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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: 360º_Panorama_Saulakopf.jpg (22921×4510). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 22921 × 4510 isn't a whole number of MCU blocks (16 × 16), so trailing-MCU padding could smear into visible pixels (crbug.com/890745, libjpeg-turbo#297). The IDCT renders below show what would happen if the gate were lifted; in stock Firefox with this commit, output matches the pure-Lanczos column.
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 4584 × 902 · IDCT 1/2 → 11461×2255 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 4584×902
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 2297 × 452 · IDCT 1/2 → 11461×2255 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 2297×452
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 2292 × 451 · IDCT 1/4 → 5731×1128 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 2292×451
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 1148 × 226 · IDCT 1/4 → 5731×1128 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1148×226
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 1146 × 226 · IDCT 1/8 → 2866×564 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1146×226
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 574 × 113 · IDCT 1/8 → 2866×564 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 574×113
diff abs diff ×10
pixel delta, amplified 10×