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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Blick_auf_den_Grimselsee_und_den_Raeterichsbodensee_0981.jpg (14056×5240). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 14056 × 5240 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 2811 × 1048 · IDCT 1/2 → 7028×2620 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 2811×1048
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 1408 × 525 · IDCT 1/2 → 7028×2620 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1408×525
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 1406 × 524 · IDCT 1/4 → 3514×1310 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1406×524
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 704 × 263 · IDCT 1/4 → 3514×1310 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 704×263
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 703 × 262 · IDCT 1/8 → 1757×655 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 703×262
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 352 × 131 · IDCT 1/8 → 1757×655 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 352×131
diff abs diff ×10
pixel delta, amplified 10×