More multiresolution and datatype questions

[Lowell Johnson]

Hello Frank,

I have been continuing my analysis of GDAL as a replacement for the
image I/O library in LAS. So far it is looking like a pretty good fit,
with multiresolution support being the biggest concern at this time. (See
below.) I have written a prototype driver set for the "old" LAS image
format and am in the process of modifying one of the existing LAS
applications to use GDAL. So now the implementation details are starting
to unearth more questions.

GDAL handles the image data type at the band level, thus allowing for
multi-datatype images. But I haven't seen an API call that allows for the
creation of multi-datatype images. For instance, GDALDriver::Create() 
passes in a GDALDataType eType parameter. For multiple datatype images, I
expected that this would be an array of size nBands. How are
multi-datatype images created?

As stated in earlier messages, we will need support for multiresolution
images (all bands sharing common corner coordinates, but pixel ground
distances varying). I am probably still misunderstanding something from
your earlier responses, but it is important that I understand the
capabilities and limitations of GDAL.

In an earlier response you said [bracketed comments added by me],
> In theory that [interpretation of GDALRasterBand XSize, YSize values
> to support multiresolution images] might be possible; however, GDAL was
> developed with the assumption that all bands on a given GDALDataset are
> of the same size and same geographic area. Any breaking of this
> assumption could have dramatic consequences.

In the Raster Band documentation, it is stated

A raster band has the following properties: 

* A width and height in pixels and lines. This is the same as that
defined for the dataset, if this is a full resolution band. 

What is the purpose of the band-level width and height if not for
multiresolution? Are they only used for overviews? Before GDAL, I haven't
encountered overviews, so I'm not familiar with their history and usage.
But overviews seem to be smaller representations (i.e., lower
resolution thumbnails?) of a full-resolution band.

I note that you earlier described your concept for subdatasets and how they
can be used to support multiresolution images. This does look like it
would be a solution, but our team would really like to have a library that
handles each band as having a distinct resolution (pixel size), with all
image bands sharing the same geographic corner coordinates. GDAL seems to
have the needed elements (Xsize, YSize per band) in place, but the design
interpretation does not support multiresolution?

I apologize for my need to have it spelled out. I'll continue with my
investigations, but any further explanations that you can give me will
hopefully shorten my analysis period so that we can get on with our
implementation process.

Thanks for you help (and, of course, your library).

Lowell
-- 
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Lowell D. Johnson ldjohn at u...
Software Engineering
Raytheon Systems
EROS Data Center
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