← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: La_Chambre_à_Arles,_by_Vincent_van_Gogh,_from_C2RMF.jpg (15016×11741). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 15016 × 11741 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 3003 × 2348 · IDCT 1/2 → 7508×5871 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 3003×2348
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 1505 × 1176 · IDCT 1/2 → 7508×5871 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1505×1176
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 1502 × 1174 · IDCT 1/4 → 3754×2936 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1502×1174
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 752 × 588 · IDCT 1/4 → 3754×2936 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 752×588
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 751 × 587 · IDCT 1/8 → 1877×1468 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 751×587
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 376 × 294 · IDCT 1/8 → 1877×1468 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 376×294
diff abs diff ×10
pixel delta, amplified 10×