What's the best OUTPUTFORMAT for ECW-files using WMS

[Richard Greenwood]

On 4/23/07, Thy, Kristian <Kristian.Thy at atkinsglobal.com> wrote:
>
>
>
> From: Richard Greenwood
> > It may not be your output format that is slowing things down. I found
> > the ECW source format to be much slower than TIFF.
>
> Interesting, as that contradicts the findings in
> http://www.agile2006.hu/papers/a382.pdf. Do you have any
> empirical
> data for your case - I would be extremely interested in any sort of
> input on this.
>
> best regards,
> Kristian Thy

I do not have a record of my results, but in any case, it is best to
test in your own environment whenever possible. Both the input and
output format effect the performance of your mapserver. The input and
output formats are pretty much independent of one another, but they
are effected by the type of source and output image types that you are
reading and writing (greyscale, 24 bit color, 8 bit color).

I tested on two different Windows machines, one with a SCSI RAID and a
1GHz processor, the other with a SATA drive and a 3GHz processor. It
was a few years ago, so probably MapServer version ~4.6. My
recollection was that the TIFF was markedly faster (maybe >2x). My
image set is 24 bit color, 1 foot ortho-pixel. About 25GB uncompressed
w/o overviews.

I tested ECW (which always returns a 24 bit image), 24 bit TIFF and 8
bit TIFF, with 8 bit TIFF by far the fastest, and 24 bit TIFF faster
than ECW. I used Photoshop to create an optimized 8 bit palette,
although there a a number of other programs than can do an equally
nice job. If you are using an 8 bit OUTPUT format, and you have
colored vector layers also in your map, you may want to restrict your
8 bit image palette to less than 256 colors. This leaves some slots
open in the output palette for your vectors.

If you have a slow disk drive and a fast processor, I suppose that the
reduced file size of the ECW might make the ECW format faster than
TIFF. Also, if you do not have overviews (pyramids) the TIFF will
likely be slower. It is also possible that the ECW driver is now
faster than the one I was using when I did my tests.

Rich

-- 
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com