Fix PDF path collection culling for hexbin offsets#32000
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tacaswell
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Jul 10, 2026
| # Optimization: Fast path for markers with centers inside canvas. | ||
| # This avoids the dictionary lookup for the common case where | ||
| # markers are visible, improving performance for large scatter plots. | ||
| if 0 <= xo <= canvas_width and 0 <= yo <= canvas_height: |
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Is it possible to look at offset_trans condition the fast path on that as well (my suspicion is that this is Identity by default and that is the only safe case).
Alternatively, would it make sense to apply the offset_trans when doing the check?
tacaswell
reviewed
Jul 10, 2026
| facecolors, edgecolors, linewidths, linestyles, | ||
| antialiaseds, urls, offset_position, hatchcolors=hatchcolors): | ||
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| # Optimization: Fast path for markers with centers inside canvas. |
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Do we have any benchmarks on what we are giving up performance-wise here?
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PR summary
This PR fixes a PDF backend regression where visible
hexbincells can be incorrectly skipped in Matplotlib 3.11.0.Closes #31999.
PR #30746 added path collection culling to the PDF backend to avoid bloated PDFs when many colored
scattermarkers are fully outside the canvas. That optimization works forscatter, where transformed offsets correspond to marker centers in canvas coordinates. However,hexbinuses a repeated hexagon path with offsets transformed byAffineDeltaTransform(self.transData). Those transformed offsets are displacements, not absolute canvas-space markercenters.
The PDF backend was culling collections by checking the transformed offset plus an approximate marker extent. For
hexbin, this can classify visible cells as off-canvas, especially when offsets are negative, so the PDF output loses part of the plot while raster output renders correctly.This PR keeps the optimization from #30746, but changes the culling check to use the actual transformed reusable path bounds translated by each offset. This preserves PDF size improvements for truly off-canvas
scattermarkers while correctly renderinghexbinand other collections whose transformed offsets are not absolute marker centers. This fix make the example in #31999 work as expected.A PDF regression test for
hexbinwith negative offsets, compared against an equivalent shifted reference.AI Disclosure
Agentic AI was employed to identify the origin of the issue, make the original fix, add the original test, and write the original issue and the PR description. The AI suggestions were reviewed and polished by human.
PR checklist
Documentation complies with general and docstring guidelines