bug in transform handling

[SebKuzminsky]

I'm playing around with the new Document and group/transform feature that @mxgrey added, and I love it -- it's a huge usability improvement.

But I think I've found a bug.

Here's my input SVG:
bones.svg.txt

The 5 paths in that SVG are in a group that uses this transform:

<g id="Sketch" transform="translate(18.120601,518.120601) scale(1,-1)">

Loading that SVG via svg2paths2() and Document reveals the problem:

In [1]: import svgpathtools

In [2]: paths, attr, svg_attr = svgpathtools.svg2paths2('bones.svg')

In [3]: paths[0]
Out[3]: 
Path(Arc(start=(63-1.55117330333e-12j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(93+6.20469321331e-12j)),
     Line(start=(93+6.20469321331e-12j), end=(93+350j)),
     Arc(start=(93+350j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(63+350j)),
     Line(start=(63+350j), end=(63+0j)))

In [4]: doc = svgpathtools.Document('bones.svg')

In [5]: results = doc.flatten_all_paths()

In [6]: results[0]
Out[6]: 
FlattenedPath(path=Path(Arc(start=(81.120601+518.120601j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(111.120601+518.120601j)),
     Line(start=(111.120601+518.120601j), end=(111.120601+168.120601j)),
     Arc(start=(111.120601+168.120601j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(81.120601+168.120601j)),
     Line(start=(81.120601+168.120601j), end=(81.120601+518.120601j))), element=<Element '{http://www.w3.org/2000/svg}path' at 0x7fe75ede9050>, transform=array([[  1.      ,   0.      ,  18.120601],
       [  0.      ,  -1.      , 518.120601],
       [  0.      ,   0.      ,   1.      ]]))

In [7]: results[0].path
Out[7]: 
Path(Arc(start=(81.120601+518.120601j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(111.120601+518.120601j)),
     Line(start=(111.120601+518.120601j), end=(111.120601+168.120601j)),
     Arc(start=(111.120601+168.120601j), radius=(15+15j), rotation=0.0, large_arc=True, sweep=True, end=(81.120601+168.120601j)),
     Line(start=(81.120601+168.120601j), end=(81.120601+518.120601j)))

In [8]: output = [result.path for result in results]

In [9]: svgpathtools.paths2svg.wsvg(paths=output)

The arcs turned inside out! This is what the flattened paths look like:
disvg_output.svg.txt

I haven't debugged it, but I'm going to guess that the negative Y scale may be implicated here.

[swolebro]

Yuuuuup, hit this one myself yesterday, and arrived at the same conclusion as you. Just wanted to chime in and give you that vote of confidence.

(If only I had seen your ticket earlier! Hahah.)

[tatarize]

Yeah, I can also confirm mathwise that for each of the scale_x or scale_y parameters you have negative, you must negate the sweep. So if they are both flipped you negate twice and get positive sweep.

[SebKuzminsky]

Yup, that's exactly what the linked PR does: flip the sweep if either but not both are negative.

https://github.com/mathandy/svgpathtools/pull/112/files