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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Porto_Covo_January_2011-2b.jpg (5169×2598). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 5169 × 2598 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 1034 × 520 · IDCT 1/2 → 2585×1299 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1034×520
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 518 × 260 · IDCT 1/2 → 2585×1299 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 518×260
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 517 × 260 · IDCT 1/4 → 1293×650 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 517×260
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 259 × 130 · IDCT 1/4 → 1293×650 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 259×130
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 258 × 130 · IDCT 1/8 → 647×325 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 258×130
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 129 × 65 · IDCT 1/8 → 647×325 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 129×65
diff abs diff ×10
pixel delta, amplified 10×