Road Train !!!!!!!!!!

by Russell McMahon

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I want to take a series of photos of table lamps (some on, some off) and later superimpose them on a picture of a different table (taken at the same distance and angle). What I want to know is how should I process the original images, so that I can simply layer them on top of the new backgrounds, and have them look 'right'.

I realize I could simply cut out the lamp part of the source image, leaving it on a transparent background. However, I think this would lose the shadows of the lamp on the table, and also any light cast by the lamp on the table. Is there a different technique that I'm missing?

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The folks at Savant AV (savantav.com) do this for one of their home automation products. I know that doesn't help you, but it does let you know it is possible. (And if you are doing this for a product, it lets you know that the competition does it, but if you are, you probably already knew that...) –  Paul Cezanne Nov 22 '11 at 16:11

2 Answers 2

Normally this is achieved by placing the object you wish to isolate against a "green screen", or possibly within a lightbox/lightroom of some kind that is surrounded with green or white. In your case, you would probably be best off placing each lamp against a green screen with the right kind of lighting to produce the shadows at the angles you need. Photographs of objects taken against a green screen can then easily be isolated by using a chroma-key tool to filter out the green background, leaving only the object...and its shadow...with all the necessary translucency in place.

Once you have isolated your lamps, you should then be able to composite those images with the image of your table as you please, and all the shadows and everything will be in place. You will need to take care with the lighting and placement of the objects you wish to isolate. To ensure shadows blend properly, it would be best to set up a table covered in the same kind of green cloth, at a proper distance from the green screen background, to guarantee shadows fall correctly. You will also need to make sure that the angles and diffusion of the light you use to illuminate the green-screen scene match that of the table shot you wish to compose the lamps into. That might take some trial and error, and things probably don't need to be 100% exact to get an acceptable result.

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Green screen? Why green? I thought blue was used for those things, since it's easier to isolate it from anything else digitally afterwards. –  MainMa Nov 22 '11 at 23:08
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Blue can be used, although green tends to be used more often. Both are easy to filter, since they generally only affect a single color channel from an RGB triplet. –  jrista Nov 23 '11 at 0:09

In addition to the green-screen technique described by jrista, you can look at the high-end masking plugins. Corel's Knockout 2 (formerly Ultimatte Knockout), for instance, did shadows rather nicely (although it's not fully compatible with recent Photoshop releases). Extracting masked elements from a scene that is similar to the target environment can also eliminate the colour casts one often gets when working with chroma-key (or, really, in any environment that is substantially different in colour from the target environment).

But let's not kid ourselves: photo compositing is hard. It's relatively easy to transpose a complete subject grouping to a new environment/background, but it's devilishly difficult to assemble individual subjects into a coherent, believable grouping. The problem with shadows and emitted light is that neither of them can be taken in isolation. Shadows fall on other objects as well as on the background, and light emitted from one object will illuminate other objects in the scene.

That means either painting some of the light and shadows manually or recreating the scene as a 3D model and extracting light and shadow maps to render onto the completed scene. (It usually takes a combination of the two to make something that passes deep scrutiny.) How much work you'll need to do depends on how complex the composition is, how big the resulting image will be, and how believable the result needs to be.

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