Global light illumination and light effects in
Adobe Photoshop (part 1)

Global illumination, reflections or light effects are usually features of 3D software like Maya, Lightwave or 3D Max. Basically building a 3D scene resumes to this three simple steps:
- choose 3D objects
- choose material and textures
- define light sources for 3D scene
Finally, you can set a point of view by defining a camera angle for that scene. I said simple steps, but in fact they are not simple at all, a lot of work has to be done to achieve a realistic lighting effect. But what about 2D software, is there a chance to get some of those stunning effects using Adobe Photoshop, for example ? The answer is yes, but one has to have some knowledge of optical physics and a good mastering of some Photoshop tools and techniques. That’s because all that is usual the result of an algorithm computation, Adobe user has to draw by hand using knowledge and talent. There is also a tool called ‘lighting effects’ , but works only in 2D environments, and the most you can get is the behavior of a light spreading on some surface, no shadows, reflections. Anyway a clever use of this tool can help for emulating a 3D environment. Let’s have some definitions first.

Global illumination (from Wikipedia) is an algorithm that take into account not only direct light coming from some source light (also called direct illumination) , but subsequent effects in which light rays from this source are reflected by other surfaces in the scene (also called indirect illumination). Remember that light itself is invisible (!) and what we actually see are iluminated objects that reflects light in some way. Basically there are to opposite ways in which a light ray can reflect when it reach a point of some surface.

Specular reflection is the perfect mirror like reflection, when incoming light direction has a single outgoing light reflected direction. The process is governed by the physical law of reflection and the result is a clear real or virtual reflected image, like you see in a mirror.

Diffuse reflection is happening when incoming light ray is reflected in a broad range of directions. In real world how much specular or diffuse reflections are, depends of the very structure of the material reflecting the light. Usually always there is a combination of both, for example polish surfaces have a greater proportion of specular, while raw surfaces exhibits diffuse reflections.

Radiosity Is the phenomen occuring when some unreflective surfaces are hitted by reflected bouncing rays of light from other objects. It seems that reflected light is transferred to other surfaces, giving the illusion of light bleeding or glowing around these surfaces.

Ambient light refers to overall illumination. The source for ambient light can be an environmental source of light (daylight, or some global hidden light sources).

Spot light refers to a definite direct source of light (directional light from a light bulb, flashlight, directional sun rays etc.) Some of these effects can be emulated via ‘lighting effects’ tool, some not. We actually see how, and what tools we can use to achieve this, in the second part of this articol.

May, 02, 2007


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