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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Raftsundet_panorama,_2010_09.jpg (7780×1810). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 7780 × 1810 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 1556 × 362 · IDCT 1/2 → 3890×905 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1556×362
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 780 × 181 · IDCT 1/2 → 3890×905 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 780×181
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 778 × 181 · IDCT 1/4 → 1945×453 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 778×181
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 390 × 91 · IDCT 1/4 → 1945×453 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 390×91
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 389 × 90 · IDCT 1/8 → 973×227 · Lanczos ratio ˜ 2.52×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 389×90
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 195 × 45 · IDCT 1/8 → 973×227 · Lanczos ratio ˜ 5.04×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 195×45
diff abs diff ×10
pixel delta, amplified 10×