Re: [Paraview] VTK XML Binary Output

Wed, 01 Apr 2009 02:56:02 -0700

Hello David,

the offset parameter is relevant if you append mulitple data sections into a single file.
Every DataArray tag with append format specifies the offset as the position in the
binary data in bytes.
To write the integer I use the following:

std::ofstream os; // binary mode (for windows ...) ofstream to your file
int N; // size in bytes
os.write(reinterpret_cast<const char*>(N), 4);

Thorsten

David Mulholland schrieb:
Hi Thorsten,

That's really helpful. I don't quite know how to write out the number of bits as a char? If you could explain a way of how to do that it would be greatly appreciated.

Also, how do I calculate what value goes in for the offset? I had a look at a couple of output files from paraview and it wasnt obvious how it was calculated.

Thanks
David

Sent: Tuesday, March 31, 2009 2:23 PM
Subject: Re: [Paraview] VTK XML Binary Output

Hello,

the solution might be to append your data as

< DataArray ... format="appended">
</DataArray>
< AppendedData encoding="raw">
_NNNNData
</AppendedData>
where you should replace NNNN with a four
byte integer containing the number of bytes
in the data array as 4 chars.
Setting the encoding to "raw" tells ParaView, that
the data is simply a stream of bytes rather than
base64 encoded.
If you want to write Image I can supply a small
writer project for 2D data.

    Thorsten
 
David Mulholland schrieb:
Hello,

I am currently trying to write my own quick output routine to generate a
binary file using C++. I am currently trying to avoid having to use the vtk
libraries because I only need a very basic output that doesn't change much.
I so far have the ASCII output working no problem but I can't get the binary
working.

I am using file.write() function to output the binary data and that seems to
be working no problem and when used in a legacy format application it works
perfectly except that the data is LittleEndian instead of BigEndian. That's
why I have moved to BigEndian because I don't want to have to make the
output routine for the data anymore complicated that file.write.

I started using the xml format and considering my output is raw binary and
not base64 I changed it to be in the AppendedData section instead of
DataArray. It still isn't working right, I get the following error when I
try to load in ParaView:

ERROR: In ..\..\..\ParaView3\VTK\IO\vtkXMLStructuredDataReader.cxx, line 349
vtkXMLImageDataReader (0ADEC7F8): Error reading extent 0 422 0 242 0 1 from
piece 0


ERROR: In ..\..\..\ParaView3\VTK\IO\vtkXMLDataReader.cxx, line 537
vtkXMLImageDataReader (0ADEC7F8): Cannot read cell data array "pressure"
from PointData in piece 0.  The data array in the element may be too short.

The format of my output file is as follows:

<?xml version="1.0"?>
<VTKFile type="ImageData" version="0.1" byte_order="LittleEndian">
<ImageData WholeExtent="0 422 0 242 0 1" Origin="0 0 0" Spacing="1 1 1">
<Piece Extent="0 422 0 242 0 1">
<PointData>
</PointData>
<CellData Scalars="pressure">
<DataArray type="Float32" Name="pressure" format="appended" offset="0"/>
</CellData>
</Piece>
</ImageData>
<AppendedData encoding="raw">
_BINARYDATA
</AppendedData>
</VTKFile>

Note following an error message I have added an _ at the start of the binary
data. Is this required?

One final thing, when using appended data, how do you calculate what the
offset should be?

Thanks
David

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