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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Cymbium_cymbium_01.JPG (7305×6525). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 7305 × 6525 isn't a whole number of MCU blocks (8 × 8), 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 1461 × 1305 · IDCT 1/2 → 3653×3263 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1461×1305
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 732 × 654 · IDCT 1/2 → 3653×3263 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 732×654
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 730 × 652 · IDCT 1/4 → 1827×1632 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 730×652
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 366 × 327 · IDCT 1/4 → 1827×1632 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 366×327
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 365 × 326 · IDCT 1/8 → 914×816 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 365×326
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 183 × 163 · IDCT 1/8 → 914×816 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 183×163
diff abs diff ×10
pixel delta, amplified 10×