[Rtk-users] Fwd: About the newspacing flag

Sat Feb 20 08:28:02 EST 2016

Hi Chao,
Thanks very much for your kind help. i see, now i know there is no
upsampling at all.
But i am still a little confused about "then you do not even need to use
the newspacing flag" . Since if i want to use same geometry(sid, sdd, fixed
detector size) after dowmsampling (from P1 to P2).
The size of each projection image changes from 256^2 to 128^2. so to
maintain the same detector size, the spacing must be 2, i.e, the newspacing
should set to 2. Is that correct???
Actually, the question i have now is that , if before the downsample, each
image size is 256^2. and in forward projecting part in SART alglrothm, say
the number of ray generated is 256^2.
Then after downsampling,i.e., when using P2 as input for rtkSART, what is
the number of ray generated in forward projections part?? is it 128^2 since
the number of pixel is 128^2 ???
Thanks very much!
Regards Guangming

On Sat, Feb 20, 2016 at 4:17 PM, Chao Wu <wuchao04 at gmail.com> wrote:

> Hi Guangming,
>
> There is no upsampling at all. It simply overwrites the pixel size of the
> projection images stored in the image files. If for instance when you
> downsampled your P1 to P2 with a factor 2, you also changed the pixel size
> from 1 to 2 already, then you do not even need to use the newspacing flag.
>
> Regards,
> Chao
>
>
> 2016-02-19 23:50 GMT+01:00 Guangming Zang <guangming.zang at kaust.edu.sa>:
>
>> Hi Simon,
>> Thanks for your kind reply and information provided.
>> Basically the problem i encountered is that without changing any geometry
>> (i.e., sid, sdd, detector physical size), if i downsample my original
>> projection images, how can i reconstruct corresponding volume correctly.
>> In RTK, it seems that i just need to change the input projection file
>>  and newspacing parameter (as same as the factor of downsample), then i got
>> promising results. but in my own application, i simulate the situation, but
>> got some aliasing. So i really want to know how it work behind our RTK.
>> I thought that this parameter is to simulate the detector's spacing size.
>> And in my experiment, i set the detector physical size as 256mm*256mm,
>> and i got projection images P1.mha with size of 256^2 , now i downsample
>> these images with factor of 2( i.e., the images size change from 256^2 to
>> 128^2 ), from which i got P2.mha
>> in our rtkfdk, after the dowmsample. i just need change input projections
>> from P1.mha to P2.mha, besides, i need change flag newspacing from 1 to 2.
>> both of the results from RTK  are pretty good.
>> my questions are:
>> 1) can i just regard the newspacing as the upsample operation with
>> nearest neighbor filter??
>> 2) how many rays RTK generated in forward projecion (i.e. ray tracing
>> part ) for each image in P2.mha??? is it 256^2????
>>
>> To sum up,in my own application, can i reproduce the result from RTK for
>> flag "newspacing as 2"  by doing:
>> upsample image by factor of 2 (got a 256mm*256mm image ) and just
>> generate half number of rays in raytracing (forward priject ) part???
>>
>> Thanks for your time and help in advance,
>> -Regards Guangming
>>
>>
>>
>> On Sat, Feb 20, 2016 at 1:15 AM, Simon Rit <
>> simon.rit at creatis.insa-lyon.fr> wrote:
>>
>>> Hi Guangming,
>>> The parameter is processed in rtkGgoFunctions
>>> <https://github.com/SimonRit/RTK/blob/master/code/rtkGgoFunctions.h>.h.
>>> It's passed to an itk::ChangeInformationFilter
>>> <http://www.itk.org/Doxygen/html/classitk_1_1ChangeInformationImageFilter.html>
>>> at the beginning of the rtk::ProjectionsReader.h
>>> <http://www.openrtk.org/Doxygen/classrtk_1_1ProjectionsReader.html>.
>>> But in the end, there is nothing to explain, it's simply changes the
>>> spacing of the input projections.
>>> I hope this helps,
>>> Simon
>>>
>>> On Fri, Feb 19, 2016 at 8:52 PM, Guangming Zang <
>>> guangming.zang at kaust.edu.sa> wrote:
>>>
>>>>
>>>> ---------- Forwarded message ----------
>>>> From: Guangming Zang <guangming.zang at kaust.edu.sa>
>>>> Date: Fri, Feb 19, 2016 at 10:49 PM
>>>> Subject: About the newspacing flag
>>>> To: rtk-users-request at public.kitware.com, Chao Wu <wuchao04 at gmail.com>,
>>>> Simon Rit <simon.rit at creatis.insa-lyon.fr>, "
>>>> rtk-users at public.kitware.com" <rtk-users at public.kitware.com>,
>>>> rtk-users at openrtk.org
>>>>
>>>>
>>>> Dear RTK community,
>>>> i have a question about the parameter "newspacing " in rtkFDK and
>>>> rtkSART algorithm.
>>>> it says:--newspacing  new spacing of input projections (before
>>>> pre-processing).
>>>>  I checked in the code, and it seems not  easy to find the correct part
>>>> in the code about the newspacing .
>>>> So could anyone  help me and explain about this parameter?? i want to
>>>> simulate this preprocess step in my own project and compare to our RTK.
>>>> -best regards Guangming
>>>>
>>>> --
>>>> *Guangming Zang (Alex)*
>>>> *King Abdullah University of Science and Technology(KAUST)*
>>>> *University of Chinese Academy of Sciences(UCAS)*
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> *Guangming Zang (Alex)*
>>>> *King Abdullah University of Science and Technology(KAUST)*
>>>> *University of Chinese Academy of Sciences(UCAS)*
>>>>
>>>>
>>>>
>>>>
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>>>
>>
>>
>> --
>> *Guangming Zang (Alex)*
>> *King Abdullah University of Science and Technology(KAUST)*
>> *University of Chinese Academy of Sciences(UCAS)*
>>
>>
>>
>>
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>

-- 
*Guangming Zang (Alex)*
*King Abdullah University of Science and Technology(KAUST)*
*University of Chinese Academy of Sciences(UCAS)*

-- 

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