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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Coenonympha_pamphilus_qtl1.jpg (3556×2551). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3556 × 2551 isn't a whole number of MCU blocks (8 × 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 711 × 510 · IDCT 1/2 → 1778×1276 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 711×510
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 356 × 256 · IDCT 1/2 → 1778×1276 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 356×256
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 356 × 255 · IDCT 1/4 → 889×638 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 356×255
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 178 × 128 · IDCT 1/4 → 889×638 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 178×128
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 178 × 128 · IDCT 1/8 → 445×319 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 178×128
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 89 × 64 · IDCT 1/8 → 445×319 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 89×64
diff abs diff ×10
pixel delta, amplified 10×