Christopher - Let me add the "evidence" that I have found that reducing the strip size in LZW-compressed GeoTIFFs has, not surprisingly, a VERY large effect on read performance - about a factor of 10 in the particular cases I used. That indicates that the data you report might not be a good 'order of magnitude' measure of performance. I'm not talking about "subtler" effects of memory, caching, etc. but the substantial effect of changing the LZW strip size in a large image; the amount of work required to decompress a portion of that image is very, very different from the case in which the entire file is treated as one strip, due to the dictionary-building nature of the LZW algorithm.
You're welcome to draw conclusions based on one data point; I am concerned that readers of your post may unreasonably extrapolate from them. You're welcome to stick to them, but I'm not sure I'd recommend that other users stick to them, at least in situations outside of your test environment. - Ed -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Christopher Schmidt Sent: Monday, February 25, 2008 9:18 PM To: discuss@lists.osgeo.org Subject: Re: [OSGeo-Discuss] 'lossless' JPEG2000 On Mon, Feb 25, 2008 at 09:04:59PM -0500, Ed McNierney wrote: > Christopher - > > You will very likely find that using different LZW compression options (particularly setting a small strip size) will slightly degrade compression performance while significantly improving read time. While I think your test data are valid, they only address one of many possible configurations and I wouldn't necessarily make broad generalizations about LZW from them. > > However, I have generally found that LZW compression for photographic data is indeed not a good choice; I'm surprised you got it to perform as well as you did (in compression). Yeah, I think we've stumbled back and forth across these numbers before. I'm aware that they're essentially 'back of the envelope': they weren't run entirely in isolation, they were only repeated a couple of times (half dozen rather than an order of magnitude more), they might have been cached in memory, etc. etc. etc. However, they do seem to serve as a good 'order of magnitude' measure of size and performance for compressing aerial imagery based on other similar experiments, and I have no evidence to seriously discount them, so I'm sticking to them. Regards, -- Christopher Schmidt Web Developer _______________________________________________ Discuss mailing list Discuss@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/discuss _______________________________________________ Discuss mailing list Discuss@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/discuss