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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Still_life_fleamarket_amk.jpg (1810×2765). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 1810 × 2765 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 362 × 553 · IDCT 1/2 → 905×1383 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 362×553
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 181 × 277 · IDCT 1/2 → 905×1383 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 181×277
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 181 × 276 · IDCT 1/4 → 453×692 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 181×276
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 91 × 139 · IDCT 1/4 → 453×692 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 91×139
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 90 × 138 · IDCT 1/8 → 227×346 · Lanczos ratio ˜ 2.52×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 90×138
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 45 × 69 · IDCT 1/8 → 227×346 · Lanczos ratio ˜ 5.04×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 45×69
diff abs diff ×10
pixel delta, amplified 10×