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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: African_Bush_Elephant.jpg (2592×3888). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 2592 × 3888 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 518 × 778 · IDCT 1/2 → 1296×1944 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 518×778
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 260 × 390 · IDCT 1/2 → 1296×1944 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 260×390
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 259 × 389 · IDCT 1/4 → 648×972 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 259×389
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 130 × 195 · IDCT 1/4 → 648×972 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 130×195
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 130 × 194 · IDCT 1/8 → 324×486 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 130×194
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 65 × 97 · IDCT 1/8 → 324×486 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 65×97
diff abs diff ×10
pixel delta, amplified 10×