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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Plan_de_Paris_vers_1550_color.jpg (6108×4444). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 6108 × 4444 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 1222 × 889 · IDCT 1/2 → 3054×2222 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1222×889
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 612 × 445 · IDCT 1/2 → 3054×2222 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 612×445
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 611 × 444 · IDCT 1/4 → 1527×1111 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 611×444
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 306 × 223 · IDCT 1/4 → 1527×1111 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 306×223
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 305 × 222 · IDCT 1/8 → 764×556 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 305×222
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 153 × 111 · IDCT 1/8 → 764×556 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 153×111
diff abs diff ×10
pixel delta, amplified 10×