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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Friesenbergsee_Panorama.jpg (8445×2890). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 8445 × 2890 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 1689 × 578 · IDCT 1/2 → 4223×1445 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1689×578
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 846 × 290 · IDCT 1/2 → 4223×1445 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 846×290
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 844 × 289 · IDCT 1/4 → 2112×723 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 844×289
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 423 × 145 · IDCT 1/4 → 2112×723 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 423×145
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 422 × 144 · IDCT 1/8 → 1056×362 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 422×144
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 212 × 72 · IDCT 1/8 → 1056×362 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 212×72
diff abs diff ×10
pixel delta, amplified 10×