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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Alpine_Ibex_in_Gornergrat.JPG (2000×3000). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 2000 × 3000 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 400 × 600 · IDCT 1/2 → 1000×1500 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 400×600
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 200 × 301 · IDCT 1/2 → 1000×1500 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 200×301
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 200 × 300 · IDCT 1/4 → 500×750 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 200×300
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 100 × 150 · IDCT 1/4 → 500×750 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 100×150
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 100 × 150 · IDCT 1/8 → 250×375 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 100×150
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 50 × 75 · IDCT 1/8 → 250×375 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 50×75
diff abs diff ×10
pixel delta, amplified 10×