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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Marek_Szufa_Jenny_(lighten).jpg (4599×2694). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4599 × 2694 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 920 × 539 · IDCT 1/2 → 2300×1347 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 920×539
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 461 × 270 · IDCT 1/2 → 2300×1347 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 461×270
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 460 × 269 · IDCT 1/4 → 1150×674 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 460×269
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 230 × 135 · IDCT 1/4 → 1150×674 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 230×135
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 230 × 135 · IDCT 1/8 → 575×337 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 230×135
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 115 × 67 · IDCT 1/8 → 575×337 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 115×67
diff abs diff ×10
pixel delta, amplified 10×