[Gvsig_english] Raster pyramids and INTEROPERABILITY

[Frank Warmerdam]

ernesto sferlazza wrote:
> Hello developers,
> in the county government organization where I work we currently work
> both with commercial (ESRI ArcGIS Desktop) and Open Source (gvSIG,
> adbToolbox, the latter being a customization of OpenJump) GIS
> software.
> All raster layers in the geographic archive are stored together with
> the pyramids, built by arcGIS, to optimize display rendering and
> zooming with this software.
> The problem is that when the a raster layer is loaded into a gvSIG
> view,  the existence of pyramids causes problems in visualization
> (e.g. for a B/W - 1 bit - image, zooming to the whole raster extent it
> appears displayed all black, while zooming in, at a certain extent,
> the image is displayed correctly).
> The trick is to brutally delete the pyramid file  (*.rrd) associated
> to the raster, but when we load tagain the raster on a ArcGIS View
> Frame the image is displayed with a coarse resolution (compared with
> the high quality resolution and zoom speed obtained with pyramids).
> I suggest two alternative solution:
> a) force gvSIG to ignore pyramids;
> b) syncronize the behaviour of gvSIG with that of ArcGis regarding the
> pyramids rendering.

Ernesto,

I'm willing to assist a bit on this problem.

ArcGIS rrd's for 1 bit images use 8bit overviews so that greyscale
values can be used to provide a smoother view to decimated line work.

In GDAL I set the special RESAMPLING: AVERAGE_BIT2GRAYSCALE metadata
on such overviews.  In GDALBandGetBestOverviewLevel(), used by the
default GDAL RasterIO machinery, I avoid using such overviews for
default decimation.

So GDAL by default tries to avoid using these overviews automatically.
But applications that fetch the list of overviews for bands, and use
them directly may still end up utilizing these overviews.

I must confess to not knowing gvSig's raster architecture well, but if it
is reading .rrd's I'm guessing GDAL is being used.  In that case I suspect
gvSig is directly reading the overviews.  It may be sufficient for gvSig
to avoid overviews with the above metadata set, or alternatively to use
a different display strategy for them, but to still utilize them (the
optimal case).

If gvSig is not using GDAL for this at all, then it still might want to
implement a similar strategy.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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