← back to index

IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Vexillum_vulpecula_01.JPG (5571×6000). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 5571 × 6000 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 1114 × 1200 · IDCT 1/2 → 2786×3000 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 1114×1200
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 558 × 601 · IDCT 1/2 → 2786×3000 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 558×601
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 557 × 600 · IDCT 1/4 → 1393×1500 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 557×600
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 279 × 301 · IDCT 1/4 → 1393×1500 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 279×301
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 279 × 300 · IDCT 1/8 → 697×750 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 279×300
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 140 × 150 · IDCT 1/8 → 697×750 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 140×150
diff abs diff ×10
pixel delta, amplified 10×