue screen in the background .what they will do with those blue and green screens ?? how they useful to add the special effects ??
i will clarify your doubts .this is for the beginners and newbies to the vfx industry so i am not going into use any technical terms .i will give you a brief idea about chroma key (blue/ green screen keying) .

Chroma key compositing, or chroma keying, is a post-production technique for compositing (layering) two images or video streams together, used heavily in many fields to remove a background from the subject of a photo or video.
 A color range in the top layer is  made transparent, revealing another image behind.

Why green and blue backgrounds are more commonly used ?

because they differ most distinctly in hue from most human skin colors.
This keyer technique mostly used y all the national and international news channels and live TV shows.

 wherein the news presenter appears to be standing in front of a large map during live television newscasts, though in actuality it is a large blue or green background.

If the news presenter wears blue clothes, his clothes will also be replaced with the background video.that is .no part of the subject being filmed with the color (blue/green) used in the background. 

The chroma keying technique is commonly used in video production and post-production also. This technique is also referred to as color keying, color-separation overlay,

Chroma keying is also used in the entertainment industry for special effects in movies and video games.
you knew about Roto (If not please refer my previous post),  Suppose Director would like to film a scene in the amazon forest with main lead of the movie  .it is very expensive and tough  shot .where another alternative for this is  Rotoscoping.
.after the shot we can Roto for all the cast(selected parts of the frame as per the director guide lines) and we can place amazon forest background images or videos .but again here Roto is time consuming process .the very good alternate for this is Chroma key .
 To use the blue/green screen technique, we have to film the amazon videos (footages ) on location. This shot is called the background plate.

 Then we have to film the main cast sequences as per the script in front of blue/green screen in a studio(or in any affordable location).

The chroma key technique lets you combine two or more pieces of film into one piece that looks very real.
You end up result look like this:

As the availability of many vfx softwares computer software, such as Pinnacle Combustion,after effects, Adobe Premiere, and dozens of other computer programs, makes it possible and relatively easy for the average home computer 
user to create videos using the "chroma key" function and easily affordable green screen or blue screen kits.
Thank you , will be back with next concept from the vfx work flow .