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Chromakey  and Lumakey Explained

There are two facilities chromakey and lumakey that are able to be used to allow the replacement of backgrounds with other information. Chromakey uses a plain coloured background while lumakey uses a difference in brightness, usually replacing a white background.

Chromakey is the more frequently used and is used a large amount in TV dramas and the production of movies as well as for weather forecasts. The background is usually either blue or green and these colours are chosen as they do not appear in the human skin, however it does present a few limitations, in that nothing of the chosen replacement colour can be used. As the two options are generally available and the same equipment can handle both, most film and TV production will use both. If for example superman wearing an outfit with a lot of blue was to fly in front on a blue chromakey background he would become half way to the invisible man.

In some situations this effect can also be used, for example operating large puppets reacting with people on a children’s show, can be easily done by the operators and the control sticks being in the chromo key colour, and therefore they are not seen.

Where TV dramas involve computer generated monsters then the actors are acting in relation to a man in a chromakey outfit holding  a stick showing the monsters head, again in the chromakey colour, these become invisible and the monsters are just edited in. This technique has allowed all sorts of special effects to now be included in productions without a film budget. People can fly, fall from high places, meet monsters or creatures or present parts of programmes from locations they have never visited. The BBC Westminster studio, that you see MP's and the like interviewed from, that appears to have windows in, in fact has no windows, its just an illusion.

With TV and video you can put a simple device in the video line to mix the two images replacing the selected chomakey colour. Editing systems are also able to do this, and of course would also allow a number of layers to be built up this way.

One place where you can go and see this yourself is the National Space Centre, Leicester www.spacecentre.co.uk, here they have a room where you can put on a presentation in front of a green wall and it is shown as a weather forecast on monitors, of course you will have a job getting in to do it, as its so popular with the kids. TV weather forecasts are done the same way, and you may have spotted they tend to make generally wide gestures avoiding pointing at any pace, as its just a blank wall that they see.

In photography we can also use both chromakey and lumakey, and background cloths are available for chromakey, either as spring up backgrounds, cloths to put on a stand or just introduced, an extra to add to the Lastolite HiLite background.  Lumakey can be done using either a well lit white background or better still the HiLite studio background (we have one of these). You can also hire special studios that have a cove painted in a chromakey colour. The cove has a rounded off back that takes it smoothly from the wall to the floor.

While it's common practice to use the blue and green widely used for TV and movies, any colour can be used.  So what can you do with chromakey and lumakey, well you can photograph people and put them in any location, or add them to a scene with others. You could also do sci-fi and similar. Perhaps you would like to have a giant hand and stand or sit someone in it…. There are many possibilities.  It also opens up many more possibilities, for example if you want to produce advertising photos, its simpler and cheaper to get stock shots of a resort and add models than fly the models and photographer around the world. In some cases you can do things by this method that would be too dangerous in reality. It can also be used for recovery situations. Perhaps someone booked a local photographer for their wedding and has been let down, now getting all the guests together and back to the church may not be possible but it would be possible to photograph the church and then in a small number of sessions photograph the people and then build the photograph.

An image is filmed or photographed in front of a chromakey background.

Let us give you an example, here we are using an animated character and mixing this with a photograph, in practice we have used a web technique rather than editing the images together but you get the same effect.

The trick, if you want to attempt something in this line, is to concentrate on the lighting, the light for all parts of the image needs to come from the same direction, or as in the case we have chosen be flat lighting. Models can often be fairly flatly lit using a glamorous effect without it looking out of place, even when there are shadows present.

To put the image together you need an editing package and this is a task that Capture NX cannot do. We use Photoshop CS3 for this type of task, but you would probably find a range of others could also do this.  A program that uses layers just has to be able to convert a defined colour to become transparent. One way to make characters fit better into an image, and become a part of it, is to drop a shadow from them to line up with the other shadows present. This is easily done.

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So what have we here

The chromakey background becomes transparent allowing whatever image we put behind to show through. This can be done with TV, film and digital photography. Here we have used another technique where the picture is in a table background and the animated Gif (collection of still images running in rotation) is in the table. The table borders are turned off so you can't see them. Although I have used an animated character, I could have used a series of photographs of a model to get exactly the same effect.  I could also have played a voice file as well, to have her talking, perhaps telling you about the carved head on the right, but then again if you are looking at this at work, this might not have made me popular.

The picture is one off the page on the Stanton Drew stone circle.