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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Mona_Lisa,_by_Leonardo_da_Vinci,_from_C2RMF_retouched.jpg (7479×11146). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 7479 × 11146 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 1496 × 2229 · IDCT 1/2 → 3740×5573 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1496×2229
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 749 × 1117 · IDCT 1/2 → 3740×5573 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 749×1117
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 748 × 1115 · IDCT 1/4 → 1870×2787 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 748×1115
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 375 × 558 · IDCT 1/4 → 1870×2787 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 375×558
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 374 × 557 · IDCT 1/8 → 935×1394 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 374×557
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 187 × 279 · IDCT 1/8 → 935×1394 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 187×279
diff abs diff ×10
pixel delta, amplified 10×