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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Locomotive_ChS4-072_2011_G1.jpg (4800×3200). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4800 × 3200 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 960 × 640 · IDCT 1/2 → 2400×1600 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 960×640
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 481 × 321 · IDCT 1/2 → 2400×1600 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 481×321
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 480 × 320 · IDCT 1/4 → 1200×800 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 480×320
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 240 × 160 · IDCT 1/4 → 1200×800 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 240×160
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 240 × 160 · IDCT 1/8 → 600×400 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 240×160
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 120 × 80 · IDCT 1/8 → 600×400 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 120×80
diff abs diff ×10
pixel delta, amplified 10×