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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Skyline_Frankfurt_2011-01.jpg (3439×2292). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3439 × 2292 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 688 × 458 · IDCT 1/2 → 1720×1146 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 688×458
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 345 × 230 · IDCT 1/2 → 1720×1146 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 345×230
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 344 × 229 · IDCT 1/4 → 860×573 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 344×229
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 172 × 115 · IDCT 1/4 → 860×573 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 172×115
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 172 × 115 · IDCT 1/8 → 430×287 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 172×115
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 86 × 57 · IDCT 1/8 → 430×287 · Lanczos ratio ˜ 5.04×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 86×57
diff abs diff ×10
pixel delta, amplified 10×