incorrect dash offset position about thorvg

[capnm]

@mgrudzinska nice to see you again!

but I haven't found any information in the SVG standard specifying where this point should be.

Only found this about the path start of the stroke for an SVG ellipse (circle): :

https://svgwg.org/svg2-draft/shapes.html#EllipseElement
Mathematically, an ‘ellipse’ element is mapped to an equivalent ‘path’ element that consists of four elliptical arc segments, each covering a quarter of the ellipse. The path begins at the "3 o'clock" point on the radius and proceeds in a clock-wise direction (before any transformation).

I have taken some screenshots to show how the various applications view the SVG:
Dash-offset-test.pdf

[capnm]

• inline SVG == Inkscape == another viewers
  

[mgrudzinska]

after #1985:

This is almost perfect except for the ellipses. It is because the starting point for the dashed lines being chosen differently in TVG and Inkscape. This could be easily changed in the appendCircle function, but I haven't found any information in the SVG standard specifying where this point should be. So, changing it just because it is set differently in another tool doesn't seem like the best idea, in my opinion.

Just to see whether the result would be exactly the same, I changed it, and yes, then everything looks like it does in Inkscape:

[mgrudzinska]

@mgrudzinska nice to see you again!

nice to be back and see all of you here!

https://svgwg.org/svg2-draft/shapes.html#EllipseElement Mathematically, an ‘ellipse’ element is mapped to an equivalent ‘path’ element that consists of four elliptical arc segments, each covering a quarter of the ellipse. The path begins at the "3 o'clock" point on the radius and proceeds in a clock-wise direction (before any transformation).

Thx! applied in #1987

Now the results from Inkscape/gimp/browsers and ThorVG are the same.

just to be sure I check more precisely:

<svg width="512" height="512" version="1.1" viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg">
<g fill="none" stroke="#f00" stroke-linejoin="bevel" stroke-width="15">
  <ellipse cx="50" cy="245" rx="45" ry="75" stroke-dasharray="29.3"
stroke-dashoffset="0" />
<ellipse cx="250" cy="245" rx="45" ry="75" stroke-dasharray="29.3"
stroke-dashoffset="7" />
<ellipse cx="450" cy="245" rx="45" ry="75" stroke-dasharray="28.1"
stroke-linecap="square" />
</g>
</svg>

before:

ThorVG after #1985 and #1987 (the expected results):