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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Anemone_purple_anemonefish.jpg (1803×2254). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 1803 × 2254 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 361 × 451 · IDCT 1/2 → 902×1127 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 361×451
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 181 × 226 · IDCT 1/2 → 902×1127 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 181×226
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 180 × 225 · IDCT 1/4 → 451×564 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 180×225
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 90 × 113 · IDCT 1/4 → 451×564 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 90×113
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 90 × 113 · IDCT 1/8 → 226×282 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 90×113
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 45 × 56 · IDCT 1/8 → 226×282 · Lanczos ratio ˜ 5.04×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 45×56
diff abs diff ×10
pixel delta, amplified 10×