rflynn
Chroma Downsampling Lowers Visual Quality
Line 337 of imgmin.c contains:
(void) MagickSetImageProperty(mw, "jpeg:sampling-factor", "2x2");
I find that the quality is noticeably and significantly better if I comment/remove that line (although the size is increased some).
Here are some test images.
graphic made in GIMP, saved as quality 100 JPEG: original imgmin output, chroma downsampling 2x2 imgmin output, chroma downsampling off
random image from rawsamples.ch, resized, saved as quality 100 JPEG: original imgmin output, chroma downsampling 2x2 imgmin output, chroma downsampling off
random image from Google Images (and ultimately from thewallgans.com, a wallpaper site), resized, saved as quality 100 JPEG: original imgmin output, chroma downsampling 2x2 imgmin output, chroma downsampling off
Every area with fine detail is blurred significantly, and colors tend to look less vivid, in the chroma-downsampled versions.
