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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Hexaplex_radix_01.jpg (4098×2784). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4098 × 2784 isn't a whole number of MCU blocks (8 × 8), so trailing-MCU padding could smear into visible pixels (crbug.com/890745, libjpeg-turbo#297). The IDCT renders below show what would happen if the gate were lifted; in stock Firefox with this commit, output matches the pure-Lanczos column.
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 820 × 557 · IDCT 1/2 → 2049×1392 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 820×557
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 411 × 279 · IDCT 1/2 → 2049×1392 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 411×279
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 410 × 278 · IDCT 1/4 → 1025×696 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 410×278
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 205 × 139 · IDCT 1/4 → 1025×696 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 205×139
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 205 × 139 · IDCT 1/8 → 513×348 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 205×139
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 103 × 70 · IDCT 1/8 → 513×348 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 103×70
diff abs diff ×10
pixel delta, amplified 10×