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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Into_the_Jaws_of_Death_23-0455M_edit.jpg (2963×2385). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2963 × 2385 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 593 × 477 · IDCT 1/2 → 1482×1193 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 593×477
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 297 × 239 · IDCT 1/2 → 1482×1193 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 297×239
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 296 × 238 · IDCT 1/4 → 741×597 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 296×238
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 148 × 119 · IDCT 1/4 → 741×597 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 148×119
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 148 × 119 · IDCT 1/8 → 371×299 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 148×119
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 74 × 60 · IDCT 1/8 → 371×299 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 74×60
diff abs diff ×10
pixel delta, amplified 10×