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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Tenodera_sinensis_5_Luc_Viatour.jpg (3954×2438). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3954 × 2438 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 791 × 488 · IDCT 1/2 → 1977×1219 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 791×488
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 396 × 244 · IDCT 1/2 → 1977×1219 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 396×244
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 395 × 244 · IDCT 1/4 → 989×610 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 395×244
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 198 × 122 · IDCT 1/4 → 989×610 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 198×122
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 198 × 122 · IDCT 1/8 → 495×305 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 198×122
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 99 × 61 · IDCT 1/8 → 495×305 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 99×61
diff abs diff ×10
pixel delta, amplified 10×