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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Asperitas_inquinata_penidae_01.JPG (6000×3289). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 6000 × 3289 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 1200 × 658 · IDCT 1/2 → 3000×1645 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1200×658
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 601 × 330 · IDCT 1/2 → 3000×1645 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 601×330
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 600 × 329 · IDCT 1/4 → 1500×823 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 600×329
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 301 × 165 · IDCT 1/4 → 1500×823 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 301×165
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 300 × 164 · IDCT 1/8 → 750×412 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 300×164
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 150 × 82 · IDCT 1/8 → 750×412 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×82
diff abs diff ×10
pixel delta, amplified 10×