Which Flat Field Offsets Should be Used?

[calvin.harman@…]

Description says "...the function shall use the offsets in the image (in->x0
and in->y0) to determine the appropriate offsets to obtain the correct pixel on
the flat-field."

How should in->image->row0, in->row0, flat->image->row0, and flat->row0 be used
to set the image value and mask value? It isn't clear which reference point
should be used when looping on the indices.

[eugene]

I've clarified this now a bit more in the current draft of the document:

The detector pixel coordinates of pixel (x,y) in
an image are:

(x + in->image->x0 + in->col0, y + in->image->y0 + in->row0)

the readout->col0, row0 values define the offset of the top-level parent
relative to the origin of the chip, while the subimage offsets (image->x0,y0)
define the relative offset between the subimage and *its* parent. Note that
means that you need to follow the image children up the chain to the image
without a parent, summing the offsets as you go.

Also note that the same relationship holds for pixels in the mask and the
flat-field image. You need to relate detector pixels to detector pixels no
matter how many layers of subimaging, etc, there are.

(Also note the reason for the levels: frequently the camera will only be read
out of a small portion of the detector, ie to speed up the readout. In these
cases, we don't have a parent that represents the full image, so we need to
carry around the first level of offset. But, you may need to operate on only
part of an image (since some chips have multiple amps, etc), so we need to be
able to address a subarray of a subraster readout!

draft SDRS has been updated, but not posted.

[eugene]

updated SDRS posted