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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Pieris_rapae_edit2.jpg (3360×2648). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3360 × 2648 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 672 × 530 · IDCT 1/2 → 1680×1324 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 672×530
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 337 × 265 · IDCT 1/2 → 1680×1324 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 337×265
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 336 × 265 · IDCT 1/4 → 840×662 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 336×265
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 168 × 133 · IDCT 1/4 → 840×662 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 168×133
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 168 × 132 · IDCT 1/8 → 420×331 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 168×132
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 84 × 66 · IDCT 1/8 → 420×331 · Lanczos ratio ˜ 5.02×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 84×66
diff abs diff ×10
pixel delta, amplified 10×