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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Leeds_Castle_-_side_view.jpg (7945×1590). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 7945 × 1590 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 1589 × 318 · IDCT 1/2 → 3973×795 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1589×318
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 796 × 159 · IDCT 1/2 → 3973×795 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 796×159
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 794 × 159 · IDCT 1/4 → 1987×398 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 794×159
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 398 × 80 · IDCT 1/4 → 1987×398 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 398×80
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 397 × 80 · IDCT 1/8 → 994×199 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 397×80
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 199 × 40 · IDCT 1/8 → 994×199 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 199×40
diff abs diff ×10
pixel delta, amplified 10×