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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Green_Flag_West_11-08.jpg (4256×2832). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4256 × 2832 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 851 × 566 · IDCT 1/2 → 2128×1416 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 851×566
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 426 × 284 · IDCT 1/2 → 2128×1416 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 426×284
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 426 × 283 · IDCT 1/4 → 1064×708 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 426×283
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 213 × 142 · IDCT 1/4 → 1064×708 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 213×142
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 213 × 142 · IDCT 1/8 → 532×354 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 213×142
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 107 × 71 · IDCT 1/8 → 532×354 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 107×71
diff abs diff ×10
pixel delta, amplified 10×