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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Solar_annular_eclipse_of_January_15,_2010_in_Jinan,Republic_of_China.JPG (4288×2848). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4288 × 2848 is a whole number of MCU blocks (8 × 8). The renders below reflect what would ship.
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1/2 barely target 858 × 570 · IDCT 1/2 → 2144×1424 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 858×570
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 430 × 285 · IDCT 1/2 → 2144×1424 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 430×285
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 429 × 285 · IDCT 1/4 → 1072×712 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 429×285
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 215 × 143 · IDCT 1/4 → 1072×712 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 215×143
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 214 × 142 · IDCT 1/8 → 536×356 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 214×142
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 107 × 71 · IDCT 1/8 → 536×356 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 107×71
diff abs diff ×10
pixel delta, amplified 10×