Visual comparison of Firefox-style IDCT pre-scaling (libjpeg-turbo's 1/2, 1/4, 1/8 IDCT)
followed by Skia Lanczos3, against pure Skia Lanczos3 from the full-resolution decode.
Each image page shows up to six configs covering the IDCT denom-selection windows
(
--idct-min-factor 2.5).
Click a thumbnail to open its page.
Firefox's commit gates the IDCT path on the JPEG having a whole number of MCUs in
both dimensions, to avoid trailing-MCU padding smearing into visible pixels
(
crbug.com/890745,
libjpeg-turbo#297).
IDCT path marks images Firefox would actually
pre-scale via IDCT (21 of 100);
Lanczos only marks the 79 where
Firefox would skip IDCT — the renders on those pages show what the IDCT path
would produce if the gate were removed, kept for curiosity.
