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IDCT prescale + Lanczos3  vs  pure Lanczos3

Input: Callyspongia_vaginalis_(Branching_Vase_Sponge_-_pink_variation).jpg (2434×3240). --idct-min-factor 2.5. Images render at intrinsic pixel size — no CSS scaling.
Firefox would skip the IDCT path on this image — 2434 × 3240 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 487 × 648 · IDCT 1/2 → 1217×1620 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 487×648
diff abs diff ×10
pixel delta, amplified 10×
1/2 heavy target 244 × 325 · IDCT 1/2 → 1217×1620 · Lanczos ratio ˜ 4.99×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 244×325
diff abs diff ×10
pixel delta, amplified 10×
1/4 barely target 243 × 324 · IDCT 1/4 → 609×810 · Lanczos ratio ˜ 2.51×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 243×324
diff abs diff ×10
pixel delta, amplified 10×
1/4 heavy target 122 × 162 · IDCT 1/4 → 609×810 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 122×162
diff abs diff ×10
pixel delta, amplified 10×
1/8 barely target 122 × 162 · IDCT 1/8 → 305×405 · Lanczos ratio ˜ 2.50×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 122×162
diff abs diff ×10
pixel delta, amplified 10×
1/8 heavy target 61 × 81 · IDCT 1/8 → 305×405 · Lanczos ratio ˜ 5.00×
IDCT pure
IDCT ● click to flick PURE ● click to flick
IDCT+Lanczos ⇄ pure Lanczos @ 61×81
diff abs diff ×10
pixel delta, amplified 10×