Masking with Channels

In the following steps, we'll use the special channels pasted into the document in the previous section to make selections. Often the ObjectID and Normal channels are both used to select a particular surface that you'd like to protect with a mask. The selection is then converted to a masked layer, ready to receive paint. You'll mask a couple of layers manually before saving an action that automates some of the prep work to save time.

1. If you have Farmhouse.psd open from the previous exercise, you can continue here; if not, open it from your hard drive before continuing. This file is also provided on the companion CD if you are jumping in here.

2. Click the Layers palette if it is not already selected. Toggle the Shadow and Tree layers off by clicking their eye icons. Select the Farmhouse layer. Create a layer set by clicking the Create A New Set button at the bottom of the Layers palette, and rename the new set to Walls.

Select one of the wall surfaces in the tower using the Normal channel.

3. Click the Channels palette. Select the Normal channel or press Ctrl+6. Press W to select the Magic Wand tool. On the Options bar, click the New Selection Mode button, type 20 into the Tolerance text box, and check Anti-aliased and Contiguous. Clear the Use All Layers option.

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4. Click a point inside the tower wall, as shown in Figure 8.21. The marching ants appear on the selected wall surface.

Notice that part of the window is selected. We can subtract these areas from the selection using the information contained in the ObjectID channel.

5. Press Ctrl+5 to select the ObjectID channel. Click the Subtract From Selection button on the Options bar.

Click inside the window and shutter area that is within the current selection. You'll have to click three times to remove each shutter and the glass from the selection (see Figure 8.22).

Figure 8.21

Click with the Magic Wand in the Normal channel, within the wall.

Figure 8.22

Subtracting from the selection using the ObjectID channel: click within the window pane and each of the shutter areas.

7. Click the Layers palette. Press Ctrl+— to select the RGB channels. Notice that you didn't have to be on the Channels palette to change channels. Create a new layer and then add a layer mask; click the Create A New Layer and Add Layer Mask buttons at the bottom of the Layers palette. The selection disappears once the mask is created. Deselect the layer mask by clicking the layer thumbnail; this step is necessary prior to painting or the paint goes on the mask.

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NOTE You will be creating many layers as you mask individual surfaces to receive paint. It would be too much to give each one of these layers a descriptive name. Therefore, organize at a higher level with named layer sets instead.

The masked layer you made in the previous step is ready to receive paint. However, let's hold off on painting a while longer until we mask a few more layers and learn how to automate the prep work.

Remember that the ObjectID channel shows distinct shades of gray for each object in the 3D scene. You can select every pixel of a particular shade of gray using the Magic Wand with zero tolerance in noncontiguous mode.

8. Press Ctrl+5 to select the ObjectID channel. Press W to select the Magic Wand. On the Options bar, click New Selection mode, set Tolerance to 0, and clear Contiguous.

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9. Click a point on the wall of the structure in the background, as shown in Figure 8.23.

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