Sid images

[Frank Warmerdam]

"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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