[PierreRaybaut]

Closes #82. Follow-up of #81, part of DataLab-Platform/DataLab#453 (curve readability).

Problem

Qt's raster engine draws pens wider than one device pixel much more slowly than one-pixel pens, which prevents applications from using thicker, more readable curves.

Change

• CurveItem.drawLines uses the new get_thick_line_strokes() helper on screen (raster engine, no scaling transform).
• An opaque solid curve wider than one device pixel is drawn as several cosmetic one-device-pixel strokes:
  • the strokes are offset horizontally and vertically by at most one device pixel from each other;
  • the device pixel ratio is taken into account;
  • dash patterns are scaled by the line width, as for the original pen;
  • the polyline is computed once, and the curve is filled once.
• The original pen is still used in the following cases:
  • printing and exporting (other paint engines, scaled painters);
  • translucent colors and non-solid pen brushes;
  • lines wider than 8 device pixels (THICK_LINE_MAX_STROKES_WIDTH);
  • when the new curve/fast_thick_lines option of the plot configuration section is disabled (enabled by default).

Measurements

Noisy curve, 2000 points, width 2, antialiased, CurveItem.drawSeries on a QImage:

Line style	DPR	Original pen	Strokes
Solid	1.0	366 ms	9.8 ms
Solid	1.5	569 ms	18.0 ms
Dashed	1.0	502 ms	9.6 ms
Dashed	1.5	742 ms	15.9 ms

Fidelity compared with the original pen (noisy sine curve, widths 1.5/2/3, DPR 1 and 1.5):

• total ink within 4 %;
• pixel difference below 8 % of the total ink.

Tests

New plotpy/tests/unit/test_curve_thick_strokes.py:

• stroke offsets, including with the device pixel ratio;
• dash pattern scaling;
• cases keeping the original pen (thin, translucent, very wide, scaled painter, other paint engine, option disabled);
• rendering fidelity against the original pen;
• translucent curves rendered exactly as before.

plotpy/tests/unit and plotpy/tests/items pass locally: 325 passed, 5 skipped. test_edit_point_tool was deselected because it also fails locally without this change (mouse simulation).