← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Gatineau_-_QC_-_Museum_of_Civilisation.jpg (3662×2497). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3662 × 2497 isn't a whole number of MCU blocks (16 × 16), 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 732 × 499 · IDCT 1/2 → 1831×1249 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 732×499
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 367 × 250 · IDCT 1/2 → 1831×1249 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 367×250
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 366 × 250 · IDCT 1/4 → 916×625 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 366×250
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 183 × 125 · IDCT 1/4 → 916×625 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 183×125
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 183 × 125 · IDCT 1/8 → 458×313 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 183×125
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 92 × 63 · IDCT 1/8 → 458×313 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 92×63
diff abs diff ×10
pixel delta, amplified 10×