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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Hot_air_balloon_and_moon.jpg (2962×2320). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2962 × 2320 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 592 × 464 · IDCT 1/2 → 1481×1160 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 592×464
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 297 × 232 · IDCT 1/2 → 1481×1160 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 297×232
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 296 × 232 · IDCT 1/4 → 741×580 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 296×232
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 148 × 116 · IDCT 1/4 → 741×580 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 148×116
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 148 × 116 · IDCT 1/8 → 371×290 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 148×116
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 74 × 58 · IDCT 1/8 → 371×290 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 74×58
diff abs diff ×10
pixel delta, amplified 10×