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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Wind_Lift_I,_Emder_Hafen_WhiteBalanced.jpg (2680×1800). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 2680 × 1800 is a whole number of MCU blocks (8 × 8). The renders below reflect what would ship.
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 536 × 360 · IDCT 1/2 → 1340×900 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 536×360
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 269 × 180 · IDCT 1/2 → 1340×900 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 269×180
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 268 × 180 · IDCT 1/4 → 670×450 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 268×180
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 134 × 90 · IDCT 1/4 → 670×450 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 134×90
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 134 × 90 · IDCT 1/8 → 335×225 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 134×90
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 67 × 45 · IDCT 1/8 → 335×225 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 67×45
diff abs diff ×10
pixel delta, amplified 10×