[Gdal-dev] Overviews questions

[Jeff L.]

Hi,

I have few questions about overviews in GDAL as I'm trying to understand 
them. My current understanding of overviews in GDAL is that their are stored 
as an auxiliary 'geotiff' file with an .ovr extension or an Erdas .rrd file.

My question is really how a band know that has overviews generated for it 
and how the bands overviews are stored in a tiff file ? is there any 
assumption about the order ?
example: overview1_ band 1, overview2_band1,...overview1_band2, 
overview2_band2.... ?

Let say I have a bitmap of three band (RGB) and I want to generate overview 
levels 2, 4, 6 and 8 for band 2 only. Once this done and I open my bitmap 
(through the code), during the open operation, an .ovr file will be searched 
and found. How we know from the .ovr (tiff) that the overviews there are 
those of band2 ? If one can explain how bands overviews are linked to their 
original bands it will be much appreciated.

Also, is there any details about the overviews storage within a Tiff, are 
they simply bands or have a different storage mechanism ?

Thanks for any insight on this.

Jeff