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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Eurasian_Spoonbill.jpg (4896×3264). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4896 × 3264 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 979 × 653 · IDCT 1/2 → 2448×1632 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 979×653
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 491 × 327 · IDCT 1/2 → 2448×1632 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 491×327
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 490 × 326 · IDCT 1/4 → 1224×816 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 490×326
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 245 × 164 · IDCT 1/4 → 1224×816 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 245×164
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 245 × 163 · IDCT 1/8 → 612×408 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 245×163
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 123 × 82 · IDCT 1/8 → 612×408 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 123×82
diff abs diff ×10
pixel delta, amplified 10×