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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Gyps_rueppellii_-Nairobi_National_Park,_Kenya-8-4c.jpg (2134×1733). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2134 × 1733 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 427 × 347 · IDCT 1/2 → 1067×867 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 427×347
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 214 × 174 · IDCT 1/2 → 1067×867 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 214×174
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 213 × 173 · IDCT 1/4 → 534×434 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 213×173
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 107 × 87 · IDCT 1/4 → 534×434 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 107×87
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 107 × 87 · IDCT 1/8 → 267×217 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 107×87
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 53 × 43 · IDCT 1/8 → 267×217 · Lanczos ratio ˜ 5.05×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 53×43
diff abs diff ×10
pixel delta, amplified 10×