[ITK] confused about itk io factory

[weiehome at sina.com]

Hi, Dženan

All i want to do is : ImageIOBase* ioPointer = dynamic_cast<ImageIOBase*>(ObjectFactoryBase::CreateInstance(classname).GetPointer());
when i give classname "class itk::PNGImageIO", this line shall return me a ioPointer points to a PNGImageIO instance.  As you explained, the only way to get a sub-class pointer is to call a subclass's CreateObject method. But what i am looking for this a reflection mechnism so i can create objects with classnames.

Is there a way to do that?

btw: i did concern about CPU consumtion in looping registered IO factory everytime reading files, that is why i wrote own map of classname and IO objects to use setImageIO specificly. 
Best Wishes
Xie




weiehome at sina.com
 
From: Dženan Zukić
Date: 2016-03-22 22:14
To: 谢玮宜
CC: community at itk.org
Subject: Re: [ITK] confused about itk io factory
Hi Xie,

the purpose of CreateInstance method is to be implemented (over-ridden) by sub-classes. Each sub-class would create instance of itself. I guess the base class just creates an instance of the first registered factory.

Normally, reader->setImageio is not necessary. Try it. The reader asks all the imageIOs whether they can read the file  (based on the filename extension) until one IO can read. If files have no extension or an un-matching extension, than setting imageIO should be used. Also, if you will only work with one type of files, you can manually set that imageIO to save a few CPU cycles by not checking different imageIOs when opening files.

Regards,
Dženan

On Tue, Mar 22, 2016 at 4:14 AM, 谢玮宜 <weiehome at sina.com> wrote:

Dear ITK experts,

I recently have some problems with using ITK ImageIO factory functions.
When I tried to use ObjectFactoryBase::CreateInstance(const char
*itkclassname) with itkclassname like itkPNGImageIO, it is not working. The
purpose is to create an image io instance by classname using RTTI.
I did debug into the code and then I found itkclassname has to be
itkImageIOBase, then it will create always the first ImageIO instance in
override map. I got really confused what is the purpose of this API
"CreateInstance(classname)".

I also print out all registered ImageIO factories and I found gdcmImageIO is
in it. So what is the purpose that in example of dicom images reading and
write, everytime we have to manually call reader->setImageio(
gdcmIO::New())?
As I understood, if an imageIO is registered already, the reader will
automatically check when it is reading a file to see if the format is
supported by an underlying IO.

Please correct me. I think I somehow got seriously wrong with the underlying
concept of itk IO mechanism.
Thank you all in advance!

BR,xie


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