[vtkusers] vtkImageReader much faster than vtkImageReader2??

[David Gobbi]

Hi Marco,

When used the way they are in your example code, vtkImageReader and
vtkImageReader2 should be exactly the same speed.

Can you explain your testing methodology in more detail?  I suspect that
when you tested vtkImageReader2, it was reading from disk, and when
testing vtkImageReader, it was reading from disk cache.  The only part
of the "read" operation that actually requires any CPU time is the byte
swapping, but that should be identical for the two readers.

 - David


On Thu, Jun 12, 2014 at 11:00 AM, Marco Sambin <m.sambin at gmail.com> wrote:
> Hi all.
> I am reading a volume made up of a set of "slices" stored in a single large
> raw file, containing just the raw pixels of each slice (unsigned short
> values), one slice after the other. I know in advance image size, position,
> orientation and spacing information related to each slice.
> I used to read my input raw volume through the vtkImageReader2 class, as
> documentation of vtkImageReader states:
>
> "vtkImageReader provides methods needed to read a region from a file. It
> supports both transforms and masks on the input data, but as a result is
> more complicated and slower than its parent class vtkImageReader2."
>
> so I always assumed vtkImageReader2 to be faster than vtkImageReader to read
> a simple volume made up of a set of raw slices.
> This until today, when I tried replacing my instance of vtkImageReader2 with
> an instance of vtkImageReader. Guess what? My volume reading code now
> executes around 10 times faster!
>
> Is this expected? Can you guess what may be the reason for such a
> significant difference in performance between vtkImageReader2 and
> vtkImageReader, the latter being much faster in my scenario (despite what
> the documentation states)?
>
> For completeness, I am using VTK 6.1 64-bit from Java 7 64-bit, under
> Windows 7 Professional 64-bit. Here is a code fragment showing how I
> configure my reader:
>
> [...]
> imageReaderVTK =  new vtkImageReader(); // much slower if I use
> "vtkImageReader2" here
> imageReaderVTK.FileLowerLeftOn();
> imageReaderVTK.SetFileDimensionality(3);
> imageReaderVTK.SetFileName(curRawVolFile.getAbsolutePath());
> imageReaderVTK.SetDataExtent(0, sliceCols - 1,
>                       0, sliceRows - 1,
>                       0, numOfUsedImages - 1);
> imageReaderVTK.SetDataSpacing(xSpacing, ySpacing, zSpacing);
> imageReaderVTK.SetDataOrigin(0.0, 0.0, 0.0);
> imageReaderVTK.SetDataScalarTypeToUnsignedShort();
> imageReaderVTK.SetDataByteOrderToBigEndian();
> imageReaderVTK.UpdateWholeExtent();
> [...]
>
> My typical input raw volume is made up of around 500 slices, each one
> 512x512 pixels large.
>
> Thanks in advance for your feedback.
> Best regards,
>
> Marco Sambin