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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Triple_Portrait_of_Cardinal_de_Richelieu_probably_1642,_Philippe_de_Champaigne.jpg (5274×4225). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 5274 × 4225 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 1055 × 845 · IDCT 1/2 → 2637×2113 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1055×845
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 528 × 423 · IDCT 1/2 → 2637×2113 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 528×423
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 527 × 422 · IDCT 1/4 → 1319×1057 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 527×422
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 264 × 212 · IDCT 1/4 → 1319×1057 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 264×212
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 264 × 211 · IDCT 1/8 → 660×529 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 264×211
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 132 × 106 · IDCT 1/8 → 660×529 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 132×106
diff abs diff ×10
pixel delta, amplified 10×