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 <[email protected]>: > 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 <[email protected]> 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 <[email protected]>: >> >>> 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 < >>> [email protected]> 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 < >>>> [email protected]> wrote: >>>> >>>>> >>>>> ---------- Forwarded message ---------- >>>>> From: Guangming Zang <[email protected]> >>>>> Date: Fri, Feb 19, 2016 at 10:49 PM >>>>> Subject: About the newspacing flag >>>>> To: [email protected], Chao Wu <[email protected]>, >>>>> Simon Rit <[email protected]>, " >>>>> [email protected]" <[email protected]>, >>>>> [email protected] >>>>> >>>>> >>>>> 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)* >>>>> >>>>> >>>>> >>>>> >>>>> ------------------------------ >>>>> 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 >>>>> distribution is prohibited. Please consider the environment before >>>>> printing >>>>> this email. >>>>> >>>>> _______________________________________________ >>>>> Rtk-users mailing list >>>>> [email protected] >>>>> http://public.kitware.com/mailman/listinfo/rtk-users >>>>> >>>>> >>>> >>> >>> >>> -- >>> *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 >>> distribution is prohibited. Please consider the environment before printing >>> this email. >>> >>> _______________________________________________ >>> Rtk-users mailing list >>> [email protected] >>> http://public.kitware.com/mailman/listinfo/rtk-users >>> >>> >> > > > -- > *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 > distribution is prohibited. Please consider the environment before printing > this email. >
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