← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Elisabethkirche_Schneeberg_color_corr.jpg (4236×2817). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4236 × 2817 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 847 × 563 · IDCT 1/2 → 2118×1409 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 847×563
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 424 × 282 · IDCT 1/2 → 2118×1409 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 424×282
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 424 × 282 · IDCT 1/4 → 1059×705 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 424×282
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 212 × 141 · IDCT 1/4 → 1059×705 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 212×141
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 212 × 141 · IDCT 1/8 → 530×353 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 212×141
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 106 × 71 · IDCT 1/8 → 530×353 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 106×71
diff abs diff ×10
pixel delta, amplified 10×