← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Panurus_biarmicus_male_portrait_01_(MK).jpg (2817×1585). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2817 × 1585 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 563 × 317 · IDCT 1/2 → 1409×793 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 563×317
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 282 × 159 · IDCT 1/2 → 1409×793 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 282×159
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 282 × 158 · IDCT 1/4 → 705×397 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 282×158
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 141 × 79 · IDCT 1/4 → 705×397 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 141×79
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 141 × 79 · IDCT 1/8 → 353×199 · Lanczos ratio ˜ 2.52×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 141×79
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 71 × 40 · IDCT 1/8 → 353×199 · Lanczos ratio ˜ 4.97×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 71×40
diff abs diff ×10
pixel delta, amplified 10×