[Rtk-users] Fwd: About the newspacing flag

Sat Feb 20 08:53:28 EST 2016

Hi,
Yes, that's correct, we simulate one ray per pixel.
Simon

On Sat, Feb 20, 2016 at 2:50 PM, Chao Wu <wuchao04 at gmail.com> wrote:

> Hi Guangming,
>
> In many image file format the pixel size (spacing) can be stored within
> the file data. If the pixel size in the projection files are correct (e.g.
> spacing=1 in your 256x256 P1, or spacing=2 in your 128x128 P2), then you
> can use them without giving the newspacing flag. The newspacing flag just
> gives you a chance to force RTK to use the pixel size you specified instead
> of the one that stored in the image files.
>
> I am not into rtkSART but I guess the forward projection works with the
> input grid, so 128x128 if the input projections are 128x128. Please correct
> me if I am wrong.
>
> Regards,
> Chao
>
> 2016-02-20 14:28 GMT+01:00 Guangming Zang <guangming.zang at kaust.edu.sa>:
>
>> 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)*
>>
>>
>>
>>
>> ------------------------------
>> This message and its contents, including attachments are intended solely
>> for the original recipient. If you are not the intended recipient or have
>> received this message in error, please notify me immediately and delete
>> this message from your computer system. Any unauthorized use or
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>>
>
>
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