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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Lanzarote_3_Luc_Viatour.jpg (4266×2716). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4266 × 2716 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 853 × 543 · IDCT 1/2 → 2133×1358 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 853×543
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 427 × 272 · IDCT 1/2 → 2133×1358 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 427×272
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 427 × 272 · IDCT 1/4 → 1067×679 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 427×272
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 214 × 136 · IDCT 1/4 → 1067×679 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 214×136
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 213 × 136 · IDCT 1/8 → 534×340 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 213×136
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 107 × 68 · IDCT 1/8 → 534×340 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 107×68
diff abs diff ×10
pixel delta, amplified 10×