Folks,
I have done an extensive amount of work with the Mr.Sid format and it is
fantastic. However there is currently no support for using it in GRASS
though there are modules in Arcview. I did contact Lizardtech last fall
about developing a module for GRASS and they do have a FREE developers
kit to do this (or they did then). I unfortunately do not have the time
to follow up on that, but the contacts there were keen on seeing GRASS
be able to utilize the format.
While Mr.Sid is a lossy technology, it does however have a broad scope of
applications in what we do. THe most critical benifit of Mr.Sid over
JPEG is that when you zoom in the things still looks sharp as a tack (or
blurry as a wavelet as the case may be), where a JPEG would be pixelated
to all get out. This quality is nearly (and I mean darn close) as good
as you get with a TIFF, at 1/25 the file size. I have put up a good
example of this at
http://www.cast.uark.edu/mrsid/
If you lack the free Mr.Sid viewer then the server also will delvier
the data in GIF format on the fly.
The bottom line is anyone up to the challenge of building a viewer for
GRASS? Just check out there site and put a query into the contact, I do
know as of fall 98 they were interested in having folks develop a viewer
for GRASS.
R. Joe
On Thu, 6 May 1999, Frank Warmerdam wrote:
"Luis F. Franco" wrote:
>
> Hi to all.
>
> Has anybody heard of the SID imagem format? I've come across some maps in
> that format and it's really great. The compression of it is about 25 to 1
> and gives images in diferente scales according to the needs.
Luis,
The MrSID format (see http://www.lizardtech.com) is proprietary and based
on patented wavelet related algorithms (as I understand it).
> I suppose it works by having a low resolution image (1:16 of the original)
> and then saving the diferences for the levels below. As the image is beying
> displayed it gets better. When you zoom in the details are increasingly
> better... etc.
A JPEG compressed, tiled TIFF file with overviews can be used to achieve
similarly fast display for overviews, albeit with less compression and/or
more compression artifacts. The JPEG2000 format is based on wavelets, and
may (or may not) provide a more open solution to the same problem. I
should also point out that MrSID (and wavelets in general?) don't have to
store overviews. To extract a low resolution image from wavelet data
you just expand less of the wavelet equation or something like that.
I don't believe that GRASS has any support for overviews when displaying
images, and furthermore the native GRASS raster format has no lossy
compression for image data, like JPEG. Am I right on this count?
> Something like that could be done with raster maps in grass? One thing I'd
> like to see in grass is a dithering that would make the image look nicer on
> screen. When you zoom out you lose all the details of roads etc...
Do you mean some sort of averaging downsampling? This can give much more
meaningful, and visually pleasing displays for some sorts of images.
Dithering is normally just used to approximate 24bit colour on 8bit systems,
and doesn't relate much to the issue of overview downsampling.
Best regards,
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I set the clouds in motion - turned up | Frank Warmerdam, warmerda@home.com
light and sound - activate the windows | http://members.home.com/warmerda
and watch the world go round - Rush | Geospatial Programmer for Rent
R. Joe Brandon
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University of Arkansas rjoe@cast.uark.edu
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