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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Keri_majakas.jpg (6000×2788). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 6000 × 2788 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 × 558 · IDCT 1/2 → 3000×1394 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1200×558
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 601 × 279 · IDCT 1/2 → 3000×1394 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 601×279
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 600 × 279 · IDCT 1/4 → 1500×697 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 600×279
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 301 × 140 · IDCT 1/4 → 1500×697 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 301×140
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 300 × 139 · IDCT 1/8 → 750×349 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 300×139
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 150 × 70 · IDCT 1/8 → 750×349 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×70
diff abs diff ×10
pixel delta, amplified 10×