Chroma Keying Underwater

Chroma keying can be a great technique for shooting action or objects in an underwater experience. It can be very expensive to get all the equipment, location and lighting needed for real underwater shooting. A special underwater housing will be needed for the camera, and light travels differently underwater and more will be necessary. Also, for the actors, a breathing apparatus is needed and the shots tend to be very short. This type of green screen is perfect, as the shots can be done in a water tank and then replaced with the great underwater background needed for the story.

Step 1: Screen Color

When shooting underwater scenes, a green screen is best over a blue screen. The green channel is used frequently, and is best as most underwater shots will have a lot of blue overtones which can make separating the different chroma keys difficult. A bright green that has only 1 shade is best. Chroma paint is the best option and comes in a very vivid hue that is easy to extract from the footage.

Step 2: Lighting

The lighting for underwater shooting can be tricky, as there is a lot of shadow. Light travels differently through water. It is best to have plenty of light, and then to add any shadows that are needed in post production. It is best to have plenty of backlighting, and some light applied to the subject and any props used in the shot.

Step 3: Match Lighting

Something to think about is to match the foreground lighting with the lighting used in the background replacement. If the foreground does not look like it belongs in the background, then the shot will not be believable. Matching the lighting will also reduce the amount of work that will need to be done in post production.

Step 4: Distance

To get the best effect, the subject should be as far away from the screen as possible. This can be difficult for underwater shooting because the space may be limited by the tank or pool size used. It is not necessary to have a huge green screen in the back by simply have the green screen behind the shot for the foreground. The screen reduction will also reduce any reflection or glare, which helps to reduce shadows.

Step 5: Underwater Background Footage

It is not necessary to go shoot the underwater background that will be used in the chroma keying. There are many libraries that contain stock footage which can be used. Most of the time, this footage is available for a fee and then the entire library is available for your use. There are also some common use photos that can be used as backgrounds. These are available for free. It is also possible to create your own background digitally.

Step 6: Final Effect

The video editing software will be able to create the final effect. A layer will be used that contains the chroma key of the green screen. This color will then be removed and the subjects will be overlaid onto the new background. This chroma key can also be set to be transparent and then layered over the background. Then, many other filtering effects can be used to ensure that the 2 images look like they were one shot.