### Author:  [@qzxuhui](https://discourse.vtk.org/u/qzxuhui)
#### Post date: [October 26, 2020, 12:11pm UTC](https://discourse.vtk.org/t/how-to-understand-binary-dataarray-in-xml-vtk-output/4489/1 "2020-10-26T12:11:07Z")
Hello everyone, I have a problem with how to understand the binary `DataArray`.
The manual said if the format of `DataArray` is `binary`,
```auto
The data are encoded in base64 and listed contiguously inside the
DataArray element. Data may also be compressed before encoding in base64. The byte-
order of the data matches that specified by the byte_order attribute of the VTKFile ele-
ment.
```
I can not fully understand that, so I have obtain ascii file and binary file for same model.
# ASCII file
```auto
int main()
{
int x = 10;
int len;
// first arg: binary buffer
// second arg: length of binary buffer
// third arg: length of ascii buffer
char *ascii = base64((char *)&x, sizeof(int), &len);
std::cout << ascii << std::endl;
std::cout << len << std::endl;
free(ascii);
return 0;
}
```
Can someone give me an explanation of how to convert?
Another relate topic can be see in
- [error when writing binary VTK files](https://discourse.vtk.org/t/error-when-writing-binary-vtk-files/4487/7)
Thanks for your time.
---
### Author:  [@Yohann\_Bearzi](https://discourse.vtk.org/u/Yohann_Bearzi)
#### Post date: [October 26, 2020, 10:00pm UTC](https://discourse.vtk.org/t/how-to-understand-binary-dataarray-in-xml-vtk-output/4489/2 "2020-10-26T22:00:21Z")
You will find your answer in `IO/Core/vtkBase64Utilities.cxx`. Bytes are encoded by triplets, pairs, or singletons. Whenever you find one isolated `=`, it is encoded by pair. When you find the pattern `==`, it is encoded by singleton. Everywhere else, it is encoded by triplet.
I suppose that the reason for this custom mapping was to be able to read more easily binary data when you open your file in a text editor, with a set of 64 human readable symbols, and no occurrence of `Enter`, `Tab`, or this kind of symbols that could pop up.
The file I pointed you to allows you to encode and decode data.
---
### Author:  [@Yohann\_Bearzi](https://discourse.vtk.org/u/Yohann_Bearzi)
#### Post date: [October 26, 2020, 10:18pm UTC](https://discourse.vtk.org/t/how-to-understand-binary-dataarray-in-xml-vtk-output/4489/3 "2020-10-26T22:18:31Z")
I suspect that you have a lot of what seems like extra data because you are using a compressor. When you do so, there is a need to write a compressor header at the beginning of each data array you are writing. If you do `SetCompressor(nullptr)` prior to writing your file, you should have raw binary data, following the encoding rule of `vtkBase64Utilities.cxx`.
---
### Author:  [@qzxuhui](https://discourse.vtk.org/u/qzxuhui)
#### Post date: [October 27, 2020, 12:06am UTC](https://discourse.vtk.org/t/how-to-understand-binary-dataarray-in-xml-vtk-output/4489/4 "2020-10-27T00:06:50Z")
Thanks for sharing sir.
I now can have a better understanding now: grinning: .
It is a complicated process. I think I should directly call the vtk API in my code other than writing the my own base64 encoder, compressor et al.
---
### Author:  [@Yohann\_Bearzi](https://discourse.vtk.org/u/Yohann_Bearzi)
#### Post date: [October 27, 2020, 5:36pm UTC](https://discourse.vtk.org/t/how-to-understand-binary-dataarray-in-xml-vtk-output/4489/5 "2020-10-27T17:36:35Z")
@qzxuhui If you want to create a custom XML reader / writer, I highly recommend writing classes inheriting from `vtkXMLReader` and `vtkXMLWriter`. XML elements are easy to handle, and there are ready-to-use methods that write / read arrays from an XML file, with the possibility to read an array given an input segment of indices.
Edit: Plus, all this encoding is hidden under the rug. You don’t have to worry about compressed / uncompressed data. The only tedious thing is that if you write your data in `Appended` mode or in `Binary` / `ASCII` mode (which are inlined), you have to handle it explicitly in the writer.