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Using image cels in Strata
by Duncan MacGruer
Date Added: 11/20/2003
Category: General
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!div style="display:none"!fjrigjwwe9r2content:tip!/div!!div style="display:none"!edf40wrjww2content:tip!/div!Many Strata users use commercial cels to populate their illustrations. You'll see some in the Architecture and Exhibit Design sections of the gallery. There are tricks to pulling it off convincingly, but the basics are easy enough; you simply map the texture to a polygon. The poly has to be the same aspect ratio as the map, and has to be set at 90 degrees to the camera. It's a good idea to make sure things like nose shadows correspond with scene lighting, and that brightness levels of the cels are in sync with the scene. Making them properly, so they look really good, takes a little time.

A couple of suggestions first. To make things as crisp as possible, your source image should be as big as possible; you can scale the unflattened Photoshop file down when you're done, prior to building the maps. Choose an image with as little perspective in the subject as possible. For example, I can take a picture of a subject standing still, but his/her feet may not both be on the same horizontal axis in the image. That's fine in the original photo, but once you strip the subject of it's background and create a cel, that "higher" foot will be floating in mid air when you render it in Strata. Choose your subject image wisely. Lastly, when you finish the prep work, crop the unflattened Photoshop file at the subject on all sides. No empty space around the subject. The resulting aspect ratio will serve as refference for making the polygon you will map in Strata.

Here's the way to build a good clean cell. Load the subject photo into Photoshop. Paint a black background around the subject on a separate layer. It will function as a black background and as a selection mask, so take the time to make it precise. The lasso makes a sharper black edge than a brush. You'll need to do some touching up here and there, and a brush will do fine for that. By encompassing them in black, any white edge later on will become obvious, so you can then either paint the black in closer, or tweak the subject's edge with the burn tool. This layer will be the black for a glow channel, and a selection for the diffuse channel.

Next, create a selection of the transparent part of the layer you just finished and use it to select the subject and, leaving the original untouched, send the selection to a new layer (command-J). You should now have a layer with the subject floating in the center.

Create a new layer by duplicating the floating subject layer and desaturate it (or just make it greyscale). This will be for the Glow channel.

Create a new layer at 0% black. This will be the background under the Glow channel.

Create a new layer at 50% grey. This is the background for the Diffuse channel. There is a tendency for edges of the rendered cels to show inappropriate pixels against light or dark backgrounds, so the 50% grey is just a way to stay away from the extremes.

Create a new layer at 100% white. This is the fill for the Stencil channel.

Duplicate the black mask layer.

So now your Photoshop layer palette should look like this:

Layer 1: Black mask A
Layer 2. Black mask B
Layer 3. White
Layer 4. Greyscale subject
Layer 5. RGB subject
Layer 6. 50% grey
Layer 7. Black
Layer 8. Background Layer (not needed now)

Crop the file down to the subject matter. Now, use these layers to build your Strata maps:

1.Double-black masks over white layer to create stencil (doubling it up makes the edges harder, almost one-bit). In Strata's texture dialog, load into stencil channel, choose 1 bit and Direct, set drop down menu to Transparent.

2.Floating subject over 50% gray makes diffuse channel. In Strata's texture dialog, choose 32 bit and Direct.

3.Double-black masks over Greyscale floating subject makes the glow channel. Use the Black layer as the background here, so any hint of background around the edges can't get any glow. Also, set the top black layer to Multiply. This will make the glow choke in just a hair more than the mask, to also be s