Choking a Mask

In Chapter 1, "Selections," we saw how we can contract a selection to bring the selection edge slightly inside the subject shape. This prevents color fringing by excluding any background colors. A more visual and flexible way of achieving the same end is to choke a mask. I prefer to perform this step on a layer mask, but you can also choke a Quick Mask or an alpha channel. Here's a simple example:

Figure 4.12. The source image.

1.	Open the Metro image.
2.	Make a selection of the Metro sign. Because of its graceful curves and straight lines, the Pen tool is a good choice, but the Magnetic Lasso also works. If you use the Pen tool, convert your Work Path to a selection by choosing Make Selection from the Paths palette. Choose a feather radius of 0we'll be adding the feathering later.
3.	Switch to Quick Mask to refine your selection as necessary.
4.	When satisfied with the selection, switch back to Standard mode and convert the selection to a layer mask. Option/Alt-double-click the Background layer and then click the Add Layer Mask icon at the bottom of the Layers palette.
5.	To evaluate the quality of your mask, it can help to see your subject against a contrasting color. Create a new layer. Choose a bright color from your Swatches palette and press Option/Alt-Delete to fill this layer with the color. Drag the layer beneath the Metro layer in the Layers palette. Figure 4.13. Putting the subject on a temporary color layer highlights inaccuracies in the mask.
6.	Select the layer mask and choose Filter > Blur > Gaussian Blur. I used an amount of 2 pixels.
7.	With the layer mask selected, choose Image > Adjustments > Levels or Command/Ctrl-L and move the Black point slider and Mid point slider to the right. Watch the document window carefully and you should see the mask contracting around the Metro sign. The farther you drag the sliders the more the mask contracts. Note that moving the White point slider and Mid point slider to the left has the opposite effect, i.e., spreading or expanding the selection.
8.	Discard the temporary color layer by dragging its thumbnail to the Trash icon at the bottom of the Layers palette. Figure 4.14. Choking the mask before (example A) and after moving the Levels Input sliders as indicated (examples B and C).