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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Opal_Pool_YNP2_filtered_noise.jpg (3688×2370). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 3688 × 2370 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 738 × 474 · IDCT 1/2 → 1844×1185 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 738×474
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 370 × 237 · IDCT 1/2 → 1844×1185 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 370×237
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 369 × 237 · IDCT 1/4 → 922×593 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 369×237
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 185 × 119 · IDCT 1/4 → 922×593 · Lanczos ratio ˜ 4.98×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 185×119
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 184 × 118 · IDCT 1/8 → 461×297 · Lanczos ratio ˜ 2.52×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 184×118
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 92 × 59 · IDCT 1/8 → 461×297 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 92×59
diff abs diff ×10
pixel delta, amplified 10×