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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: C._Bechstein_Poster,_about_1920_edit.jpg (4205×6341). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4205 × 6341 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 841 × 1268 · IDCT 1/2 → 2103×3171 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 841×1268
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 421 × 635 · IDCT 1/2 → 2103×3171 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 421×635
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 420 × 634 · IDCT 1/4 → 1052×1586 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 420×634
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 211 × 318 · IDCT 1/4 → 1052×1586 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 211×318
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 210 × 317 · IDCT 1/8 → 526×793 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 210×317
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 105 × 159 · IDCT 1/8 → 526×793 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 105×159
diff abs diff ×10
pixel delta, amplified 10×