← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Jerusalem_Holy_Sepulchre_BW_22.JPG (2338×3507). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2338 × 3507 isn't a whole number of MCU blocks (8 × 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 468 × 701 · IDCT 1/2 → 1169×1754 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 468×701
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 234 × 351 · IDCT 1/2 → 1169×1754 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 234×351
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 234 × 351 · IDCT 1/4 → 585×877 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 234×351
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 117 × 176 · IDCT 1/4 → 585×877 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 117×176
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 117 × 175 · IDCT 1/8 → 293×439 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 117×175
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 59 × 88 · IDCT 1/8 → 293×439 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 59×88
diff abs diff ×10
pixel delta, amplified 10×