Difference between revisions of "PythonEffectTutorial"

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Effect extensions in Inkscape means a simple programs or scripts that reads an SVG file from standard input, transforms it somehow and prints it to the standart output.  Usually Inkscape sits on both ends, providing the file with some parameters as input first and finally reading the output, which is then used for further work.
[extensions under review]
[[Image:Effect_flow.svg|thumb|right|400px]]
 
We will write a simple effect extension script in Python that will put "Hello World!" or "Hello <value of --what option>!" string in the center of document and inside a new layer.
 
<blockquote>
''This script won't work with the current version of inkscape, it might still help as a useful guide how to create your own scripts though.  Especially the methods to access the xml structure have changed, please refer to the [http://codespeak.net/lxml/ lxml] documentation.''
</blockquote>
 
== Effect Extension Script ==
 
First of all create a file ''hello_world.py'' and make it executable with the Python interpreter with the well-known directive:
<pre>
#!/usr/bin/env python
</pre>
 
If you're going to put the file somewhere else than into inkscape's installation directory, we need to add a path so that python can find the necessary modules:
<pre>
import sys
sys.path.append('/usr/share/inkscape/extensions') # or another path, as necessary
</pre>
 
Import the ''inkex.py'' file with the ''Effect'' base class that will do most of the work for us and the ''simplestyle.py'' module with support functions for working with CSS styles. We will use just the ''formatStyle'' function from this module:
<pre>
import inkex
from simplestyle import *
</pre>
 
Declare a ''HelloWordEffect'' class that inherits from ''Effect'' and write a constructor where the base class is initialized and script options for the option parser are defined:
<pre>
class HelloWorldEffect(inkex.Effect):
    def __init__(self):
        inkex.Effect.__init__(self)
        self.OptionParser.add_option('-w', '--what', action = 'store',
          type = 'string', dest = 'what', default = 'World',
          help = 'What would you like to greet?')
</pre>
 
The complete documentation for the ''OptionParser'' class can be found at http://docs.python.org/lib/module-optparse.html. Here we just use the ''add_option'' method which has as first argument a short option name, as second argument a long option name and then a few other arguments with this meaning:
 
* ''action'' - An action which should be done with option value. In this case we use action ''store'' which will store option value in ''self.options.<destination>'' attribute.
* ''type'' - Type of option value. We use string here.
* ''dest'' - Destination of option action specified by ''action'' argument. Using ''what'' value we say that we want to store option value to self.options.what attribute.
* ''default'' - Defalut value for this option if it is not specified.
* ''help'' - A help string that will be displayed if script will be given no arguments or some option or argument will have wrong syntax.
 
Inkscape will create a GUI form with widgets for all specified options and prefill them with the default values specified using the ''.inx'' file for this extension which we will write later.
 
We need to override only one ''Effect'' class method to provide the desired effect functionality:
<pre>
    def effect(self):
        what = self.options.what
</pre>
As you can imagine we just stored the ''--what'' option value to the ''what'' variable.
 
Now we will finally start to do something. We will have to work with the XML representation of the SVG document that we can access via ''Effect'''s ''self.document'' attribute.
It is of lxml's '' _ElementTree'' class type.  Complete documentation for the lxml package lxml can be found at http://codespeak.net/lxml/.
 
First we get SVG document's ''svg'' element and its dimensions:
<pre>
        svg = self.document.getroot()
        # or alternatively
        # svg = self.document.xpath('//svg:svg',namespaces=inkex.NSS)[0]
 
        # Again, there are two ways to get the attibutes:
        width  = inkex.unittouu(svg.get('width'))
        height = inkex.unittouu(svg.attrib['height'])
</pre>
The ''xpath'' function returns a list of all matching elements so we just use the first one of them.
 
We now create an SVG group element ('' 'g' '') and "mark" it as a layer using Inkscape' SVG extensions:
<pre>
        layer = inkex.etree.SubElement(svg, 'g')
        layer.set(inkex.addNS('label', 'inkscape'), 'Hello %s Layer' % (what))
        layer.set(inkex.addNS('groupmode', 'inkscape'), 'layer')
</pre>
This creates ''inkscape:label'' and ''inkscape:groupmode'' attributes, which will only be read by Inkscape or compatible applications.  To all other viewers, this new element looks just like a plain SVG group.
 
