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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Iglesia_de_san_francisco_300_dpi.jpg (4000×4819). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4000 × 4819 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 800 × 964 · IDCT 1/2 → 2000×2410 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 800×964
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 401 × 483 · IDCT 1/2 → 2000×2410 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 401×483
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 400 × 482 · IDCT 1/4 → 1000×1205 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 400×482
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 200 × 241 · IDCT 1/4 → 1000×1205 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 200×241
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 200 × 241 · IDCT 1/8 → 500×603 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 200×241
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 100 × 121 · IDCT 1/8 → 500×603 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 100×121
diff abs diff ×10
pixel delta, amplified 10×