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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Strombus_sinuatus_2010_G1.jpg (5600×4200). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 5600 × 4200 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 1120 × 840 · IDCT 1/2 → 2800×2100 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1120×840
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 561 × 421 · IDCT 1/2 → 2800×2100 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 561×421
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 560 × 420 · IDCT 1/4 → 1400×1050 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 560×420
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 281 × 210 · IDCT 1/4 → 1400×1050 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 281×210
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 280 × 210 · IDCT 1/8 → 700×525 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 280×210
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 140 × 105 · IDCT 1/8 → 700×525 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 140×105
diff abs diff ×10
pixel delta, amplified 10×