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Export Filters
Skencil uses export filters to write documents to files.
Configuration Variables
For export filter configuration, Skencil uses the following variables:
typeThe type must be Export. This identifier is defined in the
globals() dictionary.
tk_file_typeunload (optional)format_nameThese variables have the same meaning as devguide-22.html#N2in an import filter .
extensionsA string with the standard file extension of the export format.
Alternatively this can be a tuple of file extension strings.
Skencil uses the file extension to find the export filter.
standard_messages (optional)A boolean indicating whether the messages in the plugin are
standard messages that are defined in Skencil's message catalogs.
For third-party plugins this should be omitted.
External Interface
Given a document object and a filename for the output file, Skencil
does the following to find an export filter and to export the drawing:
Take the extension of the output filename and find the
export filter that uses that extension.
Load the module of the plugin
Call the module's save function.
The save function
The function save is the main entry point for an export plugin. It
is called with four positional parameters 
document, file,
filename and options.
document is the document that is to be saved. file is the
output file object open for writing and 
filename is the
corresponding filename. Finally, 
options is a dictionary with
(user specified) options. Currently 
options is always an empty
dictinary as there is no way yet to specify such options.
Implementation Strategy
Apart from the save function entry point Skencil requires very
little from the plugin. The only other thing it expects is that the
plugin traverses the object tree by itself and uses the interface
provided by the 
devguide-14.html#N2Protocols  class to distinguish the
different object types.
In the following, the svg export filter serves as the basis for a a
brief outline of this technique.
Both the svg and the ai filter use the same basic approach. They
implement the filter as a class and use an instance of that class to do
the work:
def save(document, file, filename, options = {}):
    saver = SVGSaver(file, filename, document, options)
    saver.Save()
    saver.close()
The interesting parts happen in the class. The constructor doesn't do
much, it just saves the parameters in instance variables:
class SVGSaver:
    def __init__(self, file, filename, document, options):
        self.file = file
        self.filename = filename
        self.document = document
        self.options = options
The Save method is where the real work begins.
First it writes a kind of file header:
    def Save(self):
        self.file.write('<?xml version="1.0" standalone="yes"?>\n')
        left, bottom, right, top = self.document.BoundingRect()
        width = right - left
        height = top - bottom
        self.trafo = Trafo(1, 0, 0, -1, -left, top)
        self.file.write('<svg width="%g" height="%g"' % (width, height))
        self.file.write('>\n')
The document method 
BoundingRect returns the bounding rectangle of the
entire document
After the header it loops over all the layers in the document...
        for layer in self.document.Layers():
...and if the layer is not a special layer, such as grid or guide layers,
and if the layer is printable...
            if not layer.is_SpecialLayer and layer.Printable():
... save the layer as a group:
                self.BeginGroup()
                self.save_objects(layer.GetObjects())
                self.EndGroup()
GetObjects() returns a list of all objects in the layer. All
compound objects have this method. In the SVG filter, the 
BeginGroupand 
EndGroup methods write the <g> and </g>tags that mark groups in SVG.
Finally, the trailer to close the <svg> tag.
        self.file.write('</svg>')
The next interesting method is save_objects. It loops over the list of
objects...
    def save_objects(self, objects):
        for object in objects:
...and if the object is a compound object...
            if object.is_Compound:
...save it as a group
                self.BeginGroup()
                self.save_objects(object.GetObjects())
                self.EndGroup()
otherwise, if the object is a bezier object, a rectangle or an ellipse...
            elif object.is_Bezier or object.is_Rectangle or object.is_Ellipse:
...save the object as bezier curves.
                self.PolyBezier(object.Paths(), object.Properties(),
                                object.bounding_rect)
The Paths method returns a tuple of devguide-8.htmlpath
objects
. Properties returns an object that has members like
fill_pattern and line_width that define the properties of the
object.
devguide-22.htmlImport Filters
devguide-20.htmlPlugins
devguide-24.htmlPlugin Objects
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