[Insight-users] AddPixelAccessor set function confusion

[robert tamburo]

Thanks for the response Luis. I'm still confused though because none of the
other pixel accessors use the Set() method for inverse operations,
e.g., SqrtPixelAccessor,
all of the trigonometric accessors, etc. I wrote some code using
SqrtPixelAdaptor (AddPixelAccessor will not compile, see below) to test the
Set() and Get() methods.

For an image set to 100:
- Get() returns 10 as expected.
- Set(10) sets the pixel to sqrt(10), and followed by Get() returns the
sqrt(sqrt(10)), which is what you warned me about. But is this the intended
use of Set()?

Back to your example with AddPixelAccessor:
It is my understanding that image adaptors behave like images. So SetPixel
would set the pixel value according to the pixel accessor used. So, by your
example, SetPixel(12) would ultimately just set the pixel to "7", which is
the value of the input image. It allows Get() to be used correctly, but
doesn't seem too useful.

Considering that itkAddPixelAccessor will not compile and there is no test
for it, and that its Set() method does not follow the other Accessors, is it
possible that the functionality for Set() was changed some time ago
and itkAddPixelAccessor
 fell through the cracks?

Thanks.

********
I attempted to write test code for AddPixelAccessor, but am getting compile
errors:
typedef itk::AddPixelAccessor<float> AddAccessorType;
itkAddPixelAccesor.h: No such file or directory

I made a copy of itkAddPixelAccesor.h (itkTestAddPixelAccessor.h), and
received the following error:
error: expected initializer before'<' token
which usually means the header wasn't included...

I moved #include "itkTestAddPixelAccessor.h" to the top of my include list
and got the following error:
itkTestAddPixelAccessor.h:39: error: invalid function declaration

Consider me perplexed...

On Fri, Dec 10, 2010 at 5:53 AM, Luis Ibanez <luis.ibanez at kitware.com>wrote:

> Hi Robert, David
>
>
> The implementation is correct.
>
>
> You are confusing the role of the Set()  method with
> the role of the Get() method. One must apply the
> inverse operation of the other.
>
>
> An ImageAdaptor is intended to present an image
> as if the image had been processed by a filter.
>
> The PixelAccessor for that ImageAdaptor implements
> the trick of transforming the pixel on the fly.
>
> The AddImageAdaptor must present the image A
> as A+k (where "k" is a constant that it is added to
> every pixel in A).
>
>                           B  =  A + k
>
> The addition of K is implemented in the "Get" method.
>
> So, if your input image has a pixel with value "7",
> and "k" has been set to "5",
>
> then calling "Get" in that pixel should give you "7+5=12"
>
> To be consistent, the "Set" operation must be the
> inverse of the "Get" operation.
>
> So, if you "Set" a pixel to value "12", the "Set" method
> must store in "A" the value "12-5 = 7".
>
> In that way,  if you call "Get" later in that same pixel,
> you get your "12" back.
>
>
>   Regards,
>
>
>      Luis
>
>
> ---------------------------------------------
> On Thu, Dec 9, 2010 at 4:33 PM, robert tamburo <robert.tamburo at gmail.com>
> wrote:
> > I'm confused with the set function of AddPixelAccessor. It subtracts a
> value
> > from the input rather than adding it.
> > Relevant code:
> > inline void Set( InternalType & output, const ExternalType & input )
> const
> >     { output = static_cast<InternalType>( input - m_Value ); }
> > Shouldn't it add m_Value to the input? Or is there something else going
> on?
> > -Robert
> >
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PROJECT(TESTCODE)

cmake_minimum_required(VERSION 2.6)

INCLUDE_DIRECTORIES(
${TESTCODE_SOURCE_DIR}
)

# Find ITK
INCLUDE (${CMAKE_ROOT}/Modules/FindITK.cmake)
IF (USE_ITK_FILE)
  INCLUDE(${USE_ITK_FILE})
ENDIF(USE_ITK_FILE)

LINK_LIBRARIES(
  ITKBasicFilters
  ITKCommon
  ITKIO
  itksys
  ITKNumerics
  ITKMetaIO
  ITKStatistics
  ITKAlgorithms
)

SET(TESTCODE_App_SRCS
    main.cxx)

SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -U__APPLE__")

ADD_EXECUTABLE(TESTCODE
               ${TESTCODE_App_SRCS}
)
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