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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Adansonia_grandidieri04.jpg (1944×2592). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would take the IDCT path on this image — 1944 × 2592 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 389 × 518 · IDCT 1/2 → 972×1296 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 389×518
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 195 × 260 · IDCT 1/2 → 972×1296 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 195×260
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 194 × 259 · IDCT 1/4 → 486×648 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 194×259
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 97 × 130 · IDCT 1/4 → 486×648 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 97×130
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 97 × 130 · IDCT 1/8 → 243×324 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 97×130
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 49 × 65 · IDCT 1/8 → 243×324 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 49×65
diff abs diff ×10
pixel delta, amplified 10×