← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Magellanic_penguin,_Valdes_Peninsula,_e.jpg (3033×2012). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3033 × 2012 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 607 × 402 · IDCT 1/2 → 1517×1006 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 607×402
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 304 × 202 · IDCT 1/2 → 1517×1006 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 304×202
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 303 × 201 · IDCT 1/4 → 759×503 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 303×201
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 152 × 101 · IDCT 1/4 → 759×503 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 152×101
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 152 × 101 · IDCT 1/8 → 380×252 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 152×101
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 76 × 50 · IDCT 1/8 → 380×252 · Lanczos ratio ˜ 5.04×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 76×50
diff abs diff ×10
pixel delta, amplified 10×