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Lesson 4. Advanced Masking > Creating a mask from an existing channel

Creating a mask from an existing channel

To create a mask from a channel, first you’ll evaluate each of the three channels to see which one offers the most detail and definition. Then you’ll duplicate the best channel for your purposes and adjust its contrast to remove as much gray as possible while still retaining a smooth edge.

1.
Choose File > Open, and open the 04Start.psd file, located in the Lessons/Lesson04/04PSD folder on your hard drive. In the Embedded Profile Mismatch dialog box, select Use the Embedded Profile (Instead of the Working Space), and click OK.

2.
Choose File > Save As, rename the file Jardin.psd, and save the file in the Lesson04/04PSD folder.

3.
Choose File > Open, and open the Leaf.psd file, located in the Lessons/Lesson04/04PSD folder on your hard drive. In the Embedded Profile Mismatch dialog box, select Use the Embedded Profile (Instead of the Working Space), and click OK.

4.
Choose File > Save As, rename the file Leaf2.psd, and save the file in the Lesson04/04PSD folder.

5.
Click the Channels tab to display the Channels palette.

6.
In the Channels palette, click the name of each of the Red, Green, and Blue channels to view the contents.

When evaluating channels as a possible basis for a shape mask, look for shapes that are either mostly white or mostly black pixels. In this case, the Blue channel offers the blackest leaf shape and will be the best choice for your leaf mask.

7.
Click the name of the Blue channel and drag it to the New Channel button at the bottom of the Channels palette to make a copy. The duplicate channel is actually an alpha channel (with gray values) and does not affect the colors in the document, unlike the Red, Green, and Blue channels which do.

8.
Double-click the Blue Copy name to display the Channel Options dialog box. Change the channel name to Leaf Mask, set the opacity at 20%, and click OK.

Later in this chapter, you’ll edit the mask with the paintbrush. On certain images, it’s easier to see details with the mask opacity set to a lower percentage.

Duplicating the Blue channel, renaming it, and setting its opacity to 20%

Now you’ll use the Curves command to adjust the contrast of the new channel.

9.
With the Leaf Mask channel still visible, choose Image > Adjust > Curves.

10.
In the Curves dialog box, click the Set Black Point button (the black eyedropper). The pointer changes to an eyedropper filled with black. Reposition the Curves dialog box, if necessary, so that you can also see the leaf image.

You can zoom in and out on an image while the Curves or Levels dialog box is displayed. Simply move the pointer outside the dialog box and onto the image. Hold down the spacebar and press Ctrl/Command to get the zoom tool. To zoom out, hold down Alt/Option along with the other keys.



11.
Make sure that the Preview option is selected. Position the black eyedropper over the light gray area in the lower right corner of the leaf, and click.

12.
If you can still see gray areas inside the leaf, click them until the inside of the leaf shape is black. Do not click outside of the leaf.

13.
When the leaf is black, click OK.

Using the black eyedropper tool to force the grays to black

For more information on setting the black and white points in Curves or Levels, see “Using target values to set highlights and shadows (Photoshop)” in Photoshop 6.0 online Help.


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