Although we have provided the relief maps with the oceans already masked as white, you will find that you will have more flexibility and creative opportunities by using the base version in conjunction with its corresponding masks.
The most effective way of doing this is to make use of Photoshop’s powerful “Channels” feature. This takes a little getting used to, but it is well worth spending the time learning – and it doesn’t take long.
Using channels
The concept is really quite simple; each base color of an image has a separate channel – RGB images have three channels (one each for the three video colors of red, green, and blue), plus one extra to show the combined colors, whereas CMYK images have four channels (one each for the four process colors), plus one showing them combined. Grayscale images only have one channel.
However, new channels can be added to an image, and these provide a place where you can store selections, or store masks that can be used to make selections. To see how you can do this:
1. Open the base version of a map (you may wish to use a low resolution version for purposes of experimenting, since this will provide faster results).
2. Open the “Channels” floating palette by selecting “Palettes > Show Channels” from the “Window” menu. Click on the right-pointing arrow at top right of the Channels palette and select “New Channel.”
3. In the dialog box which appears, name the channel “Ocean mask” and click “OK” (leave the other settings as they are). The map image window will now go blank, this being an “empty” new channel.
4. Open the corresponding mask for the map you are using, and choose “All” from the “Select” menu.
Then, EITHER:
5. Holding the mouse button down, drag the mask from its document window onto the window of the channel you have just created.
OR:
6. Choose “Copy” from the “Edit” menu, and close the mask window. The base map window showing the blank channel should now be active (if not, just click anywhere within it). Choose “Paste” from the “Edit” menu. This will paste the map mask into the blank channel.
Whichever method you use (steps 5 or 6), the result is the same, except that the latter uses more memory.
You will now use the “Ocean mask” channel to create a selection for manipulating your base map image. The important thing to remember about using channels for selections is that the black areas on the mask channel are those that will remain unaffected on the channel you are manipulating. Or, to put it another way, the white areas of the mask are those that will be affected when you make changes to your image. To manipulate your image:
7. Click on the word “Black” on the Channels palette (or hit “Command-1” on the keyboard”). This will display the base map channel.
8. Choose “Load Selection” from the “Select” menu. The coastline will appear as “marching ants.” This indicates the selected area.
9. Choose “Adjust > Brightness / Contrast...” from the “Image” menu. In the dialog box that opens, move the slider triangle on the Brightness bar until “+95” is displayed in the value box.
10. Click “OK.” The map will now show the oceans in a much paler shade of gray, creating a contrast between land and sea.
 
Applying effects to selections
The technique of using a selection channel can be employed for a large number of effects. The following examples show just a few simple effects you can achieve after following steps 1 to 8 above:
If, after step 8 above, you pressed the delete key, you would delete the selected area to reveal the background color, ending up with a white ocean (white is the default color for the background) – the same effect as already supplied on your sample map (as the “masked” version). However, if you hold down the “Option” key while hitting “delete,” the sea will fill with the foreground color – in this case black, which is the default color for the foreground:
 
You can fill the ocean area with a gray tint by defining a tint of black. Do this by opening the “Picker” palette and using the slider triangle on the “K” bar until it displays the desired percentage. In this case we have defined 20% black:
 
You can use Photoshop or third-party filters to create dramatic distinctions between land and sea. This image has had the “Noise > Add Noise...” filter applied with a value of 64, before being lightened with the “Brightness / Contrast...” control:
 
Blurring the ocean can give the land an illusion of “floating.” This image was given a Gaussian blur of 64 pixels radius, before being lightened with the “Brightness / Contrast...” control:
 
Gallery Effects (Classic Art Volume 2) “Texturizer” has been applied to this image, before being lightened with the “Brightness / Contrast...” control:
 
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