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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Mexican_yoyos.jpg (4152×3090). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 4152 × 3090 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 830 × 618 · IDCT 1/2 → 2076×1545 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 830×618
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 416 × 310 · IDCT 1/2 → 2076×1545 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 416×310
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 415 × 309 · IDCT 1/4 → 1038×773 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 415×309
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 208 × 155 · IDCT 1/4 → 1038×773 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 208×155
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 208 × 154 · IDCT 1/8 → 519×387 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 208×154
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 104 × 77 · IDCT 1/8 → 519×387 · Lanczos ratio ˜ 5.03×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 104×77
diff abs diff ×10
pixel delta, amplified 10×