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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Felsen_Zirmen_Seekopf_Panorama.jpg (7502×2144). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 7502 × 2144 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 1500 × 429 · IDCT 1/2 → 3751×1072 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1500×429
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 752 × 215 · IDCT 1/2 → 3751×1072 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 752×215
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 750 × 214 · IDCT 1/4 → 1876×536 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 750×214
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 376 × 107 · IDCT 1/4 → 1876×536 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 376×107
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 375 × 107 · IDCT 1/8 → 938×268 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 375×107
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 188 × 54 · IDCT 1/8 → 938×268 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 188×54
diff abs diff ×10
pixel delta, amplified 10×