Improve coordinate validator

[bertsky]

By request of @tboenig, this slightly improves useability of the coordinate checks.

Example output (scroll right to see the change):

  <error>INVALIDITY in Word ID 'w46' of 'weigel_gnothi02_1618/page/weigel_gnothi02_1618_0003.xml': coords '1270,526 1270,527 1271,527 1271,529 1272,529 1272,531 1273,531 1273,544 1283,544 1283,545 1284,545 1283,545 1283,560 1282,560 1282,561 1281,561 1281,562 1280,562 1280,563 1279,563 1279,564 1265,564 1265,565 1221,565 1221,566 1196,566 1196,541 1197,541 1197,540 1239,540 1239,532 1240,532 1240,530 1241,530 1241,529 1242,529 1242,527 1256,527 1257,526' - Self-intersection[1283 545]</error>

[codecov-io]

Codecov Report

Merging #418 into master will decrease coverage by 0.57%.
The diff coverage is 32%.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
- Coverage   82.81%   82.24%   -0.58%     
==========================================
  Files          39       39              
  Lines        2275     2292      +17     
  Branches      406      414       +8     
==========================================
+ Hits         1884     1885       +1     
- Misses        330      338       +8     
- Partials       61       69       +8

Impacted Files	Coverage Δ
ocrd_validators/ocrd_validators/page_validator.py	87.73% <32%> (-9.54%)	⬇️

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[bertsky]

Additionally, IMO the coordinate checks should be made a little less strict (and thus more compatible with Aletheia) to avoid crying wolf.

Things I see frequently:

1. very small (up to 1 pixel) violations of non-containment in parent element
   • Shapely does not have almost_within, but one could try containment within the dilated version:

     if not (child_poly.within(node_poly) or
             child_poly.within(node_poly.buffer(0.5)))

2. tiny (direct neighbour) self-intersections because of back-and-forth (probably caused by internal rounding)
   • This must be repaired on the spot, otherwise Shapely will not operate on these polygons. Possibly:

     if not node_poly.is_valid:
         if node_poly.simplify(0.8).is_valid:
             node_poly = node_poly.simplify(0.8)

But it could be more prudent to keep a strict validator, and outsource these repairs into a dedicated Aletheia postprocessor (e.g. ocrd-segment-repair with a new correct-coords=true).

@kba @wrznr @tboenig let me know if you want me to implement either option.

[kba]

Improved UX always welcome.

But it could be more prudent to keep a strict validator, and outsource these repairs into a dedicated Aletheia postprocessor

That seems wiser then relaxing the rules for validation of GT 👍