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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Giraffa_camelopardalis_head_(Profil).jpg (4041×2708). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4041 × 2708 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 808 × 542 · IDCT 1/2 → 2021×1354 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 808×542
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 405 × 271 · IDCT 1/2 → 2021×1354 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 405×271
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 404 × 271 · IDCT 1/4 → 1011×677 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 404×271
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 202 × 136 · IDCT 1/4 → 1011×677 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 202×136
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 202 × 135 · IDCT 1/8 → 506×339 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 202×135
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 101 × 68 · IDCT 1/8 → 506×339 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 101×68
diff abs diff ×10
pixel delta, amplified 10×