Forum: Inkscape Further requests for assistance with Python extensions

I'm now in the process of optimising and documenting my Inkscape 
extensions that implement the International Orienteering Federation 
mapping standards.  There are some areas of the extensions that I would 
like to improve to make them more universal / intuitive.

1)    Currently, a number of my routines apply formatting and patterns 
of tags or dots to paths (strokes).  I would like to extend the routines 
to also be compatible with rectangles and arcs / ellipses.  I assume 
that the routines to do most of what I want already exist in one or 
other of the 'helper' files like cubicsuperpath, pathmodifier and 
simpletransform, but as yet I've not worked out how to make them work 
for me.  I need to understand how to implement the following from within 
a Python extension:

    a)    Convert a rectangle to a path.        Will rectToPath in 
pathmodifier do this and, if so, what do I need to provide for self amd 
node?
    b)    Convert an arc/ellipse to a path.    Will ArcToPath in 
cubicsuperpath do this and, if so, what do I need to provide for p1 and 
params?

2)    I also need to understand how to apply a transform to a path (i.e. 
embed the transformation into the path so the transform is not required) 
so that effects applied to a heavil transformed path (as can happen when 
using ellipses / arcs) are applied correctly.  Will one of the routines 
in simpletransform do what I want and, if so, which one and what would I 
need to supply to it?

3)     I still need to provide the capability via an extension to 
convert a line into two lines of a given line width and a set distance 
apart.  This will need to work with curved paths, both open and closed. 
The way that I would do this manually in Inkscape would be to create a 
stroke, duplicate it, set the widths of the two strokes such that the 
lower is the width of the two lines plus the gap and the upper is the 
width of the gap, convert both strokes to paths using Path - Stroke to 
Path and then to use the Path - Difference function to leave two lines 
of the required width and separation.  How could I go about recreating 
this from within a extension?  Is there an easier method to do this, 
given that the two lines should not be joined at the ends?

Still outstanding from a previous request are a couple of unanswered 
questions (rephrased):

4)    Ho would I go about detecting whether a point lies within the 
bounds of a closed shape passed to a Python extension?  Is there an 
existing routine in one of the Python extension 'helper' files that can 
do this?

5)    How would I go about implementing a method to detect collision / 
distance between dots from within an extension?  I'd like to implement 
something similar to the 'Remove Overlaps' capability available within 
the 'Align and Distribute' dialog as part of the routine generating the 
dots, but with a single (polar - sqrt(x^2 * y^2)) variable related to 
the user-selected dot density determining how close the dots can be 
placed to each other.  I can see that this could get complex when the 
area, and hence the number of dots, gets large.

With regard to these last two, I believe there may be something possible 
using py2geom, but where do I find details of this?  What are its 
capabilities and are they documented anywhere?

Jon wrote:
> ...
>     a)    Convert a rectangle to a path.        Will rectToPath in 
> pathmodifier do this and, if so, what do I need to provide for self amd 
> node?

The self parameter is nothing to worry about, it has to do with Python's
class syntax (you can compare it to explicitly passing the this pointer
in C++, if that helps):
http://docs.python.org/tutorial/classes.html#class-objects

 From what I see in the code the node parameter should get an XML/SVG 
node.

>     b)    Convert an arc/ellipse to a path.    Will ArcToPath in 
> cubicsuperpath do this and, if so, what do I need to provide for p1 and 
> params?

It can probably do that, but it appears to be used internally when
parsing paths, so I'm not quite sure what the parameters are exactly.
But you could try figuring it out by looking at CubicSuperPath and
perhaps placing a few prints here and there.

> 2)    I also need to understand how to apply a transform to a path (i.e. 
> embed the transformation into the path so the transform is not required) 
> so that effects applied to a heavil transformed path (as can happen when 
> using ellipses / arcs) are applied correctly.  Will one of the routines 
> in simpletransform do what I want and, if so, which one and what would I 
> need to supply to it?

applyTransformToPath would be the likely candidate. It needs a matrix
(mat, a list of lists, see parseTransform) and a "path" (probably as
returned by CubicSuperPath).

> do this, given that the two lines should not be joined at the ends?
Possibly the python bindings for lib2geom can help you here. I'd also
look at using inset/outset.

> ...
> With regard to these last two, I believe there may be something possible 
> using py2geom, but where do I find details of this?  What are its 
> capabilities and are they documented anywhere?

In the python shell, import the python bindings and use help(...) (fill
in the module name, a class name, or a function name).