[Rtk-users] Rtk-users Digest, Vol 26, Issue 5

[Andy Shieh]

Hi Jan and Simon,

Your suggestions and comments are very inspiring and helpful. Thank you for
your help :)

Cheers,
Andy

2014-10-28 0:34 GMT+11:00 Jan Hoskovec <jean.hoskovec at gmail.com>:

> Hi Andy,
>
> I'm sorry not to have replied earlier. For the precisions you wanted,
>
> 1) your intuition is right, the upper and lower integration limits are
> the values you are "expecting", the values delimiting the angular
> range you want to cover (whatever is the actual sampling).
>
> 2) The division by two is there because in my approach, the "zone of
> influence" of each sampled projections begins and ends halfway between
> the sampled value of the gantry angle and the next / preceding sample.
> The weights of other than first and last samples simplify to " (next
> angle - previous angle) / 2" for me.
>
> Hope this would help (unless, of course, you've found a better option
> during the weekend :-) ).
>
> Cheers,
>
> Jan
>
> 2014-10-25 15:41 GMT+02:00 Andy Shieh <hsieandy at gmail.com>:
> > Hi Jan,
> >
> > Thanks for sharing.
> > This does seem useful to me, but I'm not sure if I understand your method
> > correctly.
> >
> > For your lower and upper integration limit, do you mean the limit values
> for
> > the angular range that you are "expecting"?
> > For example if you are expecting a 0-180 deg scan (although the first and
> > last angles might not be 0 and 180 due to sampling), lower/upper
> integration
> > limit would be 0 and 180 deg?
> >
> > And why is the division 2 needed there?
> > I thought in rtkFDKWeightProjectionFilter.txx, the gap value used for the
> > weighting is "nextAngle - previousAngle" for a certain projection.
> > In this case I would expect Gap_first to be
> >
> > Gap_first = second_angle - lower_integration_limit
> > (As the lower integration limit is kind of like the "virtual angle"
> > preceding the first angle?)
> >
> > Thanks for your help :)
> >
> > Cheers,
> > Andy
> >
> >
> >>
> >> Date: Fri, 24 Oct 2014 17:21:27 +0200
> >> From: Jan Hoskovec <jean.hoskovec at gmail.com>
> >> To: Andy Shieh <hsieandy at gmail.com>
> >> Cc: "rtk-users at public.kitware.com" <rtk-users at public.kitware.com>
> >> Subject: Re: [Rtk-users] itFirstAngle and itLastAngle in
> >>         rtkParkerShortScanImageFilter.txt
> >> Message-ID:
> >>
> >> <CANtP0QSnh70uETrdyTjg=u3HaUth4kRwDVfhMmKL=DhwrwzNLg at mail.gmail.com>
> >> Content-Type: text/plain; charset=UTF-8
> >>
> >> Hi Andy,
> >>
> >> I was recently dealing with a similar problem in a different context
> >> (180? backprojection with irregular sampling and how to handle the
> >> first and last gaps) and what worked for me was
> >>
> >> Gap_first = (second angle - first angle) / 2 - lower integration limit
> >>
> >> and, analogically,
> >>
> >> Gap_last = upper integration limit - (last angle - second last angle) /
> 2
> >>
> >> with the integration limits being arbitrary set where I wanted them.
> >> The idea behind this was that a continuous projection value we are
> >> miming in the discrete integral should always be represented by the
> >> closest projection we have, with a known angular segment to cover.
> >>
> >> However, that was a DBP-type algorithm, for which the exact
> >> integration limits are extremely important, it may be different in the
> >> context of a short scan. But just in case you might find this
> >> useful...
> >>
> >> Cheers,
> >>
> >> Jan
> >
> >
> > _______________________________________________
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> > http://public.kitware.com/mailman/listinfo/rtk-users
> >
>
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