[gdal-dev] VRT images and overview types

[Armin Burger]

Hi everybody

I noticed some strange behaviour of virtual images with the Erdas RRD 
overview type. I am using VRT images referencing Geotiff files and 
applying image stretching parameters (offset and scale) to improve the 
display in Mapserver applications. The VRT images are created with 
Python and GDAL bindings. This works fine if the overview is created 
e.g. with the -ro flag, the stretching parameters are then applied at 
all overview levels.

If I create the overviews using the Erdas format (--config USE_RRD YES) 
then the vrt image is applying the stretching parameters only in case 
the full Tiff image is read. At a scale level where just the overview 
file is read the stretching parameters are not applied. This causes a 
confusing display effect by enabling the stretching at a certain scale.

I'd prefer to continue with the RRD overviews since the images are also 
read via Desktop programs like Erdas or ArcMap that make use of the RRD 
.aux files. Is there a way to have RRD overviews and stretching 
parameters applied on them as well? Or would this be possible in future 
GDAL versions?

Thanks for any clarification

Armin