Using .map file as a raster layer source

[Frank Warmerdam]

On 8/25/05, Tamas Szekeres <szekeres.tamas at freemail.hu> wrote:
> It would be useful to make the possibility to use a .map file as a data
> source of a raster layer in another .map file.
> 
> The main benefits of this enhancement are:
> 
> 1. High number of maps could be joined into a single map. The maps could
> be organized hierarchically.
> 2. Tileindex support can be used to filter the maps to be drawn.
> 3. There's no need to create temporary images in the filesystem as with
> WMS layers
> 
> In order to try this functionality a patch have been added in
> http://mapserver.gis.umn.edu/bugs/show_bug.cgi?id=1445

Tamas, 

This is an interesting addition, but I am a bit uncertain whether it is a 
good general addition to MapServer.  Once concern I have is about the fact
that sub-maps can generally not be influenced by the calling map except for
setting of the target extent. Are we likely to find people wanting more control
than this, and needing to build mechanisms to satisfy it?  Does this capability
really give us something we wouldn't otherwise have? 

A few details:
 o The patch does not address RGB, RGBA or other types of render results.  It
   seems that it assumes the result of the drawMap() is an 8bit GD image result.
   At the very least it ought to error out gracefully for other return results.

 o We would really need appropriate user documentation in order to incorporate
   this as a supported feature. 

As I see it, we have three options going forward:

 1) The patch is incorporated as a fully supported feature of MapServer. 
     We would need documentation for this.  We would also need a 
     detailed proposal to the technical steering committee on the feature.
     There would likely need to be significant discussion and revision of
     the patch.

 2) We apply the patch "as is", perhaps with better error trapping for
     non-8bit imagObj results and it is an unsupported and undocumented
     feature of MapServer.  You might want to submit a specific howto for
     the web site on using it, but we wouldn't try and document it in the
     main references.  We would likely still need TSC approval for this, but
     I don't think we would need to set a high bar since it wouldn't affect
     any other functionality.

 3) The patch is not applied to MapServer, but you can of course continue
     to use it for your local use. 

I would be willing to vote "+1" on option 2 barring any compelling 
argument against it from others.  But I am concerned about the 
conceptual complexity of making it a fully supported feature of MapServer
(option 1).

Contrats on a cool hack!  (I mean hack in the most positive sense). 

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent