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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Milvus_migrans_head.jpg (4608×3072). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 4608 × 3072 is a whole number of MCU blocks (16 × 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 922 × 614 · IDCT 1/2 → 2304×1536 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 922×614
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 462 × 308 · IDCT 1/2 → 2304×1536 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 462×308
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 461 × 307 · IDCT 1/4 → 1152×768 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 461×307
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 231 × 154 · IDCT 1/4 → 1152×768 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 231×154
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 230 × 154 · IDCT 1/8 → 576×384 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 230×154
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 115 × 77 · IDCT 1/8 → 576×384 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 115×77
diff abs diff ×10
pixel delta, amplified 10×