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# Fractals in React

### An article that was originally meant to discuss why recursion doesn't work in React turns into one concerning implementing fractals in React. What could go wrong? Read on to find out more.

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So… this started as an article about why recursion doesn't work in React. It looks like it works, then you `npm run build`, and it stops working.

Curious, right? Worth looking into, eh?

That's not the article you're getting. It started as that article, then I spent 3 hours building a Pythagoras tree fractal. It's 2:30am, and is my life even real?

Who the hell accidentally spends all night building fractals? Me… I guess.

Pretty, innit? Built with React, and it's going to stop working when I `npm run build`. Still don't know why. I'll figure that out next week.

Here's how the Pythagoras tree works:

The construction of the Pythagoras tree begins with a square. Upon this square are constructed two squares, each scaled down by a linear factor of ½√2, such that the corners of the squares coincide pairwise. The same procedure is then applied recursively to the two smaller squares, ad infinitum.

That becomes four bullet points:

• 1 component called `<Pythagoras >`
• draws rectangle
• calculates props for next 2 rectangles
• `<Pythagoras><Pythagoras>`

Which turns into some 30 lines of code:

``````import React from 'react';
import { interpolateViridis } from 'd3-scale';

const Factor = .5*Math.sqrt(2);

const Pythagoras = ({ maxlvl, w, x, y, lvl, left, right }) => {
if (lvl > maxlvl || w < 1) {
return null;
}

const nextLeft = Factor*w,
nextRight = Factor*w,
d = nextLeft + nextRight + w,
A = 45,
B = 45;

let rotate = '';

if (left) {
rotate = `rotate(\${-A} 0 \${w})`;
}else if (right) {
rotate = `rotate(\${B} \${w} \${w})`;
}

return (
<g transform={`translate(\${x} \${y}) \${rotate}`}>
<rect width={w} height={w}
x={0} y={0}
style={{fill: interpolateViridis(lvl/maxlvl)}} />

<Pythagoras w={nextLeft}
x={w-nextLeft} y={-nextLeft} lvl={lvl+1} maxlvl={maxlvl}
right />

<Pythagoras w={nextRight}
x={0} y={-nextRight} lvl={lvl+1} maxlvl={maxlvl}
left />
</g>
);
};

export default Pythagoras;``````

Beautiful. Let me explain.

`interpolateViridis` is a d3-scale that gives beautiful colors. Call it with an argument in `[0, 1]` and it returns a color.

`Factor` is the constant linear factor. We use it to calculate the sides of future rectangles.

`d` is the diameter of the triangle formed by the current square and two future squares. More on that later.

`A` and `B` are angles for each future rectangle. Set to 45 degrees statically.

## Then We Start Drawing

If we're in a `left` rectangle, we set up a left rotation; if `right` then a right rotation. `rotate()` is an SVG transformation that rotates the current coordinate system.

To draw the rectangle, we:

• `translate` to `(x, y`), that means "move there"
• now our coordinate system is moved and rotate
• draw a rectangle at `(0, 0)`
• add two `<Pythagoras>` with new parameters

And that's how you build a fractal in React. It won't work in production, but it sure looks pretty on your localhost.

The animation is done in App.js with a timer that updates the `maxlvl` prop every 500ms. Calling the root node of `Pythagoras` looks like this:

``` <Pythagoras w={100} x={320-50} y={480-100} lvl={0} maxlvl={this.state.currentMax}/> ```

Start `lvl` at `0` and set the `maxlvl`. Those are important. At `maxlvl` past 12 or 13, it stops working. It takes too much CPU power to ever render.

Yes, I tried. The naive algorithm isn't good enough. You could optimize by taking calculations out of recursion and preparing them in advance.

## The Part I Can't Figure out

Look at Andrew Hoyer's Pythagoras tree. That thing is beautiful and flexible and dances like tree-shaped worm.

I can't figure out how to calculate those angles and rectangle sizes. I know that using `.5` in the `Factor` is for `45` degree angles.

You can change the ratio by using a `.3` and `.7` factor for each side. Then it stops working with `45` degree angles yeah.

Ok, that was expected. Since you know all the sides, you should be able to apply the Law of Sines to calculate the angle.

`````` const nextLeft = .3*Factor*w,
nextRight = .7*Factor*w,
d = nextLeft + nextRight + w,
A = Math.degrees(Math.asin(nextRight/d)),
B = Math.degrees(Math.asin(nextLeft/d));``````

I can't figure it out. I'm pretty sure I'm applying the Law of Sines correctly, but the numbers it throws out are wrong.

Halp!

PS: Here's a paper that describes using Pythagoras trees as data structures. Sort of.

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Topics:
linear ,tree ,fractal programming ,react ,recursion ,web dev

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