Premiere Tutorial 7: Transparency

Special Effects used to be very expensive to create, but with cheap NLE equipment some of their hi-tech gadgetry can be used by anyone. One of the most common techniques is Colour Seperation Overlay (CSO or Chroma-Key), where the actor stands in front of a blue or green screen and the background is electronically replaced with a video clip such as a beach. Premiere has Chroma-Key in its transparency settings, along with a number of other effects.

You will need 2 clips, the backdrop must be on the Video A or B track, and the blue screen footage must be on the Video 2 track (all tracks above Video 1 are transparency tracks). Select the top track and select 'transparency' from the menu, you then select the appropriate transparency setting.

In the screenshot below you will see the Video 2 track has been expanded using the arrow on the left. Underneath the video clip there is a red line which is where you set your fade levels. In this example the clip 'video mask' will fade into the 'titanic' clip and then stay at 50%. Halfway along it will jump to 100% and then fade out.

The different key types are explained here.

None		This is the default key and means no transparency is added.
Chroma (key)		This is the most common form of keying. You select a colour such as the blue screens and the computer removes the colour and replaces it with the lower clip on the timeline.
RGB Difference		This is a simpler version of the chroma-key as there are fewer sliders to control. You should use this key for brightly lit scenes with no shadows.
Luminance		This works by keying out dark parts of an image but leaving the brighter parts visible. It should be used on areas of high contrast
Alpha Channel		Using Photoshop you can create an Alpha channel which will decide what will be transparent, for example you could create a matte just by drawing around the edges of the blue screens, which will give you a much cleaner matte
Black/White Alpha Matte		Some alpha channels may have black or white borders. Using these keys eliminates the edges.
Image Matte		You need to import a still image ( ideally a greyscale image) which then selects the transparency
Difference Matte
Blue/Green Screen		These are similar to the chroma-key, but it is adapted for using the true chroma-key colours rather than a general blue or green.
Multiply		You will need to select another image to use as a matte, the key will then add transparency over brighter areas
Screen (Matte)		You will need to select another image to use as a matte, the key will then add transparency over darker areas
Track Matte
Non-Red		This will key out blue or green backgrounds and can be used when the other Mattes are producing poor results.

Creating Masks You will notice a button to create masks in some of the keys. This creates a black and white image. This can then be manipulated in Photoshop to create a clear mask, so you can erase the bits of noise often created in keys. This is especially useful for creating image and screen mattes so your mattes are very clean.

You select the key type, then alter the various levels to get the desired effect