← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Château_Chantilly_1.jpg (3038×2040). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3038 × 2040 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 608 × 408 · IDCT 1/2 → 1519×1020 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 608×408
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 304 × 204 · IDCT 1/2 → 1519×1020 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 304×204
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 304 × 204 · IDCT 1/4 → 760×510 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 304×204
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 152 × 102 · IDCT 1/4 → 760×510 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 152×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 152 × 102 · IDCT 1/8 → 380×255 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 152×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 76 × 51 · IDCT 1/8 → 380×255 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 76×51
diff abs diff ×10
pixel delta, amplified 10×