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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Schiffmühlen_von_Lorenzkirch_und_Strehla_um_1840.jpg (2430×1512). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2430 × 1512 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 486 × 302 · IDCT 1/2 → 1215×756 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 486×302
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 243 × 152 · IDCT 1/2 → 1215×756 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 243×152
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 243 × 151 · IDCT 1/4 → 608×378 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 243×151
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 122 × 76 · IDCT 1/4 → 608×378 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 122×76
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 122 × 76 · IDCT 1/8 → 304×189 · Lanczos ratio ˜ 2.49×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 122×76
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 61 × 38 · IDCT 1/8 → 304×189 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 61×38
diff abs diff ×10
pixel delta, amplified 10×