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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Ara_ararauna_01.jpg (4752×3168). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4752 × 3168 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 950 × 634 · IDCT 1/2 → 2376×1584 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 950×634
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 476 × 317 · IDCT 1/2 → 2376×1584 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 476×317
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 475 × 317 · IDCT 1/4 → 1188×792 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 475×317
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 238 × 159 · IDCT 1/4 → 1188×792 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 238×159
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 238 × 158 · IDCT 1/8 → 594×396 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 238×158
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 119 × 79 · IDCT 1/8 → 594×396 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 119×79
diff abs diff ×10
pixel delta, amplified 10×