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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Cervus_elaphus_Luc_Viatour_2.jpg (2556×1704). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2556 × 1704 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 511 × 341 · IDCT 1/2 → 1278×852 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 511×341
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 256 × 171 · IDCT 1/2 → 1278×852 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 256×171
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 256 × 170 · IDCT 1/4 → 639×426 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 256×170
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 128 × 85 · IDCT 1/4 → 639×426 · Lanczos ratio ˜ 5.01×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 128×85
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 128 × 85 · IDCT 1/8 → 320×213 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 128×85
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 64 × 43 · IDCT 1/8 → 320×213 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 64×43
diff abs diff ×10
pixel delta, amplified 10×