Making Selections with Color Channels


Making Selections with Color Channels

You can use an image s color information to make really difficult selections. The differences in the contrast levels of the red, green, and blue channels can often provide a method of isolating regions of the image that seem impossible to select. You ll use this technique and a combination of Photoshop s painting tools and filters to select from images in which the background and foreground are complex and almost indistinct. In Figure 21.1, for example, isolating the tree from the background would otherwise be virtually impossible .

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Figure 21.1: It s close to impossible to select this tree with traditional selection tools, but you can use color channels to do it.

Follow these steps to use color channels to make a selection:

  1. Open the image leaflesstree.psd in the ch21 folder on the Photoshop CS Savvy CD.

  2. Open the Channels palette (Window Channels, or F7 on the keyboard if you still have it clustered with the Layers palette). Click the Red, Green, and Blue channel thumbnails to display each channel in grayscale in the image window. Determine which channel has the most contrast between the fine branch tips of the tree and the background. In this case, it s the Red channel.

  3. Target the Red channel, and duplicate it by choosing Duplicate Channel from the Channels palette menu, or by dragging it to the New Channel icon at the bottom of the palette. The channel will be named Red Copy (see Figure 21.2).

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    Figure 21.2: The copy of the Red channel in the Channels palette.

  4. Choose Image Adjustments Levels and increase the contrast of the Red Copy channel by dragging the sliders inward. Click OK. I set the input levels in this image to 81, 1.00, and 186 (see Figure 21.3).

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    Figure 21.3: The Levels adjustment adds more contrast to the channel.

  5. Choose the Lasso tool . Encircle the tree as close to the branches as possible. Include the dark foreground at the base of the tree (see Figure 21.4).

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    Figure 21.4: The Red Copy alpha channel with the areas around the tree selected

  6. Inverse the selection. Choose white as a foreground color and press Option/Alt-Delete/Backspace to fill the areas around the tree with white (see Figure 21.5).

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    Figure 21.5: The Red Copy alpha channel with the areas around the tree filled with white

  7. Choose Image Adjustments Invert to make all the white areas of the channel black, and the black areas white. (See Figure 21.6).

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    Figure 21.6: The Inverted Red Copy alpha channel

  8. Now that you ve isolated the tree, you re going to place it on a different background. Open the file wasteland.psd from the ch21 folder on the CD.

  9. Click the leafless tree image. Target the RGB channel. Load the Red Copy by dragging it to the Load Channel As Selection icon in the Layers palette to select the tree.

  10. Choose the Move tool ; drag the tree from the image window and drop it onto the wasteland image.

  11. The extremities of the branches are a red color. You ll adjust the color with the Hue/ Saturation sliders. Choose Image Adjust Hue/Saturation. Drag the Hue slider so that it reads 33, and the Saturation slider so that it reads “41 (see Figure 21.7).

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    Figure 21.7: The Hue/Saturation dialog box

  12. Choose Layer Matting Defringe Radius 1px to eliminate any unwanted edge pixels. See Figure 21.8 for the completed image.

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    Figure 21.8: The finished image




Photoshop CS Savvy
Photoshop CS Savvy
ISBN: 078214280X
EAN: 2147483647
Year: 2004
Pages: 355

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