[Gdal-dev] Overviews questions

[Jeff L.]

Thank you very much Frank. Things are clear now.

Jeff


>From: Frank Warmerdam <warmerdam at pobox.com>
>To: "Jeff L." <nf10 at hotmail.com>
>CC: gdal-dev at lists.maptools.org
>Subject: Re: [Gdal-dev] Overviews questions
>Date: Tue, 21 Mar 2006 11:21:20 -0500
>
>Jeff L. wrote:
>>Thanks Frank for your response and your insights.
>>
>>So one need to build overviews for all the bands if the used overview 
>>format is tiff: Ok.
>>
>>I'm still a bit unclear about the internal storage of the overviews within 
>>a tiff. You mentioned that an overview .ovr file is a 'normal' image. Does 
>>is have any 'normal' bands (full resolution) or only reduced resolutions ? 
>>Can you please explain ? (If i run a 'gdalinfo' on an .ovr it stated that 
>>it has one band with attached overviews).
>
>Jeff,
>
>In the .ovr file all image directories will be marked as reduced
>resolution images.  But if you were to run gdal tools on it they will
>treat the first encountered image as the "base full res image" I think.
>
>I would encourage you to run "tiffinfo" on the .ovr file to see it's
>structure in more detail.
>
>>Once an .ovr file is created and the base file (.bmp for instance) is 
>>opened programmatically, there is a check for the .ovr file existence. If 
>>the .ovr is found how we get the overview count (number of overviews) for 
>>each band ? (Will it be the same for all the bands ?) Is it stored with 
>>the .ovr or we deduce it (example: base dataset has 3 bands and the .ovr 
>>has 12 'bands' --> 12/3 : 4 overviews for each band ?).
>
>Through the GDAL API you would call GetOverviewCount() on the 
>GDALRasterBand
>to find out how many overviews there are available, and GetOverview(int)
>to fetch them.  If the overviews are stored in TIFF format, all bands
>will have the same number of overviews.
>
>>Last question: how the overviews are stored within the tiff ? From your 
>>response I understood
>>that there will a be a set of 'images' within the tiff. Every image 
>>correspond to an overview level and has all the bands. To simplify the 
>>picture, this would look like:
>>
>>image1:
>>  overview level 1 for band1
>>  overview level 1 for band2
>>  overview level 1 for band3
>>  .... overview level 1 for last band
>>
>>image2:
>>overview level 2 for band1
>>overview level 2 for band2
>>overview level 2 for band3
>>.....
>>
>>Where level 1 can be 2, 4, 6 or any even number
>>
>>Please correct if this schema is not correct.
>>
>>Sorry about asking many questions as I'm trying to understand the details 
>>about 'overviews' within GDAL.
>
>chmod a-w base.tif
>gdaladdo base.tif 2 4
>warmerda at gdal2200[160]% ls -l base*
>-r--r--r--  1 warmerda users 787570 Mar 21 11:19 base.tif
>-rw-r--r--  1 warmerda users 246236 Mar 21 11:20 base.tif.ovr
>tiffinfo base.tif.ovr
>TIFF Directory at offset 0x8 (8)
>   Subfile Type: reduced-resolution image (1 = 0x1)
>   Image Width: 256 Image Length: 256
>   Tile Width: 128 Tile Length: 128
>   Bits/Sample: 8
>   Sample Format: unsigned integer
>   Compression Scheme: None
>   Photometric Interpretation: RGB color
>   Samples/Pixel: 3
>   Planar Configuration: separate image planes
>TIFF Directory at offset 0x116 (278)
>   Subfile Type: reduced-resolution image (1 = 0x1)
>   Image Width: 128 Image Length: 128
>   Tile Width: 128 Tile Length: 128
>   Bits/Sample: 8
>   Sample Format: unsigned integer
>   Compression Scheme: None
>   Photometric Interpretation: RGB color
>   Samples/Pixel: 3
>   Planar Configuration: separate image planes
>
>
>So, there are two overview levels, so there are two "image directories".
>Each image directory has 3 bands (samples/pixel:3).
>
>Hopefully this makes it more clear.
>
>Best regards,
>--
>---------------------------------------+--------------------------------------
>I set the clouds in motion - turn up   | Frank Warmerdam, 
>warmerdam at pobox.com
>light and sound - activate the windows | http://pobox.com/~warmerdam
>and watch the world go round - Rush    | President OSGF, http://osgeo.org
>