Creating a Channel Mask: The Basics
Let's start with a simple channel masking example. Then we can get into the details of each step of this procedure.
Figure 6.5. Using a channel mask makes it easy to maintain fine detail around the edges of the building.
Creating a Channel Mask: The (Practical) Details
Create a channel mask in six easy steps is the theory. Of course, most images aren't as straightforward. Let's look at some tricks you can use when creating channel masks for more challenging images. Because every image
Determining Your Intent
In the following example of a tree, the black
Figure 6.6. The original images (examples A and B ), the finished composite (example C ), and the mask (example D ). Not forcing the contrast on the mask to opaque black and white retains more of the details in the tree branches. This works in this context because one blue sky is replacing another.
Choosing Your Channel
Choosing your channel is
Color channels have different contrast in different color modes. If none of your RGB channels has enough contrast, try duplicating the image and converting it to CMYK to check the channels there. Once your image is in CMYK color mode, duplicate the channel with the most contrast. To use this duplicate channel, go to your original RGB image and choose Select > Load Selection, then choose the CMYK image as the source document, and select the appropriate channel from the Channel drop-down menu.
As an alternative to duplicating a channel, you may get better results using either of two options under the Image menu, Apply Image or Calculations, to make your alpha channel (a potential selection, stored in a channel as a grayscale image). Use Apply Image to combine a duplicate channel with any other channel or with itself, using the same blend mode options available for layers.
Calculations works slightly differently, letting you blend two individual channels (or two copies of the same channel) as a new channel. Both Apply Image and Calculate
For both options Overlay blend mode usually, but not always, works best, lightening values less than 50% gray and darkening values more than 50% gray.
Figure 6.7. The Apply Image (example A ) and Calculations (example B ) dialog boxes.
Figure 6.8. A copy of the Blue channel (example A ), and a more contrasty result using Calculations to blend the Blue channel with itself using the Overlay blend mode (example B ).
You can make it easier to choose your channel mask by adding more contrast to your image with a temporary adjustment layer. Add a Curves adjustment layer and pull the curve down in the shadows and up in the highlights to increase the contrast. Now, when you view your color channels one of them should have the contrast you need. Once you have made (or calculated) your duplicate channel and refined it into a mask, you can trash the adjustment layer. Note that you can also use this approach to help out any of your Selection tools.
Figure 6.9. The original image (example A ), and a duplicate of its Blue channel (example B ). Adding a temporary Curves adjustment layer to increase the contrast (example C ) and duplicating the Blue channel using Calculations yield a much more contrasty alpha channel (example D )a better starting point for your channel mask.
In step 3 of "Creating a Channel Mask," I advised that when painting on your mask you should stay well away from the edges. In some circumstances, however, you can paint up to the edges. If an edge already has a good amount of contrast, try painting in Overlay blend mode. Painting in white in Overlay blend mode turns the gray areas white, but
Make sure that what looks like black is actually black (100%) and what looks like white is actually white (0%) by opening your Info palette and taking a tour around the edges with your Eyedropper tool to note the exact gray values. You may find it helpful to Shift-click a couple of places to take color samples of your edge areas. These samples
Note that if you don't want sharp edges, then forcing your mask to opaque black and white is
Because the gray values of your edges may vary, some parts of the edge may require more force than others to
Figure 6.12. Using the Median filter to remove any stray pixels, before (example A ) and after (example B ).
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