TIF or ECW: which is faster?

[Ed McNierney]

Luca -

It is very dangerous to make general statements like "ECW is faster".
It's like saying "which is faster, a Lamborghini or a Trabant?" - if the
Lamborghini's out of fuel, the Trabant is faster!

Generally speaking, however, a well-organized set of TIFF images will be
the fastest route.  All else being equal, a compressed format like ECW
lets you (when reading images) read less data and spend more time
processing it.  TIFF = more disk I/O, less CPU, while ECW = less disk
I/O, more CPU.

In general, that tradeoff favors the TIFF approach.  For most disk
systems, the slowest task is locating the file on disk and moving the
read head to it.  After that, reading data is pretty fast.  So reading
twice as much data from a file certainly does NOT take twice as long.
If you used a compression scheme that reduced the data to 10% of its
original size, you're going to seek for that file (in both cases) and in
one case read 10X as much data as the other.  But reading 10X as much
data will not take 10X as long and may, in fact, not take any longer at
all!

The tradeoff is that decompression requires CPU and memory space.  If
you are serving multiple users, you are likely to get VERY quickly
bogged down by these requirements, and if you start swapping you're in
big trouble.

Disk space is cheap; use TIFFs if possible, and spend your time thinking
about how to organize them well and learning how your disk system
behaves.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Luca Manganelli
Sent: Thursday, January 12, 2006 5:36 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] TIF or ECW: which is faster?

Hi, I want to know which of TIF or ECW is faster.

--
"An algorithm must be seen to be believed."
   -- Donald Knuth, in "Fundamental Algorithms"
                               http://www.trapanator.com/blog