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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Dn_nasva_0854.jpg (3594×2340). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3594 × 2340 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 719 × 468 · IDCT 1/2 → 1797×1170 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 719×468
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 360 × 234 · IDCT 1/2 → 1797×1170 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 360×234
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 359 × 234 · IDCT 1/4 → 899×585 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 359×234
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 180 × 117 · IDCT 1/4 → 899×585 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 180×117
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 180 × 117 · IDCT 1/8 → 450×293 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 180×117
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 90 × 59 · IDCT 1/8 → 450×293 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 90×59
diff abs diff ×10
pixel delta, amplified 10×