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

[Frank Warmerdam]

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,
-- 
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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