Random JPEG Questions

These may be questions for Gordon Richardson, but I thought others

might be interested.

<OL>

<LI>Why do Canon digicams save JPEG with 4:2:2 chroma subsampling?

According to <A HREF="http://www.photo.net/learn/jpeg/">Gordon's

JPEG article</A>, "The 4:2:2 option is an old standard from the

television industry which is seldom used." It's 2x1x1 horizontally

but 1x1x1 vertically.

<LI>Why does Photoshop SaveForWeb use high quality factors like 98

in conjunction with 2x2 chroma subsampling? Mere stupidity, like

the failure to warn about non-sRGB colorspace, perhaps.

<LI>Is there evidence that different Q factors for qtable 1 and 2

helps? My IJG-based software can't do that; Canon and Photoshop do.

<LI>At approximately what quality factor (on the IJG) scale should

one switch from 2x2 to 1x1 chroma subsampling?

<LI>Do the new JPEG encoding routines with PaintShopPro work better

than IJG libraries, or has nobody evaluated this yet? Comparisons

between Photoshop 7 and IJG showed that Photoshop performed better

at lower Q factors, while IJG was tighter (smaller) at high Q factors

with equal or better quality.

</OL>

2. Because the huge majority of JPEG files out there are encoded like that, to the point that some decoders don't work well (or at all) with any other encoding.

 

(I have personally found that geocities had problem with some files that had no subsampling).

I've sometimes wondered about 1. myself, Jean-Baptiste's explanation seems plausible, but it might also be (at elast originally) some kind of space-vs-quality optimization made just by gut feeling.

 

In general, I find that quality settings depends surprisingly much on subject matter, but tend to go with 1x1 sampling as far as possible.

<em>>>the failure to warn about non-sRGB colorspace, perhaps.</em>

 

<p>I can't answer your questions, but if anyone from Adobe is reading this thread, PLEASE FIX THIS!

If Jean-Baptiste is correct about #2, as I suspect he is, then #1

is even harder to explain! My own answer to #4 would be around

85-90 on the IJG scale. Photoshop goes from 86 2x2 at their Q6

setting to 83 1x1 at Q7.

The JPEG Optimizer in PSP X now defaults to 1x1x1. I usually found that produced the best-looking JPEGs with earlier versions, so I guess I'm not the only one.

<i>>>the failure to warn about non-sRGB colorspace, perhaps. </i>

<p>Heck...let's take it one step further Adobe...It's called <b>Save for Web</b>, why not make the sRGB conversion either automatic or at least a choice in the Save for Web dialog!