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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Crepuscular_rays_at_Sunset_near_Waterberg_Plateau.jpg (5610×3738). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 5610 × 3738 isn't a whole number of MCU blocks (16 × 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 1122 × 748 · IDCT 1/2 → 2805×1869 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1122×748
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 562 × 375 · IDCT 1/2 → 2805×1869 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 562×375
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 561 × 374 · IDCT 1/4 → 1403×935 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 561×374
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 281 × 187 · IDCT 1/4 → 1403×935 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 281×187
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 280 × 187 · IDCT 1/8 → 702×468 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 280×187
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 141 × 94 · IDCT 1/8 → 702×468 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 141×94
diff abs diff ×10
pixel delta, amplified 10×