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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Saami_Family_1900.jpg (3320×2440). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3320 × 2440 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 664 × 488 · IDCT 1/2 → 1660×1220 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 664×488
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 333 × 244 · IDCT 1/2 → 1660×1220 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 333×244
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 332 × 244 · IDCT 1/4 → 830×610 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 332×244
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 166 × 122 · IDCT 1/4 → 830×610 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 166×122
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 166 × 122 · IDCT 1/8 → 415×305 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 166×122
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 83 × 61 · IDCT 1/8 → 415×305 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 83×61
diff abs diff ×10
pixel delta, amplified 10×