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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Polypedates_leucomystax_weissbart_ruderfrosch.jpg (3000×1732). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3000 × 1732 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 600 × 346 · IDCT 1/2 → 1500×866 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 600×346
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 301 × 174 · IDCT 1/2 → 1500×866 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 301×174
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 300 × 173 · IDCT 1/4 → 750×433 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 300×173
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 150 × 87 · IDCT 1/4 → 750×433 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×87
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 150 × 87 · IDCT 1/8 → 375×217 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 150×87
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 75 × 43 · IDCT 1/8 → 375×217 · Lanczos ratio ˜ 5.05×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 75×43
diff abs diff ×10
pixel delta, amplified 10×