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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Havne_udsigt_fra_dokken-CN.jpg (3478×2296). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3478 × 2296 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 696 × 459 · IDCT 1/2 → 1739×1148 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 696×459
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 348 × 230 · IDCT 1/2 → 1739×1148 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 348×230
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 348 × 230 · IDCT 1/4 → 870×574 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 348×230
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 174 × 115 · IDCT 1/4 → 870×574 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 174×115
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 174 × 115 · IDCT 1/8 → 435×287 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 174×115
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 87 × 58 · IDCT 1/8 → 435×287 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 87×58
diff abs diff ×10
pixel delta, amplified 10×