Draw a black dot
We start by drawing a simple 3 by 3 pixels dot. Select the Ellipse tool from the Tools panel, click your canvas and enter 3 px for the Height and Width. Hit OK. Give the dot a black fill.
Go to Object > Distort & Transform > Transform…
In the window that appears, check the Preview option, then enter 8 px in the Move Horizontal field, and 19 in the Copies field. Hit OK.
Go again to Object > Distort & Transform > Transform…. Hit the OK button in the warning window that appears. Then first check the Preview option again, to see what will happen. Now enter 4 px in both the Move Horizontal and Vertical fields, and enter 1 in the Copies field. Hit OK.
With the dot still selected, go to Object > Expand Appearance. Then go again to Object > Distort & Transform > Transform…. Hit the OK button in the warning window that appears. Then check the Preview option again in the Transform window. Now enter 8 px in the Move Vertical field and 19 in the Copies field. Hit OK. Go to Object > Expand Appearance again, so all dots are fully editable objects.
Adjust Document Raster Effects Settings
Next we'll be applying a Photoshop Effect, which means that our dots will be rasterised. So we need to make sure that the resolutions of our document is set high enough so we get a proper end result. Go to Effect > Document Raster Effects Settings and change the Resolution to High (300 ppi).
Apply Ocean Ripple effect
Make sure all dots are selected. Now go to Effect > Distort > Ocean Ripple.
Set the value for both the Ripple Size and the Ripple Magnitude to 15, and hit OK. You should have a similar result as shown in the image above.
First go to Object > Expand Appearance, then go to Window > Image Trace. To see the result live check the Preview option. You can play with the settings if you prefer, but the default settings should give you a pretty good result already (see the image above as a reference). Just make sure Ignore White is checked. Otherwise Illustrator will draw a white box around your dots and you'll have to delete it manually. If all looks good, click the Expand button in the Options bar at the top.
Create a pattern swatch
We're almost there. Now it's only a matter of transforming this into a pattern swatch, so we can apply it as a pattern fill. Make sure your object is fully selected. Now go to Object > Pattern > Make. Illustrator will give you a warning message that a pattern swatch is created in the Swatches panel. Normally, you don't have to edit a thing here and the dimmed area around the dots should look perfectly seamless. The only thing you need to do is entering a name for your pattern, click the Done link at the top of your document, and you're all set. Your pattern is ready to be applied from the Swatches panel.
Apply the pattern to your illustration
Adding a pattern fill
There are different ways in how you can apply your pattern to your illustration. In my example at the top of the article, I wanted to add texture to some parts of the illustration. I apply the pattern as an extra fill on top of my color fill in Overlay blending mode, using 60% opacity. I'm usually using Overlay as Transparency blending mode, because the hue of the pattern adapts perfectly with the color below. You can add an extra fill via the Appearance panel by clicking the Add Fill icon at the bottom of the panel.
In the example above I added the pattern to the entire tree. To do this I copied the 3 objects of the tree, pasted this in front (cmd/ctrl + F) so the objects are placed into the exact same position. Then I merged the objects together into 1 object (Pathfinder > Unite), and added the pattern fill to this object. In the Layers panel I clicked the circle icon on the right of the layer, and applied 60 % Opacity in Overlay mode (via the Appearance or Transparency panel).
Resizing the pattern fill
In a lot of cases the pattern might appear too big (or maybe too small). You can easily resize the pattern on an object, without scaling the object itself. You do this by only checking the Transform Patterns option in the Scale window, after you've selected the Scale Tool and after you've clicked on your canvas.
Adding an Opacity Mask
Sometimes you want to go a little bit further and have the pattern fade out into a certain direction. You can achieve this by adding an Opacity Mark to your object that holds the pattern fill. So in this case you need to have the pattern fill as a separate object on top of your object that has the color fill. To create such Opacity Mask, you need to draw a shape on top of your pattern fill object. It can be a simple rectangle. As long as your object is fully covered. Then you give this shape a white to black gradient. The white part is where your pattern will be still visible. The black part is where the pattern will fade out. You can edit this gradient at any time, so it doesn't matter how you create it when you set this up.
Next, you select both the gradient object and the pattern fill object, and click the Make Mask button in the Transparency panel (or via the Appearance panel - see also previous image). If the result is not exactly right, you click the mask icon to go into Mask mode where you can edit the gradient. Don't forget to click the image icon on the left (of the link icon) in the Transparency panel to leave this mode when you're done.
Create a set of patterns
Most of the time you'll need more than one pattern to work with. So you need to create a small set that you can use together in your illustration.
The easiest one is creating a white version. You can do this by saving a copy of the one you've already created. To do this, double click the pattern in the Swatches panel to go into Pattern mode. Next, click the Save a copy link at the top and give the new pattern a name. A copy of the original pattern swatch is now saved into the Swatches panel. Click the Done link to exit Pattern mode first, because you are still in edit mode of your original pattern swatch. Now double click the new pattern swatch from the Swatches panel, fully select it, give it a while fill, and click the Done link at the top.
In the image above I have applied this new white texture pattern to the water by adding an extra (pattern) fill on top of the color fill, at 75% opacity.
To create more different patterns, change the spacing and size of the dots, and try out other effects to transform & tweak the shape of the dots. Save a white version of each black version, and give each pattern a logical name that helps you separate them from each other.
Save a Swatch Library file
Once you have created a bunch of different swatches, you can save a Swatches Library File. To do this you need to clean out the Swatches panel first by removing the ones you don't want to include in the set. Once only the pattern swatches are left in the panel, go to the panel's option menu (the small arrow button at the top right of the panel), and choose Save Swatch Library File as AI. You can save it at any location you want. If you want the swatches to load each time you start Illustrator, than the file should be saved into the Swatches folder which is located in the Presets folder. This location is shown by default when you save this file. I usually save my Swatches Library files with the project I've used it for. When I need this file later on, I load it via Open Swatch Library > Other Library…, located in the Swatches panel's options menu.
That's it! Hope you liked it and had some fun. Don't forget to try out other shapes, other effects, etc. Happy texture pattern experimenting!