contour direction is not correct

[typemytype]

The first drawing the source, second the result through skia, the third with booleanOperations

<?xml version="1.0" encoding="UTF-8"?>
<glyph name="B" format="2">
  <outline>
    <contour>
      <point x="375" y="24" type="curve" smooth="yes"/>
      <point x="567" y="24"/>
      <point x="723" y="180"/>
      <point x="723" y="372" type="curve" smooth="yes"/>
      <point x="723" y="563"/>
      <point x="567" y="719"/>
      <point x="375" y="719" type="curve" smooth="yes"/>
      <point x="184" y="719"/>
      <point x="28" y="563"/>
      <point x="28" y="372" type="curve" smooth="yes"/>
      <point x="28" y="180"/>
      <point x="184" y="24"/>
    </contour>
    <contour>
      <point x="375" y="147" type="curve" smooth="yes"/>
      <point x="252" y="147"/>
      <point x="151" y="248"/>
      <point x="151" y="372" type="curve" smooth="yes"/>
      <point x="151" y="495"/>
      <point x="252" y="595"/>
      <point x="375" y="595" type="curve" smooth="yes"/>
      <point x="499" y="595"/>
      <point x="599" y="495"/>
      <point x="599" y="372" type="curve" smooth="yes"/>
      <point x="599" y="248"/>
      <point x="499" y="147"/>
    </contour>
    <contour>
      <point x="175" y="180" type="line"/>
      <point x="232" y="120" type="line"/>
      <point x="386" y="226" type="line"/>
      <point x="805" y="-96" type="line"/>
      <point x="918" y="-22" type="line"/>
      <point x="379" y="332" type="line"/>
    </contour>
  </outline>
</glyph>

[anthrotype]

yeah, the handling of contour direction in the resulting path sometimes is completely wrong, I still haven't figured out why. I have to say I've lost most of my initial enthusiasm.
I'm not looking forward to fix skia bugs.

[khaledhosny]

@anthrotype If you think Skia is not suitable for our needs, we better kill this experiment now and start over with something else.

[anthrotype]

I don't know, Khaled. Maybe we are using the API incorrectly... I need help from you guys debugging this.
I can expose more SkPath methods to Python to help debugging, but I don't know if I have the ability/time to dig deeper in the C++ implementation.

[adrientetar]

It's fine, we just need to shift the start point to the next on curve and reverse each contour in the resulting path.

[anthrotype]

We can also try reproducing the issue using this interactive page https://fiddle.skia.org/c/@pathops
you can write some C++ code that uses skia and see the rendering live.
If we can repro then we can file a bug and hope they fix it upstream.

[adrientetar]

It's not a bug: https://groups.google.com/d/msg/skia-discuss/9xieDIhMoxk/XbfbmHOTAAAJ

But the transform to what we want is (afaict) deterministic.

[anthrotype]

so if skia cannot give us a result path with nonzero winding fill type, but always returns a path with even-odd fill type, then we would need a reliable way to change it back to non-zero winding...

[adrientetar]

Actually yeah, with the example of the OP it doesn't work, one contour needs not be reverse.

[khaledhosny]

Can someone give me a python snippet that I can test with?

[adrientetar]

Anyway yeah assuming it's a valid even odd contour (which I assume it is, for proper rendering) there must be an algorithm to change the contour according to the other fill rule.

[anthrotype]

in TruFont, you can do

from pathops import *
glyph = CurrentGlyph()
contours = list(glyph)
glyph.clearContours()
union(contours, glyph.getPen())

[anthrotype]

note that the union function is internally calling the binary Op function with a UNION operator and the second operand being another empty Path.

You could alternatively try using a OpBuilder instance (wrapper for SkOpBuilder.cpp), call add methods on it, and then finally the resolve method, to see if anything changes.

And I noticed sometimes one gets it right, while the other doesn't. I still haven't understood the difference between the two though.

[adrientetar]

Seems converting from even odd to winding might be quite complex. @behdad thoughts?

[anthrotype]

also, in the functions defined in operations.py module, we are accumulating multiple contours in a single Path, and pass that to Op function as a whole.

We could alternatively try calling builder.add for as many contours in a glyph and see if that changes the result. In my tests, it didn't help much (some inner contours would even disappear altogether!)

[khaledhosny]

Interestingly, Skia’s canvas is able to draw the glyph correctly after the union operation https://fiddle.skia.org/c/4873ecb267c61ff616953a3b3887ad9e