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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Szopienice_wieża_ciśnień_huty_cynku.jpg (3093×1546). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3093 × 1546 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 619 × 309 · IDCT 1/2 → 1547×773 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 619×309
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 310 × 155 · IDCT 1/2 → 1547×773 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 310×155
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 309 × 155 · IDCT 1/4 → 774×387 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 309×155
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 155 × 77 · IDCT 1/4 → 774×387 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 155×77
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 155 × 77 · IDCT 1/8 → 387×194 · Lanczos ratio ˜ 2.52×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 155×77
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 77 × 39 · IDCT 1/8 → 387×194 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 77×39
diff abs diff ×10
pixel delta, amplified 10×