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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Lunenburg_-_NS_-_Lunenburg_Academy_edit.jpg (3303×2547). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3303 × 2547 isn't a whole number of MCU blocks (16 × 16), 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 661 × 509 · IDCT 1/2 → 1652×1274 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 661×509
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 331 × 255 · IDCT 1/2 → 1652×1274 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 331×255
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 330 × 255 · IDCT 1/4 → 826×637 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 330×255
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 165 × 128 · IDCT 1/4 → 826×637 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 165×128
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 165 × 127 · IDCT 1/8 → 413×319 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 165×127
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 83 × 64 · IDCT 1/8 → 413×319 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 83×64
diff abs diff ×10
pixel delta, amplified 10×