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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Ducati_748_Studio.jpg (3067×2045). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3067 × 2045 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 613 × 409 · IDCT 1/2 → 1534×1023 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 613×409
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 307 × 205 · IDCT 1/2 → 1534×1023 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 307×205
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 307 × 204 · IDCT 1/4 → 767×512 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 307×204
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 154 × 102 · IDCT 1/4 → 767×512 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 154×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 153 × 102 · IDCT 1/8 → 384×256 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 153×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 77 × 51 · IDCT 1/8 → 384×256 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 77×51
diff abs diff ×10
pixel delta, amplified 10×