Lack of precision ImageData finding procedures

[Odet Christophe]

I think that there is a problem with the function vtkImageData::ComputeStructuredCoordinates
because the 

ijk[i] = (int) floatLoc 

statement rounds towards zero the floatLoc value. When the floatLoc equals -0.5, for example,
it is rounded to 0 and the x point seems to belongs to the cell beginning at point 0,0,0. The 
problem arises when dealing with x values greater or lower than than the extent.

I suggest to change some reference to ijk[i] with floatLoc in the code lines:

######### OLD CODE
        if ( ijk[i] >= this->Extent[i*2] && ijk[i] < this->Extent[i*2 + 1] )
      {
      pcoords[i] = floatLoc - (float)ijk[i];
      }

    else if ( ijk[i] < this->Extent[i*2] || ijk[i] > this->Extent[i*2+1] ) 
      {
      return 0;
      } 
########## END OF OLD CODE

to obtain 

######### NEW PROPOSED CODE
        if ( floatLoc >= this->Extent[i*2] && floatLoc < this->Extent[i*2 + 1] )
      {
      pcoords[i] = floatLoc - (float)ijk[i];
      }

    else if ( floatLoc < this->Extent[i*2] || floatLoc > this->Extent[i*2+1] ) 
      {
      return 0;
      } 
########## END OF NEW CODE

It test the patch with the following little tcl/tk script and it seems to work well. The dataset implicit 
clipping is more precise with the new code (this is visible with small objects as used in the script).
In the original version a too large piece of the sphere is selected. In the new version, only some
part of the cells clipped by the dataset are selected. I hope this will help, but I think that the same problem
could appear in some other functions of the VTK library (FindCells, FindPoints....).

##########

# Object to be cut. A simple little sphere

vtkSphereSource sphe
sphe SetRadius 2

vtkPolyDataMapper spheMapper
    spheMapper SetInput [sphe GetOutput]
    spheMapper ScalarVisibilityOff
vtkActor spheActor
    spheActor SetMapper spheMapper

# The cutter... a simple parallelogram

vtkStructuredPoints sp
sp SetDimensions 2 2 2
sp SetSpacing 5 0.1 2
sp AllocateScalars
set spsc [[sp GetPointData] GetScalars]
for {set i 0} {$i<8} { incr i } {
 $spsc SetScalar $i 1
}
vtkDataSetMapper spMapper
  spMapper SetInput sp
vtkActor spActor
  spActor SetMapper spMapper 

vtkImplicitVolume spds
spds SetVolume sp
spds SetOutValue 0
spds SetOutGradient 1 0 0

# Cutting...

vtkClipPolyData clipper
 clipper SetInput [sphe GetOutput]
 clipper SetClipFunction spds
 clipper GenerateClipScalarsOn
 clipper SetValue 0.5

vtkPolyDataMapper clipMapper
    clipMapper SetInput [clipper GetOutput]
vtkActor clipActor
    clipActor SetMapper clipMapper


# Create graphics stuff
#
vtkRenderer ren1
vtkRenderWindow renWin
    renWin AddRenderer ren1
vtkRenderWindowInteractor iren
    iren SetRenderWindow renWin

# Add the actors to the renderer, set the background and size
ren1 AddActor spheActor
[spheActor GetProperty] SetRepresentationToWireframe
ren1 AddActor spActor 
#spActor AddPosition -3 0 0
ren1 AddActor clipActor
#clipActor AddPosition 0 3 0
ren1 SetBackground 0 0 0
[ren1 GetActiveCamera] Azimuth 30
[ren1 GetActiveCamera] Elevation 30
[ren1 GetActiveCamera] Dolly 1.5
ren1 ResetCameraClippingRange

renWin SetSize 400 400
iren Initialize

# render the image
#
iren SetUserMethod {wm deiconify .vtkInteract}


scale .c

proc toto { val } { clipper SetValue $val; renWin Render }
pack .c
.c configure -from 0 -to 1 -resolution 0.1 -command toto
.c set 0.5
 

--
********************************************************************
ODET Christophe
CREATIS, INSA 502, F-69621 Villeurbanne cedex (France)
tel 04 72 43 85 62
Fax 04 72 43 85 26
Email: christophe.odet at creatis.insa-lyon.fr
URL    :http://www.creatis.insa-lyon.fr
********************************************************************

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