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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Parc_de_Versailles,_parterre_de_Latone,_bassin_de_Latone_05.jpg (8500×2507). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 8500 × 2507 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 1700 × 501 · IDCT 1/2 → 4250×1254 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1700×501
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 852 × 251 · IDCT 1/2 → 4250×1254 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 852×251
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 850 × 251 · IDCT 1/4 → 2125×627 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 850×251
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 426 × 126 · IDCT 1/4 → 2125×627 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 426×126
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 425 × 125 · IDCT 1/8 → 1063×314 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 425×125
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 213 × 63 · IDCT 1/8 → 1063×314 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 213×63
diff abs diff ×10
pixel delta, amplified 10×