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Filed under: Lighting •Motion Graphics •Visual Effects •

Gobos and Gels

Finishing our tour of 3D lighting in After Effects, we’ll discuss gobos and gels. For those new to lighting, a gobo is an opaque object that blocks off some of the rays cast by a light, either to more carefully control where they fall, or to give the impression of light streaming through an object such as a window blind or the leaves of a tree. A gel is a translucent object placed in front of a light, which colorizes the rays cast by it. It is generally a solid color, but can be a graphic. Here are a few different approaches to replicating these inside After Effects.

The most straightforward way to create a gobo in After Effects is to place a 3D layer in front of a light. The alpha channel of the layer determines where the light passes through, and where the light is blocked. This effect is easiest to control when the Light Type of Spot – this type allows you to reduce the Angle of the light rays being cast so they don’t leak around the edges of the gobo layer which is supposed to be blocking them. To soften the pattern cast by a gobo, don’t use the light’s Cone Feather parameter; instead, soften the gobo pattern itself, or use the light’s Shadow Diffusion parameter.

Layers that already have alpha channels – such as stock footage of objects with their alphas already cut out, or geometric shapes created in a program such as Adobe Illustrator – will block light where the object or shape is present, resulting in a shadow the same shape as your object. To cast light in the same pattern as your object or shape, you will need to invert the layer’s alpha channel. To do this, go into the File > Interpret Footage > Main dialog for the layer’s Footage item, and enable the Invert Alpha option. If you want to soften edges of the gobo pattern, apply a blur to the gobo layer.

You can also create your own patterns inside After Effects by creating a Solid, and then using masks or paint strokes. To soften the edges of the resulting pattern, increase the Mask Feather or use a feathered brush (respectively). In all of these cases, the colors present in the layer don’t matter; only the alpha channel does.

If you have a grayscale image that you want to convert into a gobo, place the image into the file’s the alpha channel using a paint program such as Adobe Photoshop, or use a Track Matte in a precomp to luma matte the image onto a Solid. Other tricks such as using the Set Matte or Shift Channels effects are also possible, but keep in mind that the trickier you get, the harder it is to remember what you did and why when you come back to a project later.

We used an image of leaves (left) as a gobo. When used as a gobo to block one of the lights being projected onto another layer (below left), the result was the leaf pattern being cast across our footage (below). Leaves from Digital Vision’s Quiet Form CD; lion footage courtesy of Harry Marks.

Obviously, the gobo must be placed between the light and the layers receiving the shadows cast by it, as shown below. This means you have two objects to manage as you begin to move layers around. We find it easier to use Parenting to attach the gobo to the light – that way, moving the light will then move the gobo with it. Plan ahead by first setting the gobo layer, the light’s Position, and the light’s Point of Interest to the same X and Y Position values, and then Parent the gobo to the light. Once you have done this, you can edit just the gobo’s Z Position value to set how far away the gobo is from the light, no matter where you move the light to later. This distance in turn controls how large of a pattern is cast.

You may be surprised to learn that In After Effects (as well as most dedicated 3D programs), layers do not by default block light. To have them block some of your light’s rays, you must enable the Casts Shadows option for both the light and your gobo layer. If you run into a problem where the gobo layer becomes visible in the scene, as of version 5.5 you could set a layer’s Cast Shadows option to Only, which means you will only see the light pattern – not the layer.

next page: creating “gel” effects

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