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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Spilosoma_canescens_caterpillar.jpg (2883×1923). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2883 × 1923 isn't a whole number of MCU blocks (16 × 16), 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 577 × 385 · IDCT 1/2 → 1442×962 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 577×385
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 289 × 193 · IDCT 1/2 → 1442×962 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 289×193
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 288 × 192 · IDCT 1/4 → 721×481 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 288×192
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 144 × 96 · IDCT 1/4 → 721×481 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 144×96
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 144 × 96 · IDCT 1/8 → 361×241 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 144×96
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 72 × 48 · IDCT 1/8 → 361×241 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 72×48
diff abs diff ×10
pixel delta, amplified 10×