Changes between Version 6 and Version 7 of BottomLeftPixelProposalII:

09/04/2008 05:02:08 PM (11 years ago)

Comment added.


  • BottomLeftPixelProposalII:

    v6 v7  
    113113 * Specify the position of the lower left corner of the CCD as (0.0,0.0), but increase pixel position linearly with actual physical position of the CCD. Thus the center position of pixels on the next amplifier will probably not end in .0. One will have to use a function to translate pixel index to pixel position (which I have long advocated) but the offset to this function will be stored in the metadata for each amplifier image (rather than being a project-wide constant), and a lookup table for this for a full CCD image. I suspect this proposal and the original proposal both will have very similar headaches if one is trying to process an entire CCD image at once (if the algorithm cares about position). 
     115==== Comment by rowen on Thu 04 Sep 2008 12:02:08 PM CDT ==== 
     116In my opinion LSST code should NOT assume a convention for the mapping between pixel index and pixel position. There are inline functions in afw for converting between index and position, plus a 
     117constant describing the difference between the two. In my opinion all LSST code should be required to use these. This avoids the following common problems: 
     118 - A coder assumes the wrong convention and codes accordingly. RHL notes many examples of this. There are two conventions already in common use and RHL is proposing to adopt a third, which will exacerbate the problem. 
     119 - A coder assumes the right convention but codes it wrong. In particular, the transformation from position to index is notoriously easy to get wrong. 
     120 - A coder forgets the difference between index and position. This leads to code that is hard to understand and is often subtly incorrect.