scratch/content/html/fr/blog/SVG-and-m4-fractals.md

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isHidden:       false
menupriority:   1
kind:           article
created_at:     2011-10-20T11:39:36+02:00
title: SVG and m4 fractals
subtitle: Accroître le pouvoir des languages déficients.
author_name: Yann Esposito
author_uri: yannesposito.com
# tags:
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<%= blogimage("main.png","Yesod logo made in SVG and m4") %>

begindiv(intro)


<%= tlal %> Voilà comment faire des fractales avec m4 et SVG.

enddiv

You might want to know how I made the logo for yesod in my preceding post.

First, I hate with passion most XML based languages. Because XML wasn't created to be exposed to developer. 
XML should be read and generated by some software. But you should _never_ read or edit it manually.

## The XSLT Example

The main language where XML fail terribly is XSLT. XPath is very good, but XSLT use one of the worst syntax I ever crossed in my life of developer.

In order to reduce the verbosity of such so bad languages, there is a way.
**`m4`**. Yes, the preprocessor you use when you program in `C` and `C++`.

Here are some example: 

- Variable, instead of writing the natural `myvar = value`, here is the <sc>xslt</sc> way of doing this:

<code class="xml">
<xsl:variable name="myvar" select="value"/>
</code>

- Printing something. Instead of `print "Hello world!"` here is the <sc>xslt</sc> equivalent:

<code class="xml">
<xsl:text 
    disable-output-escaping="yes"><![CDATA[Hello world!
]]></xsl:text>
</code>


- printing the value of a variable, instead of `print myvar` the <sc>xslt</sc> is:

<code class="xml">
<xslt:value-of select="myvar"/>
</code>

- Just try to imagine how verbose it is to declare a function with this language.

## The cure (m4 to the rescue)

<code class="xml">
<?xml version="1.0" standalone="yes"?> <!-- YES its XML -->
<!-- ← start a comment, then write some m4 directives:

define(`ydef',`<xsl:variable name="$1" select="$2"/>')
define(`yprint',`<xsl:text disable-output-escaping="yes"><![CDATA[$1]]></xsl:text>')
define(`yshow',`<xsl:value-of select="$1"/>')

-->
<!-- Yes, XML sucks to be read -->
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<!-- And it sucks even more to edit -->
<xsl:template match="/">
    ydef(myvar,value)
    yprint(Hello world!)
    yshow(myvar)
</xsl:template>
</code>

Now just compile this file:

<code class="zsh">
m4 myfile.m4 > myfile.xslt
</code>

And you can profit! Now <sc>xslt</sc> is more readable and easier to edit!

## The cool part: Fractals!

SVG is a format to display vector graphics, it even support animations.
At its beginning some people believed it would be the new Flash. Apparently, it will be more canvas + js.

Let me show you the result:

<object data="<%= blogimagedir %>main.svg" type="image/svg+xml" height="512" width="512"><%= blogimage("main.png","Yesod logo made in SVG and m4") %></object>

The positionning of the "esod" text with regards to the reversed "λ" was done by changing position in firebug. I didn't had to manually regenerate to test.


Making such a fractal is mostly:

1. take a root element
2. duplicate and transform it (scaling, translating, rotate)
3. the result is a sub new element.
4. repeat from 2 but by taking the sub new element as new root.
5. Stop when recursion is deep enough.

If I had to do this for each step, I had make a lot of copy/paste in my SVG, because the transformation is always the same, but I cannot say, use transformation named "titi". Then instead of manually copying some XML, I used m4

and here is the commented code:

<code class="xml" file="yesodlogo.m4">
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!--
     M4 Macros
define(`YTRANSFORMONE', `scale(.43) translate(-120,-69) rotate(-10)')
define(`YTRANSFORMTWO', `scale(.43) translate(-9,-67.5) rotate(10)')
define(`YTRANSFORMTHREE', `scale(.43) translate(53,41) rotate(120)')
define(`YGENTRANSFORM', `translate(364,274) scale(3)')
define(`YTRANSCOMPLETE', `
    <g id="level_$1">
        <use style="opacity: .8" transform="YTRANSFORMONE" xlink:href="#level_$2" />
        <use style="opacity: .8" transform="YTRANSFORMTWO" xlink:href="#level_$2" />
        <use style="opacity: .8" transform="YTRANSFORMTHREE" xlink:href="#level_$2" />
    </g>
    <use transform="YGENTRANSFORM" xlink:href="#level_$1" />
')
 -->
<svg 
    xmlns="http://www.w3.org/2000/svg" 
    xmlns:xlink="http://www.w3.org/1999/xlink"
    x="64" y="64" width="512" height="512" viewBox="64 64 512 512"
    id="svg2" version="1.1">
    <g id="level_0"> <!-- some group, if I want to add other elements -->
        <!-- the text "λ" -->
        <text id="lambda" 
            fill="#333" style="font-family:Ubuntu; font-size: 100px"
            transform="rotate(180)">λ</text>
    </g>
    <!-- the text "esod" -->
    <text 
        fill="#333" 
        style="font-family:Ubuntu; font-size: 28px; letter-spacing: -0.10em" 
        x="-17.3" 
        y="69" 
        transform="YGENTRANSFORM">esod</text>
    <!-- ROOT ELEMENT -->
    <use transform="YGENTRANSFORM" xlink:href="#level_0" />

    YTRANSCOMPLETE(1,0) <!-- First recursion -->
    YTRANSCOMPLETE(2,1) <!-- deeper -->
    YTRANSCOMPLETE(3,2) <!-- deeper -->
    YTRANSCOMPLETE(4,3) <!-- even deeper -->
    YTRANSCOMPLETE(5,4) <!-- Five level seems enough -->
</svg>
</code>

and I compiled it to <sc>svg</sc> and then to <sc>png<sc> with:

<code class="zsh">
m4 yesodlogo.m4 > yesodlogo.svg && convert yesodlogo.svg yesodlogo.png
</code>

## Conclusion

It was fun to make a fractal in <sc>svg</sc>, but the interesting part is how to augment the power of a language using this preprocessor method. 
I used the <sc>xslt</sc> trick at work for example.