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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Egretta_novaehollandiae_-_Austins_Ferry.jpg (1657×2500). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 1657 × 2500 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 331 × 500 · IDCT 1/2 → 829×1250 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 331×500
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 166 × 251 · IDCT 1/2 → 829×1250 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 166×251
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 166 × 250 · IDCT 1/4 → 415×625 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 166×250
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 83 × 125 · IDCT 1/4 → 415×625 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 83×125
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 83 × 125 · IDCT 1/8 → 208×313 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 83×125
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 42 × 63 · IDCT 1/8 → 208×313 · Lanczos ratio ˜ 4.97×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 42×63
diff abs diff ×10
pixel delta, amplified 10×