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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Dendrobates_azureus_qtl1.jpg (4144×2848). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4144 × 2848 is a whole number of MCU blocks (8 × 8). The renders below reflect what would ship.
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 829 × 570 · IDCT 1/2 → 2072×1424 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 829×570
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 415 × 285 · IDCT 1/2 → 2072×1424 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 415×285
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 414 × 285 · IDCT 1/4 → 1036×712 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 414×285
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 208 × 143 · IDCT 1/4 → 1036×712 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 208×143
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 207 × 142 · IDCT 1/8 → 518×356 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 207×142
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 104 × 71 · IDCT 1/8 → 518×356 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 104×71
diff abs diff ×10
pixel delta, amplified 10×