SODA spec doesn't tell us how to provide dataproduct_type transformation

[Bonnarel]

First implementations already provide various flavours of cutouts from cubes. Some cutout interface can extract subcubes from a large cube. Some others could create spectra or images (dimension reduction). An idea could be to use the DPTYPE parameter designed for SIAP2 to force this product_type conversion.
This has been discussed for TimeSeries there: http://www.ivoa.net/documents/Notes/TimeSeriesDiscoveryAndAccess/index.html

[Bonnarel]

This point was discussed a little but during DAL running meeting on Feb 24th 2021. Pat said we had to decide if when we want to extract a spectrum from a cube we are rebinning or not (that is only extracting a spectrum at a given position .
I thougt a little bit about this. I think we can combine DPTYPE, SPATRES and POS (or POLYGON or CIRCLE) to fix that
if DPTYPE is "spectrum" and SPATRES encompass the POS region we will rebin and have only one spectrum. If it is smaller (down to the pixel size) then the result is several spectra. The way we package several spectra would be provided by FORMAT

Examples : if we have a spectral cube
POS= CIRCLE lon lat rad(deg)
BAND = min max
DPTYPE= cube
is a subcube

POS=CIRCLE lon lat rad(deg)
SPATRES = rad(arcsec)
BAND = min max
DPTYPE = spectrum
FORMAT = FITS
is a FITS spectrum summing up for each selected spectral channel the fluxes inside the CIRCLE

POS=CIRCLE lon lat rad(deg)
SPATRES = rad1(pixels sizearcsec)
BAND = min max
DPTYPE = spectrum
FORMAT = tar, VOTABLE
is a set of spectra for each pixel in the selected area formatted in VOTABLE poackeged with tar.