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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Auf_dem_Gehrenberg_6.jpg (2534×1672). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2534 × 1672 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 507 × 334 · IDCT 1/2 → 1267×836 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 507×334
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 254 × 168 · IDCT 1/2 → 1267×836 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 254×168
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 253 × 167 · IDCT 1/4 → 634×418 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 253×167
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 127 × 84 · IDCT 1/4 → 634×418 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 127×84
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 127 × 84 · IDCT 1/8 → 317×209 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 127×84
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 63 × 42 · IDCT 1/8 → 317×209 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 63×42
diff abs diff ×10
pixel delta, amplified 10×