[Mapserver-users] I have some landsat 7 data that I want to tile : overlap + transparency

[Stephen Clark]

Frank,

> If you are wanting to use transparency as a mechanism to blend overlapping
> tiles you are out of luck I am afraid.

Essentially this is what I want to do.

I will have to create a mosaic to produce the imagery results I want for
now.

Could some basic mosaicing functions be added to gdal in the future?

Thanks,
Stephen


----- Original Message ----- 
From: "Frank Warmerdam" <warmerdam at pobox.com>
To: "Stephen Clark" <stephen.clark at focus.ca>
Cc: <mapserver-users at lists.gis.umn.edu>
Sent: Monday, February 02, 2004 2:03 PM
Subject: Re: [Mapserver-users] I have some landsat 7 data that I want to
tile : overlap + transparency


> Stephen Clark wrote:
> > Q1: Is there a way of specifying a range of values for the OFFSITE
command
> > to handle the lossy compression fuzziness?
>
> Stephen,
>
> No that is not possible at this time.
>
> > Q2: The transparency values do not work for a tiled raster file (TIFF
and
> > ECW) using the links posted above.  It believe it to be an error.
>
> Hmm, if the TRANSPARENCY keyword isn't working that is a bug.
>
> > Q3: I would like the ability to specify an individual TRANSPARENCY and
> > OFFSITE value for each raster file before creating a tile.  That way I
could
> > create a mosaic from many raster files and have the best part of each
image
> > viewable.  Would this be a tremendous amount of work?
>
> I'm starting to think I see the issue with the transparency. Are you aware
> that the transparency applies to the layer as a whole relative to other
> layers, not to individual overlapping tiles within a tiled layer?  That
is,
> if you have transparency on, and there are overlapping tiles, the last
tile
> drawn will still completely overwrite any other tiles drawn.  The
transparency
> keyword basically means that the layer is drawn into a temporary image as
> per normal, and then that whole image is alpha-blended into the map view
> as so far built from other layers.
>
> If you are wanting to use transparency as a mechanism to blend overlapping
> tiles you are out of luck I am afraid.
>
> Adding per-tile OFFSITE values wouldn't be terribly hard.  My main concern
> with it is that there are *alot* of layer parameters that we might start
> wanting to put in the tile index.  The general idea is that tile indexes
> should be used for layers where all the raster tiles are "similar" in
> nature.  As we try to generalize a number of issues would arise which I am
> not keen to face at this time.
>
> My opinion would be that for your situation you should do alot of
preprocessing
> of your imagery.  Basically, build a mosaic from the landsat scenes, and
then
> either split that up into regular (non-overlapping) tiles again or leave
> as one big file.
>
> If you want to do that, you should likely do your mosaicing all with
> uncompressed data to avoid iterative degredation of the image.  Then you
> could compress to ECW as a last step if that is how you would like to keep
> stuff.  Of course, you will still have the offsite issues but at least it
> will not cause artifacts along seams between overlapping scenes, only at
the
> edges with areas of no-data.
>
> Also, if you don't have a real hard issue with disk space, I would
encourage
> you to keep the final resulting mosaic in uncompressed form.  Keeping it
as
> a tiled TIFF file with overviews (if it isn't bigger than 4GB) would be
> high performance, and precise (ie. no edge artifacts).
>
> Please let me know if my assumptions about what you want to do with
transparency
> are wrong.  And if, so we can return to setting up a test case that
> reproduces the problem.
>
> Best regards,
> -- 
> ---------------------------------------+----------------------------------
----
> I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
>
>