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Compositing Light

Part 1: Casting light in Adobe Photoshop By Dave Nagel
In our varied and manifold compositing series on Adobe Photoshop, we've looked at water, atmosphere and lens effects, each with the goal of adding detail to images with a photographic quality. Today we start a new series on compositing light and kick it off with a tutorial on casting light onto an image as if the light were coming through a multi-paned or shuttered window or door.

Before we get started, there's one preliminary for this tutorial that you should have read about last week, specifically the method for creating path-based objects in Photoshop. If you have not already followed this tutorial, you should do so now, keeping in mind that the shape you will create should be the shape of the window panes or shutters that you'll be using in the present tutorial. You can find the article on path-based objects at http://www.creativemac.com/2002/09_sep/tutorials/pspathobj020911.htm.

Beyond this initial shape creation step, there are essentially six main stages to this tutorial, plus a little cleanup work at the end, if you think any is necessary. The key to this particular approach is quasi-automation, so there shouldn't actually be too much cleanup afterwards.

But before we get started, you need to take one more preliminary action--the creation of your displacement map.

The displacement map
What is a displacement map? Well, it's a file that essentially will tell Photoshop how to distort the light effect that we create. Photoshop uses the gradations in a displacement map to determine how an image will be displaced. Allow me to show you an example. Below you see two images with a light pattern over them. The first one shows the light without displacement. The second shows the light with displacement using the background image as the displacement map.

So you see how using a displacement map can help you to achieve a better lighting effect in a photographic image. But first we need to prepare our image to be used as a displacement map.

The biggest problem with using a photographic image as a displacement map is all the little details in the image that could cause unwanted distortion--blades of grass, texture in carpeting, etc. So we want to eliminate as much detail as possible without taking too much away from the image. I think the easiest way is to apply a Gaussian Blur to your image of about 2.5 pixels.

For most of you, this will be enough. But you also might want to go in and use the dodge and burn tools over your image to help bring out some contrast where needed.

When you have an image, finally, with good definition in the major details and little definition in the minor details, choose File > Save As, and make sure you save the image in the Photoshop format. Call the image "Displacement Map 1" just for simplicity's sake.

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