- Aug 19, 2010
Working with Channels
Sculptors often say that the figures are already in the stone—they just release them. The same is often true with alpha channels. Look at each channel independently until you find the ones with highest contrast. You can use one or more color channels as the basis for creating an alpha channel.
Converting a Channel into a Mask
Often, you can use a single channel as the basis for an alpha channel or layer mask. This is especially true for scanned or flattened logos.
- Open the file Ch04_Channel_Extract.psd from the book’s DVD.
- Switch to the Channels panel or choose Windows > Channel if it isn’t visible.
- Duplicate the Gray channel by right-clicking and choosing Duplicate Channel. Name the channel “Alpha,” and click OK.
- The new channel needs to be cleaned up. The “white” paper has some discoloration. Turn on the alpha channel’s eye icon, and disable visibility for the Gray channel.
- With the new channel selected, press Cmd+L (Ctrl+L) to open the Levels dialog. Drag the black point and white point sliders toward the center until you move beyond the histogram data. This makes the black and white in the channel perfectly clean.
- The “holes” in the channel must be filled in to create clean transparency. Press B to select your Brush tool. Then press D to load the default colors of black and white, and press X to switch black to the Foreground color. Paint out the white letters in the center of the box. This will ensure that the white part of the logo is not interpreted as transparent.
- The alpha channel must be reversed to “cut the matte.” With the alpha channel selected, press Cmd+I (Ctrl+I) to invert the channel.
- To load the alpha channel, Cmd+click (Ctrl+click) its thumbnail in the Channels panel. Once you have an active selection, you can turn on the eye icon for the Gray channel and select the Gray channel so it’s active. Turn off the eye icon for the alpha channel.
- Switch to the Layers panel. The logo layer is locked because it’s a flattened document. Double-click the layer’s name, and call it “Logo.”
- Click the Add Layer Mask button at the bottom of the Layers panel to store the transparency selection with the layer. You can now save the file as a PSD to store the transparency.
Converting Multiple Channels into a Mask with Calculations
We’ll use an advanced procedure called Calculations to combine two channels into a new channel, which will function as the alpha. Depending on the source photo, you’ll generate anything from a perfect mask to a great start.
- Open the document Ch04_Calculations.tif from the chapter’s folder.
- Call up the Channels panel and look for the highest-contrasting channels. Because you want to remove the background, look
for the highest contrast between foreground and background. The red channel should stand out the most.
We’ll now use an advanced command to merge channels together. Before Photoshop had layers, it had Calculations. The name scares most people off—math is not a favorite course at most journalism and art schools. Relax. The computer does the math. All you have to do is enable the Preview function and tweak a few drop-down menus. This feature is designed to combine channels from the same document or documents of an identical size.
- Call up the Calculations command from the Image menu. Make sure the Preview box is active.
- You can now combine the Red, Blue, Green, or Grayscale composite channels to form a new channel. With this image, use the Red channel as your first source.
- Combine it with the Blue channel. Click the invert box for the Blue channel, and try the Color Dodge blend mode. The resulting image should show a high-contrast image, a white shape for the church tower, and a gray image for the sky.
- Click OK and generate the new alpha channel.
- Now make a Levels adjustment to clean up the matte. You want a high-contrast black-and-white matte. Adjust the midpoint and black point until a clean matte is generated.
With a little bit of experimentation, a perfect matte can be generated. This alpha channel can be kept and used in a video application, or it can be loaded and turned into a layer mask. Calculations aren’t a solution all the time, but they are worth a try when you have high-contrast channels.