← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Matanuska_Glacier_mouth.jpg (3206×2034). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3206 × 2034 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 641 × 407 · IDCT 1/2 → 1603×1017 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 641×407
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 321 × 204 · IDCT 1/2 → 1603×1017 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 321×204
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 321 × 203 · IDCT 1/4 → 802×509 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 321×203
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 161 × 102 · IDCT 1/4 → 802×509 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 161×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 160 × 102 · IDCT 1/8 → 401×255 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 160×102
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 80 × 51 · IDCT 1/8 → 401×255 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 80×51
diff abs diff ×10
pixel delta, amplified 10×