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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Vanadium_crystal_bar_and_1cm3_cube.jpg (4698×3588). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4698 × 3588 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 940 × 718 · IDCT 1/2 → 2349×1794 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 940×718
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 471 × 360 · IDCT 1/2 → 2349×1794 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 471×360
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 470 × 359 · IDCT 1/4 → 1175×897 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 470×359
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 235 × 180 · IDCT 1/4 → 1175×897 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 235×180
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 235 × 179 · IDCT 1/8 → 588×449 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 235×179
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 118 × 90 · IDCT 1/8 → 588×449 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 118×90
diff abs diff ×10
pixel delta, amplified 10×