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Grokking The Gimp
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8.6 Rendering Project II: A Carved Stencil

This project describes a rendering effect that looks like carving into wood. It makes use of bumpmapping and shadows.

The technique creates the carved effect using two layers. Figure  8.29(a)

Figure 8.29: The Two Layers Used to Create the Carved Rendering Effect
Figure 8.29

shows the first layer, a bit-mapped image of a boat. Figure  8.29(b) shows the second layer, a background inserted under the bitmap and filled, using the Bucket Fill tool, with a wood pattern from the Pattern Selection dialog. The visibility of the boat layer has been toggled off so that the pattern can be seen. Figure  8.29(c) illustrates the disposition of the two layers in the Layers dialog.

The first step in creating the carved effect is to obtain a selection of the boat's outline. Since the bitmap of the boat consists of only two colors, black and white, this is most easily done with the By Color selection tool found in the Image:Select menu (see Section  3.3.11). The resulting selection is used to create an inset  for the carved effect.

What is an inset? It is just the Wood Background layer darkened slightly within the perimeter of the boat selection. This helps create the illusion that this part of the image is recessed, or inset, into the wood. Figure  8.30(a)

Figure 8.30: Creating an Inset
Figure 8.30

shows the Marching Ants of the selection. The selection is seen floating over the Wood Background layer because the Eye icon in the Boat layer has been toggled off as shown in Figure  8.30(b). Notice that the color of the wood seems a little darker inside the selection. This darkening was created by sliding the middle Input Slider control of the Levels tool slightly to the right in the Value channel. After this step, the selection is canceled by typing C-S-a in the image window.

The next step is to apply the Bump map    filter to the Wood Background layer using the bitmap of the Boat layer as the embossing map. Before doing this, the bitmap of the boat is blurred slightly because it improves the effect of the bumpmapping. Figure  8.31(a)

Figure 8.31: Creating the Carved Effect Using Bump map
Figure 8.31

shows the result of applying Gaussian Blur (IIR)  with a radius of 3 pixels to the bitmap of the Boat layer, and Figure  8.31(b) shows the Bump map dialog. As already stated, the bitmap of the Boat layer has been chosen as the embossing map. The other Bump map filter parameters are as shown in Figure  8.31(b). Note that the Compensate for darkening checkbox has been toggled on. The result of the Bump map filter is shown in Figure  8.31(c).

At this point, the result shown in Figure  8.31(c) already looks carved. However, the effect can be accentuated by making clever use of a punchout. The idea is that the perimeter of the carving should cast a punchout-like shadow onto the inset. To achieve the effect, the colors of the Boat layer need to be inverted, the layer must be copied into its own layer mask, and the layer must be blurred and offset. The following list summarizes these steps:

Invert the colors of the Boat layer using Invert from the Image:Image/Colors menu.
Copy the Boat layer to the default buffer by typing C-c in the image window.
Create a layer mask for the Boat layer.
Paste the contents of the default buffer by typing C-v in the image window.
Anchor the resulting floating selection into the layer mask by clicking on the Anchor button in the Layers dialog.
Make the Boat layer active by clicking on its thumbnail in the Layers dialog.
Blur and offset the Boat layer using Gaussian Blur (IIR) with a radius of 5, and Offset  the layer by 5 in X and Y.
These steps produce the image shown in Figure  8.32(a).
Figure 8.32: Making the Punchout Effect
Figure 8.32

Figure  8.32(b) shows the associated Layers dialog.

Notice that the creation of the punchout has turned the inset white. To recover the dark colored wood of the inset the Boat layer is made active and the Multiply mode is selected from the Mode menu in the Layers dialog. As a final finesse, the shadow from the punchout is made a bit more diffuse by setting the Opacity slider, in the Layers dialog, to 65%. The final carved result is shown in Figure  8.33(a).

Figure 8.33: Using the Multiply Mode and the Opacity Slider to Obtain the Final Carved Effect
Figure 8.33

Figure  8.33(b) shows the associated Layers dialog.

Grokking The Gimp
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