[Mapserver-users] Help COLOR_MATCH_THRESHOLD

[Frank Warmerdam]

Jan Hartmann wrote:
> Frank Warmerdam wrote:
> 
>>
>> Jan,
>>
>> The SCALE=AUTO is done independently for each tile, and so it is 
>> likely that
>> different scaling ranges would be selected.
> 
> 
> Is this desirable? I can't see any use for tiled maps with differently 
> scaled tiles. Would it be very difficult to scale from the minimum and 
> maximum values of all displayed tiles combined?

Jan,

The desirable thing would be for AUTO scaling to always be based on the
whole image set for the input layer.  But it is not practical to recompute
that each time a map is rendered, so the code currently just computes the
auto-scaling on the basis of the window of imagery actually loaded to
render the map.

This means that the scaling changes tile by tile in a tiled layer, and even
that it it varies from render to render as you zoom in on a single raster file.

The solution isn't to do more work each time the map is rendered, but instead
to have map generation tools that can collect appropriate scaling information
once in advance.  In the meantime I would strongly encourage you to just
use "gdalinfo -mm <filename>" to get reasonable scaling ranges and enter
them into the map file.

The short answer is, it would be very compute intensive (and a quite a bit
of work to implement for that matter) to have auto raster scaling done on
the basis of all displayed tiles combined.

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