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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Chaunax_stigmaeus_dorsal_view2.jpg (2500×1792). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2500 × 1792 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 500 × 358 · IDCT 1/2 → 1250×896 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 500×358
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 251 × 180 · IDCT 1/2 → 1250×896 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 251×180
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 250 × 179 · IDCT 1/4 → 625×448 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 250×179
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 125 × 90 · IDCT 1/4 → 625×448 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 125×90
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 125 × 90 · IDCT 1/8 → 313×224 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 125×90
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 63 × 45 · IDCT 1/8 → 313×224 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 63×45
diff abs diff ×10
pixel delta, amplified 10×