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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Washington_Dulles_International_Airport_at_Dusk.jpg (2750×1633). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2750 × 1633 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 550 × 327 · IDCT 1/2 → 1375×817 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 550×327
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 276 × 164 · IDCT 1/2 → 1375×817 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 276×164
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 275 × 163 · IDCT 1/4 → 688×409 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 275×163
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 138 × 82 · IDCT 1/4 → 688×409 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 138×82
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 138 × 82 · IDCT 1/8 → 344×205 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 138×82
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 69 × 41 · IDCT 1/8 → 344×205 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 69×41
diff abs diff ×10
pixel delta, amplified 10×