[gdal-dev] Overview scales

[Wilfried Karel]

Dear list,

I'd like to use overviews for building image pyramids with a scale
reduction of 2 for photogrammetric applications. While
GDALDataset::BuildOverviews allows for the specification of "overview
decimation factors" as integers only, GDAL seems to interpret these
factors as hints only, and it generates overviews of slightly different
scales if the base image resolution is not divisible by the decimation
factor without remainder. The way that GDAL tries to even out the
varying offsets between the regular image grids not only results in
possibly anisotropic scales in the 2 coordinate directions of overview
images, but it also means that the transformation of image coordinates
from 1 overview to the other is not linear, but a step function (image
content is shifted between -0.5px and +0.5px), and that the sampling
distance is non-constant: there will be 2 neighboring rows and columns
close to the horizontal/vertical center lines of the base image that are
both mapped to neighboring rows and columns of the overview.

Is there any way to force an exact, constant sampling distance of 2
throughout an overview?

What is the reason for this behavior, especially as by GDAL-means, only
integer decimation factors may be specified, anyway? Also, all other
image processing tools that I know of (and that are not based on GDAL)
seem to support a constant sampling distance of exactly 2.

Thanks!
Willi.
-- 
                        _____ _____ _____  Technische Universität Wien
   DI Wilfried Karel   /____// ___//    / Dept. of Geodesy & Geoinform.
  wilfried.karel@     // __ / /__ / // / Research Groups Photogrammetry
 geo.tuwien.ac.at    //__/// /__ / // /   & Remote Sensing
+43 1 58801 12244   /____//____//____/ Gusshausstr. 27-29, A-1040 Wien