Hi,<br><br>I like to clarify my understanding on the concept of overviews. <br><br>Say if i have a geotif map 300 pix by 300 pix, then if i build a overview giving a decimation factor of 2, I will get a map 150 pix by 150 pix. This is so because 1/2 of the amount of pixels along its width and 1/2 of it along its height are being thrown away, thus resulting in a map with 1/4 the original map size in terms of total number of pixels.<br>
<br>Did I get my concept right? <br><br>Thanks again..<br>Heng Hock<br><br><div class="gmail_quote">On Fri, Feb 22, 2008 at 11:08 PM, Frank Warmerdam <<a href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</a>> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div class="Ih2E3d">lee heng hock wrote:<br>
> Hi,<br>
><br>
> Issue1:<br>
> Can we build overviews on a paletted geotiff map? or it has to apply<br>
> only to rgb maps?<br>
<br>
</div>Lee,<br>
<br>
Yes, but you must be careful only to use the default nearest neighbour<br>
resampling with non-continuous paletted images, not averaging, when<br>
building the overviews.<br>
<div class="Ih2E3d"><br>
> Issue2 :<br>
> I have a georeferenced geotiff map (with only 1 raster band) which is<br>
> large in size probably a few hundred Mb which i like to load in my app.<br>
> So i use GDALBuildOverviews to build overviews so that I can load lower<br>
> resolution maps at smaller scales to beef up performance.<br>
><br>
> However, I noticed that after the building process is done, when i do this:<br>
> rasterband->GetOverviewCount(), the result is not 0 which is normal<br>
> since i built overviews.<br>
><br>
> But when I do:<br>
> rasterband->GetOverview(0)->GetOverviewCount(); the overview count is<br>
> also not 0. Why is this so ? shouldnt there be just one layer of overviews?<br>
<br>
</div>Hmm, I thought I avoided representing non-base layers as having overviews,<br>
but apparently not. My advice to you is to ignore this and just don't ask<br>
overviews if they have overviews. But I would also encourage you to file<br>
a ticket on this issue so that the GTiff driver can be fixed up to behave<br>
more appropriately.<br>
<br>
Best regards,<br>
<font color="#888888"><br>
--<br>
---------------------------------------+--------------------------------------<br>
I set the clouds in motion - turn up | Frank Warmerdam, <a href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</a><br>
light and sound - activate the windows | <a href="http://pobox.com/%7Ewarmerdam" target="_blank">http://pobox.com/~warmerdam</a><br>
and watch the world go round - Rush | President OSGeo, <a href="http://osgeo.org" target="_blank">http://osgeo.org</a><br>
<br>
</font></blockquote></div><br><br clear="all"><br>-- <br>Regards<br>Heng Hock