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Complex Materials aren't really as complex as you'd think. It's just a couple of textures and an alpha map. Your first two selections (“A” nd “B”) are the textures you'll see in your finished render. Your third (“C”) selection is the map. Texture “A” will appear in the black sections of the “C” texture's alpha map (often used as the transparency map). Texture “B” will appear in the white ones. Simple, right? Well, let's give it a shot anyway.
We're going to start with Procedural Textures.
1.) Click on the ground plane to select it.
2.) Click on the “M” to enter the Material Lab.
3.) While holding down the Ctrl key, click on the “C” space (the third column) beside diffuse and ambient in the color section, and diffusion, ambience, and bump height in the value section, as shown. All those little dots should turn blue just like in the picture. Now it knows you're using the third material as a map for the first two. But you probably won't like the look of the randomly assigned materials, so we're going to pick better ones.
4.) Select the material for “A” by opening the drop-down menu. (Click the arrow at the top right of the “A” box.) For this example I chose “Forest” from the “Bump” menu. It's going to be our vegetation.
5.) Select the material for “B” in the same manner. For this example I chose “Marble Sand” from the “Sand” menu. It's going to be, well, sand.
6.) Select the material for “C” in the same manner. For this example I chose “BlackWhite” from the “Basic” menu. I picked BlackWhite because it has good balance and contrast between the two colors. This makes it easy to see our results clearly.
7.) Now open the editor for texture “C.” (Click the blue button in the top left corner of the “C” box.)
8.) Check to make sure that texture “C” is set to a fairly low frequency. You probably won't want anything over 20% for now. If you don't like the results, you can always come back and change this setting later.
9.) Click the check mark to exit the Material Lab.
10.) Render. You should have something that looks similar to mine. See, now that wasn't hard, was it? The forest texture went where the black parts of BlackWhite are, and the Marble Sand went where the white parts are.
Now you can return to the Material Lab and adjust the frequencies of each texture until you have something you're happy with. Save the material if you'really like it.
What do you mean that isn't quite what you had in mind? You want it to look a little more realistic, you say? OK, we can do that. Here we go. We'll do the same thing with image textures. Go ahead and click the “M” to go back into the Material Lab.
11.) Change your “A” texture type to an image texture by clicking on the “P” button toward the bottom left of the “A” box. (Yes, I know it doesn't make a whole lot of sense for “P” to stand for “Image” texture and “T” to stand for “Procedural” texture. But that's the way it is. Just go with it.) Leo will appear after you click the “P” button.
12.) Now click the pink button just above Leo's head to enter the Texture Source Editor.
13.) Click any of the empty gray boxes. (Most people pick the first one.) A dialog will appear that will allow you to browse to whatever image you want to use for the “A” texture. You can use the one of the ones I provided or choose one of your own.
14.) Click the word “Copy” which appears below your image to copy it.
15.) Click the word “Paste” which appears below the box to the right. This will paste your image in as a grayscale image, so you have a bump map.
16.) Click the check mark to exit the Texture Source Editor.
17.) - 22.) Repeat steps 11 - 16 (as shown in the image) for your “B” texture, choosing a contrasting image.
23.) Open the editor for texture “C.” (Click the blue button in the top left corner of the “C” box.)
24.) Check to make sure that texture “C” is set to a fairly low frequency. This way you can better see how your textures look together.
25.) Click the check mark to exit the Material Lab.
26.) Render and evaluate. What do you think?
If you don't like the contrast between your textures, just go back and choose a different image for one or both of them. If you don't like the frequency, you can change that, too. Experiment to find something you like.
If you're feeling really adventurous, you can even create an image in Photoshop (or your favorite paint program) and make a custom image for your alpha channel. This will allow you to make roads, rivers or whatever you can imagine. Good luck!