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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Hakaniemen_metroasema_2.jpg (3000×2000). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 3000 × 2000 is a whole number of MCU blocks (8 × 8). The renders below reflect what would ship.
Space or click the button toggles every pair at once. Click a single image to toggle just that pair. Hold Shift while clicking the toolbar button to swap continuously.
1/2 barely target 600 × 400 · IDCT 1/2 → 1500×1000 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 600×400
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 301 × 200 · IDCT 1/2 → 1500×1000 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 301×200
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 300 × 200 · IDCT 1/4 → 750×500 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 300×200
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 150 × 100 · IDCT 1/4 → 750×500 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×100
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 150 × 100 · IDCT 1/8 → 375×250 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×100
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 75 × 50 · IDCT 1/8 → 375×250 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 75×50
diff abs diff ×10
pixel delta, amplified 10×