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1. Triangulate Your Stock Photo
Let's start with the Triangulator script, downloaded from http://somestuff.ru/I. Unzip the file and launch the script within your web browser. You'll drag the image you're using to create your low-poly design from into the field that says, "Drop your bitmap here".
You'll notice triangles will form where you place anchor points, filled with gradients of the image's color scheme. If you find nothing happens when you click on the photo, hit "Add 25 vertices: randomly" and then "Delete all vertices". You'll find you can freely click around the image and add points without a problem now.
You can click around at random, adding large shapes, or click around the image's features (reduce the opacity of the triangle layer with the slider at the top of the image) in order to make your design more likely to resemble the image you chose.
2. Import into Adobe Illustrator and Photoshop
When satisfied with your design, save the SVG file by Right-Clicking the "save this link" at the top of the image.
Open your SVG file in Adobe Illustrator, Select the entire file and Copy (Control-C) and Paste (Control-V) into a new file in Adobe Photoshop. I initially choose Smart Object when pasting Illustrator files into Photoshop just in case I want to resize without losing any image quality. Afterwards, I Right-Click and choose Rasterize Layer so my image is ready to be Cut, Copied, and Pasted into the pattern file.
3. Create the Pattern
Using the Pen Tool (P), I drew a polygon of no particular shape (just something with lots of edges) and in the Paths panel, set the freshly closed path to Load path as a selection. Copy and Paste the selection into a new document of 1000 pixels square.
I wanted a more subdued color palette, so now that we're working on the pattern I've switched to another image in order to create the final pattern. Your color schemes, initial image, and how subtle of a pattern you're creating is entirely up to you.
Paste two instances of your copied shape into your new file created in the previous step. Rotate the shape by going to Edit > Transform > Rotate 180° and overlap the rotated shape onto the other one. Merge Layers (Control-E) and repeat with another set of shapes. Make sure that when you're merging layers you're not merging the copied shapes onto the background layer.
With your merged shapes, go to Filter > Other > Offset in order to wrap the shapes around the document and expose larger holes in the pattern. Make sure Wrap Around is selected for Undefined Areas. Hit OK and continue on to the next step.
Continue drawing closed path shapes with the Pen Tool on the original triangulated image and pasting them into the pattern document. Do your best to fill in empty areas. When satisfied with their placement, Merge Layers.
When your pattern is filled in, take notice of dark areas that stand out, harsh edges, and portions of the pattern that repeat, making it obvious that shapes are pasted multiple times. Use the Pen Tool to draw polygonal selections and fill them in with Linear Gradients using the Gradient Tool (G). Use the Eyedropper Tool (I) to select colors near your selection shape and make sure the gradient goes from your foreground to transparent.
In the image below, note how the darker shapes were toned down with gradient shapes sampled from nearby.
When satisfied with your design, or when you want to see it in action, make sure your layers are merged, Select All (Control-A), and create a new pattern by going to Edit > Define Pattern... and giving it a name. You'll find your pattern in the Paint Bucket Tool (G) options by selecting Pattern under the fill options and opening the Pattern Swatches. Create a new document, larger than your last, and Fill it with your new pattern.
Great Job, You're Done!
Now that you've created one or more patterns you'll find use for them as backgrounds, a basis for larger illustrations, etc.
You may find that your low-poly patterns are in need of more refinement sometimes and require multiple layers of gradients, several uses of the Offset filter, and a few patterns defined in order to choose one that's just right. With the Triangulator script, you've sped up the process it takes to choose a color palette, create polygons, and create gradient shapes within an image. Take the script for a spin and share your creations in the comment section below!