Implement adaptive anti-aliasing for polygon rendering

[jserv]

Background

Anti-aliasing (AA) reduces visual artifacts (jaggies) on rasterized edges by sampling at sub-pixel positions. Traditional fixed AA uses the same sampling grid (e.g., 4x4) for all edges, which is accurate but expensive.

Standard 4x4 AA samples each pixel at 16 sub-pixel positions to compute coverage. Adaptive AA observes that:

1. Perfectly vertical edges (dx=0) have zero horizontal variation → horizontal sub-sampling provides no benefit
2. Text and curves are high-frequency details extremely sensitive to AA degradation
3. Slope-based thresholds cannot distinguish text from geometry at runtime

Implementation Strategy

Conservative Approach: Only optimize perfectly vertical edges (dx=0)

• ✅ Mathematically safe (vertical edges don't need horizontal AA)
• ✅ Zero visual regression (verified with demo-sdl)
• ❌ Rejected: Slope-based thresholds (caused text aliasing at 30-50°)

Span-Width Threshold: Apply optimization only to wide spans (≥16 pixels)

• ✅ Avoids branch overhead on thin/medium spans
• ✅ Optimal balance: +7.9% gain on large rectangles
• ❌ Rejected: 4-pixel threshold (-30% to -35% regression due to branch overhead)

Technical Details

• Add _span_fill_vertical() for vertical edge fast path
• Use constant coverage values (0x40 for full pixels, count*0x10 for partial)
• Optimize via constant multiplication (compiler converts to shift: count << 4)
• Threshold 16 pixels * 4 samples/pixel = 64 samples minimum span width

Performance Results

Measured with mado-perf, 3-run average, Apple M1:

Test Case	Improvement	Statistical Significance
500x500 rectangles	+7.9%	p<0.05 (significant) ✅
100x100 rectangles	+4.0%	p≈0.07 (borderline)
500x500 vertical lines	+3.1%	Positive improvement
100x100 vertical lines	+2.6%	Positive improvement
Text shapes	+0-2%	Quality preserved ✅

Statistical Validation:

• Variance coefficient: 0.9% to 6.5%
• t-test analysis confirms large rectangle gains
• 3 independent test runs for reliability

Precision Trade-off

• Row 2 partial pixels: 0.4% rounding error (1/255)
• Visual impact: Imperceptible (verified with demo-sdl)
• Error source: 0x10 vs 0x0f for first sub-pixel
• Full pixel: 0x40 vs 0x3f (accepted design choice)

Threshold Validation

Empirical testing of different threshold values:

Threshold	Result	Reason
16 pixels	+2.6% to +7.9% ✅	Optimal balance
4 pixels	-30% to -35% ❌	Branch overhead dominates

Why Conservative Approach?

Initial attempts with aggressive AA optimization caused issues:

1. Slope-based thresholds (43°, 60°) caused visible text aliasing
2. Cannot distinguish text from geometry at runtime
3. Text rendering is extremely sensitive to AA quality (2-4 pixel strokes)
4. Vertical edges (dx=0) inherently don't need horizontal AA → mathematically safe

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Summary by cubic

Implemented adaptive anti-aliasing for polygon rendering that uses optimized span fill on perfectly vertical edges (dx=0) with 16-pixel span-width threshold. This delivers 2.6-7.9% speedups on common UI elements with zero visible quality loss, while preserving full 4x4 AA for text, curves, and diagonal strokes.

• New Features
  • Added _span_fill_vertical() optimized fast path for vertical edges
  • Constant multiplication (count * 0x10) instead of array lookup
  • Span-width threshold (16 pixels) to avoid branch overhead
  • Comprehensive polygon benchmark suite with 7 test scenarios
  • Performance: +2.6% to +7.9% improvement, statistically validated
  • Quality: Zero visual regression, verified with demo-sdl

^{Written for commit 5841ceb29843f8831c80ff3968588ad44a60dfc3.}

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (all 1 issues)

Understand the root cause of the following 1 issues and fix them.


<file name="src/poly.c">

<violation number="1" location="src/poly.c:389">
The vertical-span fast path is triggered by checking `edge_start-&gt;dx == 0`, but this dx has already been reduced to the Bresenham remainder, so integer-slope diagonals (e.g., 45° lines where dx == dy) also satisfy the condition and drop to 1x1 AA, causing visible aliasing. Please base the check on a flag that preserves the original slope (e.g., `aa_quality`).</violation>
</file>

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