
Images and Chroma-Subsampling - ssttoo
http://calendar.perfplanet.com/2015/why-arent-your-images-using-chroma-subsampling/
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Taek
I worked on video encoding for a long time. And chroma-subsampling is no
longer necessary.

In short, chroma subsampling was performed because it matched the data more
closely to how the eyes work, at the cost of losing certain colors.

Today, modern video and image compression algorithms can compensate without
using chroma subsampling. Starting with the full image actually gives the
encoder more to work with, and it can optimize better.

Because the software has improved (at least, the cutting edge open source
tools have), chroma subsampling is obsolete, and will produce inferior picture
qualtiy at larger byte sizes than their rgb444 compressed alternatives.

If you are seeing improvements by doing chroma subsampling, either you are
hurting quality or you should use better compression software.

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mrob
Here's a worst case test image for chroma subsampling:

[https://commons.wikimedia.org/wiki/File%3ARicardo_Quaresma_%...](https://commons.wikimedia.org/wiki/File%3ARicardo_Quaresma_%28L%29%2C_Pablo_Zabaleta_%28R%29_%E2%80%93_Portugal_vs._Argentina%2C_9th_February_2011.jpg)

I first saw this in Fabrice Bellard's BPG demonstration at
[http://xooyoozoo.github.io/yolo-octo-
bugfixes/](http://xooyoozoo.github.io/yolo-octo-bugfixes/) ("Soccer Players").
Every lossy format fails to reproduce the red dots on the green stripe because
they all use chroma subsampling. Until I see tools that can automatically deal
with images like this I think the anti-subsampling attitude mentioned in the
article is correct. "Unless you are willing to evaluate each image, the
thinking goes, you should avoid subsampling."

