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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Pyrrhosoma_nymphula_qtl4.jpg (3274×2200). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3274 × 2200 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 655 × 440 · IDCT 1/2 → 1637×1100 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 655×440
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 328 × 220 · IDCT 1/2 → 1637×1100 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 328×220
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 327 × 220 · IDCT 1/4 → 819×550 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 327×220
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 164 × 110 · IDCT 1/4 → 819×550 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 164×110
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 164 × 110 · IDCT 1/8 → 410×275 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 164×110
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 82 × 55 · IDCT 1/8 → 410×275 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 82×55
diff abs diff ×10
pixel delta, amplified 10×