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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Sella_group_panorama.jpg (5307×2000). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 5307 × 2000 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 1061 × 400 · IDCT 1/2 → 2654×1000 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1061×400
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 532 × 200 · IDCT 1/2 → 2654×1000 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 532×200
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 531 × 200 · IDCT 1/4 → 1327×500 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 531×200
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 266 × 100 · IDCT 1/4 → 1327×500 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 266×100
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 265 × 100 · IDCT 1/8 → 664×250 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 265×100
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 133 × 50 · IDCT 1/8 → 664×250 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 133×50
diff abs diff ×10
pixel delta, amplified 10×