Tuesday, 15 June 2010

10 Steps to create a Greenscreen Adventure

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At last weekends festival of nature we were really proud of our presence as the BBC Natural History Unit. The theme of our tent was to give people and insight into the technologies used to make Natural History TV, including mini-cams, thermal cameras, IR-cams and parabolic reflectors for recording sound. We couldn't take people to exotic and remote locations and so we decided that we would bring some of these locations to them. I created a greenscreen adventure to do just that. It was a very successful activity with more than 500 children (and a few adults) having a go at presenting from one of three locations; Polar Bears in the Arctic, Orangutan in the rainforest and Great White Sharks under the sea. The shark adventure was by far the most popular - giving our intrepid presenters the opportunity to swim with, and escape, this formidable predator! I've had lots of people calling and requesting information on how to create a live greenscreen and so I've put together this basic guide to get you started...

Step 1. Hire or make your greenscreen
I used two 2x1 metre strips of lime green fabric bought from a craft shop and sewed together. For best results use a single continuous piece of material as any 'joins' will create lines through your green screen image. A nice bright lime green material works best but blue has also been traditionally used in television.

Step 2. Find a location 
For best results you will need a location that is not influenced by natural light. As we were creating a greenscreen experience for the Bristol Festival of Nature 'The BBC Adventure Screen', we had to work within a white festival tent. On the first day the lighting was even and so once the chroma-key had been set (i.e. the green removed so that you can insert your custom backdrop) it did not need to be recalibrated. On the second day the sky was intermittently cloudy and the lightning conditions frequently changed. I created a chroma-key setting optimised for both light conditions so I could switch between the two as the natural light changed.

Step 3. Stretch the material
Ensure the greenscreen is pulled tight and flat, any creases could distort the greenscreen image.

Step 4. Use even lighting
As already mentioned lighting is of the most important factors to consider. For best results it's important that the greenscreen is lit evenly and is much brighter than the 'presenter'. I used two kenoflo lights, which produce a nice soft white light. I did not use hot-heads as the light produced by these tend to be concentrated at a central focal point, they also tend to become 'hot' which is not a good idea when working in tents at a public event.

Step 5. Connect Camera
You can use any camera device including the inbuilt webcam on a macbook pro. For a much better image, and to optimise the chroma-key effect, I used a Z1 camera connected to my Macbook Pro via firewire.

Step 6. Connect Sound
For sound I connected a directional microphone to the camera, and had the audio playing from external speakers connected to the cameras minijack port. These speakers where situated away from the presenter and oriented towards the audience. The video that I created to play behind the presenter as the 'video layer' had sound effects and music. To create a more immersive experience I fed this audio directly from the laptop into a second pair of speakers which were placed facing the presenter.

Step 7. Connect view screen
I connected the laptop to a plasma screen onto which the final greenscreen composite was played full-screen. The control deck of the software was visible on the laptop screen so the public could see how it all worked. Due to the limitations of the plasma screen that I was using there was a short delay in image playback. This was because I had to convert from a digital output from the mac to an analogue input into the TV. I would recommend using a digital output to digital input such as a DVI to HDMI cable.

Step 8. Create a greenscreen layer
I used BoinxTV but this is quite a pricey package, so if you just want a simple greenscreen there's plenty of free software such as: Chroma Key Live

Once the greenscreen is in place and the lighting has been set it's time to launch the chroma-key software and calibrate the image. With Chroma Key Live it's a simple case of clicking on the green within the image displayed in the software window. If all has worked well the green of the greenscreen will be replaced by a grey and white chequerboard. If the lighting is not even some of the green material may still be visible through the chequers (as in the image on the left). You can adjust hue, brightness and saturation until the greenscreen has been completely highlighted (most software will have an auto button to make this process even simpler!).

Step 9. Create a video layer

Upload a video to use as a backdrop. This will form the layer which sits beneath the greenscreen layer. This 'video layer' will be visible and should completely replace the chequers in the chroma-key layer. Using BoinxTV I was able to line up all videos as separate layers ready to be switched on depending on which 'adventure' the presenter wished to do. Most Chroma-key software only allows one background video to be loaded at a time meaning a new video has to be uploaded with each presenter.

Step 10. Added Glitz

We gave the 'presenters' clothing to suite the locations they were visiting, e.g. polar gear or a rainforest hat - children also really enjoyed using binoculars in their search for wildlife! If the presenter is wearing a green T-shirt it may appear transparent with the background, so its handy to have something that they can wear over the top... however, many children enjoy the chance to be partially invisible. We did keep an extra piece of green material so they could play with it - like Harry Potters invisibility cloak!

An added feature of BoinxTV was that I was able to create additional layers to overlay onto the composite image. The 'video layer' was the bottom layer, the presenter chroma-key layer was the middle layer and then I created a snow or rain layer as the top layer. I used an additional layer which showed their name and location e.g. 'Charlie LIVE from the Arctic'.

Hope this helps. Have fun. - Paul

1 comment:

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