Reaction Diffusion is an algorithm that mimics the chemical process that's responsible for a lot of patterns we see in the natural world. The stripes on a zebra, the spots on a cheetah or giraffe. Fish have these sorts of patterns, plants, coral, etc. And considering how often these patterns appear in nature, its amazing how little commercial software there is out there to create such patterns.
You have two choices with this lesson, watch me discuss the issue in the video below, or read the full text.
Image from Spots plus spots equals maze: how animals create living patterns 
Image from Zebra Facts 
Image from Wildscreen Arkive 
Initial: Random Dots 
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Initial: Random Dots with more distance between them 
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Initial: Photo Of A Tiger From The Internet 
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Initial: Perlin Noise With Fractal and High Contrast 
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Initial: A dot 
Final: I stopped the process after a few iterations 
Initial: A Tall Dot 
Final: I let the process keep going for a lot of iterations 

Initial: Big empty dot with smaller dots 
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Initial: Big empty dot with smaller dots 
Final: Notice how blank areas tend to fill with longer linear strands 

Initial: Lines and circles 
Final: Notice how dots and lines form slightly different patterns 
Initial: All Wavy Lines 
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Initial: Long lines 
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Initial: Long lines at different distances 
Final: Notice how more blank area leads to curvier lines, and areas where lines are closer together remain straight 

Initial: Horizontal Noise 
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Initial: Horizontal Noise 
Final: This simulation had more iterations to get a more complex pattern 

Initial: Horizontal Noise with Contrast 
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Initial: Perlin Noise With Contrast 
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Initial: A few thick lines 
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Initial: Random Dots 
Final: Only a couple of iterations 

Initial: Round Cellular Pattern 
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Initial: Square Cellular Pattern 
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Initial: Round Cellular Pattern Large 
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Initial: Round Cellular Pattern High Contrast 
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Initial: Round Cellular Pattern High Contrast and Fractal Iterations 
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Initial: Square Cellular Pattern with Noise on top 
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Initial: Tech Pattern 
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Initial: Circles 
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Initial: Random Dots 
Final: More Blur causes larger patterns to form 

Initial: A dot 
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Initial: All Wavy Lines 
Final: Thicker Wavy Lines 
Initial: Cellular Pattern 
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Initial: Perlin Noise High Contrast 
Final: Sorta Reminds Me Of Sand Dunes 

Initial: Round Cellular Pattern 
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Initial: Dots at regular intervals 
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Initial: Long lines at different distances 
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Initial: Tiny Random Dots 
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Initial: Photo Of A Tiger From The Internet 
Final: Less Blur causes finer patterns to appear 
Initial: Perlin Noise from one direction 
Final: Since the directional blur is less, you get finer patterns 

Initial: Horizontal Noise with Contrast 
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Initial: Perlin Noise High Contrast 
Final: Stuff in middle is spun less of a distance than stuff at the edge, so you have a more detailed final pattern in the middle 

Initial: Random Dots 
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Initial: Square Cellular Pattern 
Final 
Initial: Long lines at different distances 
Final 