Fix rounded rectangle rendering with thick borders and fill artifacts

[Ai-Chetan]

Fix: Rounded rectangle rendering issues with thick borders and fill artifacts

Fixes #1632

Fixes rendering bugs in rectangles with rounded corners where thick borders caused arc positioning issues and fill colors showed visible line artifacts at corners.

🐛 Problem

When rendering rectangles with rounded corners using rect(..., round_corners=True), two visual issues occurred:

1. Thick border positioning issue: With borders ≥ 2pt line width, the inner and outer arcs of corner circles were misaligned, creating visual distortions
2. Fill artifacts: Rectangles with background colors showed small visible lines at each corner where fill met the border

Reproduction code:

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()

# Thick border - shows positioning issue
pdf.set_draw_color(0, 0, 0)
pdf.set_fill_color(255, 255, 255)
pdf.set_line_width(6)
pdf.rect(50, 40, 120, 70, style='DF', round_corners=True, corner_radius=12)

# With background color - shows line artifacts
pdf.set_fill_color(230, 120, 125)
pdf.set_line_width(1)
pdf.rect(50, 160, 120, 70, style='DF', round_corners=True, corner_radius=12)

pdf.output("round_corners.pdf")

🔧 Root Cause

The _draw_rounded_rect() method drew each corner arc as a separate path operation with independent fill/stroke operations, then added separate lines for edges. This caused:

• Multiple disconnected path segments instead of one continuous closed path
• Each arc being stroked/filled independently, causing alignment issues with thick borders
• Gaps and overlaps between path segments, creating visible artifacts

✅ Solution

Rewrote _draw_rounded_rect() to construct the entire rounded rectangle as a single continuous closed path:

1. Added helper method _draw_arc_segment() that draws arc segments without starting a new path or applying styles
2. Modified _draw_rounded_rect() to:
   • Start with a single m (move) command
   • Draw all edges and corner arcs as one continuous path using l (line) and c (curve) commands
   • Close the path with h and apply the style operator once at the end

Benefits:

• ✅ Thick borders render correctly - entire shape stroked as one unit
• ✅ Fill artifacts eliminated - single fill operation for complete closed path
• ✅ More efficient PDF output (fewer operators)
• ✅ Maintains backward compatibility

🧪 Testing

Automated tests:

# Run the new tests for this fix
pytest test/shapes/test_round_corners_fix.py -v

# Run all rounded corner tests
pytest test/shapes/test_rectangle.py -k round_corners -v

# Run all shape tests
pytest test/shapes/ -v

Manual validation:

# Generate visual validation PDF
python scripts/validate_round_corners_fix.py

Added comprehensive test suite in test/shapes/test_round_corners_fix.py with 3 test cases:

• test_round_corners_thick_border - Verifies thick borders render without positioning issues
• test_round_corners_fill_no_artifacts - Verifies filled corners have no visible line artifacts
• test_round_corners_combined_issue - Tests both issues together

All existing rounded corner tests updated and passing ✅

📝 Changes

Modified:

• fpdf/fpdf.py - Rewrote _draw_rounded_rect(), added _draw_arc_segment() helper method
• test/shapes/class_round_corners_rect.pdf - Updated reference PDF
• test/shapes/class_round_corners_rect_conditionals.pdf - Updated reference PDF
• test/shapes/class_round_corners_rect_draw_color.pdf - Updated reference PDF

Added:

• test/shapes/test_round_corners_fix.py - New test file with 3 comprehensive tests
• test/shapes/round_corners_thick_border.pdf - Reference PDF for thick border test
• test/shapes/round_corners_fill_no_artifacts.pdf - Reference PDF for fill artifacts test
• test/shapes/round_corners_combined_fix.pdf - Reference PDF for combined test
• scripts/validate_round_corners_fix.py - Visual validation script

✔️ Checklist

• A unit test is covering the code added / modified by this PR
• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder (N/A - bug fix)
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged

---

Visual comparison:

Before (with bugs):

• Thick borders show misaligned corner arcs
• Fill colors show visible lines at corners

After (fixed):

• Smooth, perfectly aligned corners with thick borders
• Clean fills with no artifacts

Run python scripts/validate_round_corners_fix.py to generate a validation PDF demonstrating the fix.

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[andersonhc]

Please see comments
Also, the lint job is failing. Please run black and pylint before the next commit.

[Lucas-C]

Excellent work, good job 👍

I asked for 2 extra tests, but appart from that this seems good to me 🙂

[andersonhc]

@allcontributors please add @Ai-Chetan for code

[allcontributors]

@andersonhc

I've put up a pull request to add @Ai-Chetan! 🎉

[andersonhc]

@Ai-Chetan please add an entry on the CHANGELOG.md file, something like

### Fixed
* rendering of round rectangles that caused visual distortions and incorrect filling - thanks to @Ai-Chetan - [PR #1634](https://github.com/py-pdf/fpdf2/pull/1634)

We should be good to merge after that.

[Lucas-C]

Merged!

Thank you very much @Ai-Chetan for your contribution 👍

[Lucas-C]

This has just been released in the new published version 2.8.5:
https://github.com/py-pdf/fpdf2/releases/tag/2.8.5