Now we create an SVG text element with a text value containing the "Hello World" string:
<pre>
        text = inkex.etree.Element(inkex.addNS('text','svg'))
        text.text = 'Hello %s!' % (what)
</pre>
 
Set the position of the text to the center of SVG document:
<pre>
        text.set('x', str(width / 2))
        text.set('y', str(height / 2))
</pre>
 
If we want to center the text on its position we need to define the CSS style of the SVG ''text'' element. Actually we use the ''text-anchor'' SVG extension to CSS styles to do that work:
<pre>
        style = {'text-align' : 'center', 'text-anchor' : 'middle'}
        text.set('style', formatStyle(style))
</pre>
 
Finally we connect all created elements together and put them into the SVG document:
<pre>
        layer.append(text)
</pre>
 
We just defined a class that inherited from the original effect extension so we have to create an instance of it and execute it in the main control flow:
<pre>
effect = HelloWorldEffect()
effect.affect()
</pre>
 
== Extension Description File ==
 
To include script in Inkscape's main menu create ''hello_world.inx'' file describing script evokation.
 
<pre>
<inkscape-extension>
  <_name>Hello World!</_name>
  <id>org.ekips.filter.hello_world</id>
  <dependency type="executable" location="extensions">hello_world.py</dependency>
  <dependency type="executable" location="extensions">inkex.py</dependency>
  <param name="what" type="string" _gui-text="What would you like to greet?">World</param>
  <effect>
    <object-type>all</object-type>
    <effects-menu>
      <submenu _name="Examples"/>
    </effects-menu>
  </effect>
  <script>
    <command reldir="extensions" interpreter="python">hello_world.py</command>
  </script>
</inkscape-extension>
</pre>
 
Create ''<param>'' element for every option of a script and ''<dependency>'' for every included module which is not from Python standard library. Inkscape will search for this modules in directory with script. ''<effect>'' element and its descendants defines name of menu item evoking our new "Hello World!" extension.
 
If the inx file isn't well formed or if any of the dependencies wasn't met, the extension won't show up in the menu. If your extension doesn't show up, take a look at extension-errors.log, which may give you a hint why it wasn't loaded.
 
== Installation ==
 
To install a new extenstion just put ''hello_world.py'' and ''hello_world.inx'' files with all dependency modules  to ''<path_to_inkscape>/extensions'' or ''~/.inkscape/extensions'' directory and start Inkscape. A new menu item ''Hello World!'' in ''Effects->Examples'' menu should appear.
 
== Complete Source Code ==
 
Here is a complete commented source pre of ''hello_world.py'' script file:
 
<pre>
#!/usr/bin/env python
 
# These two lines are only needed if you don't put the script directly into
# the installation directory
import sys
sys.path.append('/usr/share/inkscape/extensions') # or another path, as necessary
 
# We will use inex module with predefined effect base class.
import inkex
# simplestyle module provides functions for style parsing.
from simplestyle import *
 
""" Example Inkscape effect extension.
Creates a new layer with "Hello World!" text centered in middle of document."""
class HelloWorldEffect(inkex.Effect):
    """ Constructor.
    Defines "--what" option of a script."""
    def __init__(self):
        # Call base class construtor.
        inkex.Effect.__init__(self)
 
        # Define string option "--what" with "-w" shortcut and default value "World".
        self.OptionParser.add_option('-w', '--what', action = 'store',
          type = 'string', dest = 'what', default = 'World',
          help = 'What would you like to greet?')
 
    """ Effect behaviour.
    Overrides base class' method and insert "Hello World" text in SVG document. """
    def effect(self):
        # Get script "--what" option value.
        what = self.options.what
 
        # Get access to main SVG document element and get its dimensions.
        svg = self.document.getElementsByTagName('svg')[0]
        width = inkex.unittouu(svg.getAttribute('width'))
        height = inkex.unittouu(svg.getAttribute('height'))
 
        # Create a new layer.
        layer = self.document.createElement('g')
        layer.setAttribute('inkscape:label', 'Hello %s Layer' % (what))
        layer.setAttribute('inkscape:groupmode', 'layer')
 
        # Create text element
        text = self.document.createElement('text')
        value = self.document.createTextNode('Hello %s!' % (what))
 
        # Set text position to center of document.
        text.setAttribute('x', str(width / 2))
        text.setAttribute('y', str(height  / 2))
 
        # Center text horizontally with CSS style.
        style = {'text-align' : 'center', 'text-anchor': 'middle'}
        text.setAttribute('style', formatStyle(style))
 
        # Connect elements together.
        text.appendChild(value)
        layer.appendChild(text)
        svg.appendChild(layer)
 
# Create effect instance and apply it.
effect = HelloWorldEffect()
effect.affect()
</pre>
 
[[User:Blackhex|Blackhex]] 11:59, 26 April 2007 (UTC)

Latest revision as of 04:07, 2 February 2020

[extensions under review]

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