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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: 2011-12-13_12-58-49-eglise-st-maimboeuf-montbeliard.jpg (2004×3598). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2004 × 3598 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 401 × 720 · IDCT 1/2 → 1002×1799 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 401×720
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 201 × 361 · IDCT 1/2 → 1002×1799 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 201×361
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 200 × 360 · IDCT 1/4 → 501×900 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 200×360
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 100 × 180 · IDCT 1/4 → 501×900 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 100×180
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 100 × 180 · IDCT 1/8 → 251×450 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 100×180
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 50 × 90 · IDCT 1/8 → 251×450 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 50×90
diff abs diff ×10
pixel delta, amplified 10×