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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Turkish_Stars_2434.JPG (4752×2673). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4752 × 2673 isn't a whole number of MCU blocks (16 × 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 950 × 535 · IDCT 1/2 → 2376×1337 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 950×535
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 476 × 268 · IDCT 1/2 → 2376×1337 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 476×268
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 475 × 267 · IDCT 1/4 → 1188×669 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 475×267
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 238 × 134 · IDCT 1/4 → 1188×669 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 238×134
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 238 × 134 · IDCT 1/8 → 594×335 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 238×134
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 119 × 67 · IDCT 1/8 → 594×335 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 119×67
diff abs diff ×10
pixel delta, amplified 10×