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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Théâtre_Bruges_1835.jpg (2827×3524). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2827 × 3524 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 565 × 705 · IDCT 1/2 → 1414×1762 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 565×705
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 283 × 353 · IDCT 1/2 → 1414×1762 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 283×353
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 283 × 352 · IDCT 1/4 → 707×881 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 283×352
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 142 × 177 · IDCT 1/4 → 707×881 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 142×177
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 141 × 176 · IDCT 1/8 → 354×441 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 141×176
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 71 × 88 · IDCT 1/8 → 354×441 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 71×88
diff abs diff ×10
pixel delta, amplified 10×