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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: 20110810_North_Face_of_Everest_Tibet_China_Panoramic.jpg (8533×2924). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 8533 × 2924 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 1707 × 585 · IDCT 1/2 → 4267×1462 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1707×585
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 855 × 293 · IDCT 1/2 → 4267×1462 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 855×293
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 853 × 292 · IDCT 1/4 → 2134×731 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 853×292
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 428 × 146 · IDCT 1/4 → 2134×731 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 428×146
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 427 × 146 · IDCT 1/8 → 1067×366 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 427×146
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 214 × 73 · IDCT 1/8 → 1067×366 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 214×73
diff abs diff ×10
pixel delta, amplified 10×