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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Fogo,_Cape_Verde_Islands.jpg (2800×2800). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 2800 × 2800 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 560 × 560 · IDCT 1/2 → 1400×1400 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 560×560
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 281 × 281 · IDCT 1/2 → 1400×1400 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 281×281
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 280 × 280 · IDCT 1/4 → 700×700 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 280×280
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 140 × 140 · IDCT 1/4 → 700×700 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 140×140
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 140 × 140 · IDCT 1/8 → 350×350 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 140×140
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 70 × 70 · IDCT 1/8 → 350×350 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 70×70
diff abs diff ×10
pixel delta, amplified 10×