# Individual coloured lines in polydata file
**URL:** https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007
**Category:** Support
**Tags:** file-formats
**Created:** [June 10, 2024, 3:11pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007 "2024-06-10T15:11:00Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@jpmorr](https://discourse.vtk.org/u/jpmorr)
#### Post date: [June 10, 2024, 3:11pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/1 "2024-06-10T15:11:00Z")
I’m trying to understand how to write a `.vtp` correctly to display different cell data for each line.
I’ve created a simple case with 7 points, which should give me a 6 lines, and therefore 6 cells, each of which can be set independently. I’ve written the `.vtp` as this which I though was correct:
```auto
0.5901740250590545 0.9947671646588059 0.7489206394212843 0.0304343354124273 0.9234538424010137 0.9798888990537769
0.3373415663707561
0.3916196439944402 0.6046746919418857 0.8792924894608538 0.1426086960632018 0.3544317346711524 0.5498989637627353
0.8375566887711513
1.0 2.0 3.0 4.0 5.0 6.0
0.0000000000000000 0.0000000000000000 0.0000000000000000 1.0000000000000000 1.0000000000000000 0.0000000000000000
2.0000000000000000 0.0000000000000000 0.0000000000000000 3.0000000000000000 1.5000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 3.0000000000000000 1.0000000000000000 1.0000000000000000 3.0000000000000000
2.0000000000000000 0.0000000000000000 3.0000000000000000
0 1 2 3 4 5
6
2 3 4 5 6 7
```
However, when I try to load into Paraview, it throws an error about not enough points:
```auto
ERROR: In vtkXMLDataReader.cxx, line 433
vtkXMLPolyDataReader (0x56178872f700): Cannot read cell data array "velocity" from PointData in piece 0. The data array in the element may be too short.
```
I’m a little confused as to what the correct data for `offsets` is to specify the 6 expected lines. I’m also confused by the warning which says that the cell data is missing from the `PointData` as I have placed this in the `CellData` section, which is what is done with polylines.
---
### Author:  [@jpmorr](https://discourse.vtk.org/u/jpmorr)
#### Post date: [June 10, 2024, 8:31pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/2 "2024-06-10T20:31:54Z")
OK, after a break from this I realised I was treating lines the same as poly-line without thinking that the lines need to be pairs of points, even if two lines share a point.
So now my working solution looks like this:
```auto
0.0304343354124273 0.9234538424010137 0.9798888990537769 0.3373415663707561 0.6533569100468444 0.5805916314301794
0.9231814078620875 0.8286460163085659 0.8841588239981764 0.8735697354630155
0.3916196439944402 0.6046746919418857 0.8792924894608538 0.1426086960632018 0.3544317346711524 0.5498989637627353
0.8375566887711513 0.5901740250590545 0.9947671646588059 0.7489206394212843
0 1 2 3 4
0.0000000000000000 0.0000000000000000 0.0000000000000000 1.0000000000000000 1.0000000000000000 0.0000000000000000
1.0000000000000000 1.0000000000000000 0.0000000000000000 2.0000000000000000 0.0000000000000000 0.0000000000000000
2.0000000000000000 0.0000000000000000 0.0000000000000000 3.0000000000000000 1.5000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 3.0000000000000000 1.0000000000000000 1.0000000000000000 3.0000000000000000
1.0000000000000000 1.0000000000000000 3.0000000000000000 2.0000000000000000 0.0000000000000000 3.0000000000000000
0 1 2 3 4 5
6 7 8 9
2 4 6 8 10
```
Which I believe is correct (I’ve removed one of the line segments from the original example for testing purposes).
However, this raises a question about duplication of data.
For a small example like this it doesn’t matter, but what if there are thousands of lines where each point could be shared between multiple lines, then there would be significant duplication making the data file much larger than required. Is there a way in the `offsets` or `connectivity` to recreate this polydata file with only the bare minimum number of 7 points in the file instead of the 10 (or 12 if I put back the deleted segment) required?
---
### Author:  [@Aron\_Helser](https://discourse.vtk.org/u/Aron_Helser)
#### Post date: [June 11, 2024, 12:14pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/3 "2024-06-11T12:14:54Z")
> [@jpmorr](#):
>
> Is there a way in the `offsets` or `connectivity` to recreate this polydata file with only the bare minimum number of 7 points in the file
Yes, absolutely! The points array can be de-duplicated. In your case, if you wanted the first two line segments to share point 1, you could specify `0 1 1 2` to share that point. You can just list your 7 points, and then use indices 0-6 in the connectivity array to refer to them.
---
### Author:  [@jpmorr](https://discourse.vtk.org/u/jpmorr)
#### Post date: [June 11, 2024, 1:46pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/4 "2024-06-11T13:46:29Z")
Sorry I think I’m being a bit thick today. If I understand correctly, I would just leave the `connectivity` to be `0 1 2 3 4 5 6` and then modify the `offsets` to include the duplicate points?
In my example, since I have 7 points, I should be able to make 6 continuous lines if they are connected end to end. I’ve added 6 `CellData` points for the velocity field and listed the connectivity as 0 though 6. I’ve tried to then edit the offsets as suggested, but importing into paraview still throws an error:
```auto
ERROR: In vtkXMLDataReader.cxx, line 433
vtkXMLPolyDataReader (0x5617862a1180): Cannot read cell data array "velocity" from PointData in piece 0. The data array in the element may be too short.
```
I believe I’ve not managed to follow your instruction correctly. Could you check this simple template for my mistake and point out how to change it? I’ve also tried with the `offsets` specified as `0 1 1 2 2 3 3 4 4 5 5 6` but that throws the same error.
```auto
0.5901740250590545 0.9947671646588059 0.7489206394212843 0.0304343354124273 0.9234538424010137 0.9798888990537769
0.3373415663707561
0.3916196439944402 0.6046746919418857 0.8792924894608538 0.1426086960632018 0.3544317346711524 0.5498989637627353
0.8375566887711513
1.0 2.0 3.0 4.0 5.0 6.0
0.0000000000000000 0.0000000000000000 0.0000000000000000 1.0000000000000000 1.0000000000000000 0.0000000000000000
2.0000000000000000 0.0000000000000000 0.0000000000000000 3.0000000000000000 1.5000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 3.0000000000000000 1.0000000000000000 1.0000000000000000 3.0000000000000000
2.0000000000000000 0.0000000000000000 3.0000000000000000
0 1 2 3 4 5 6
1 2 2 3 3 4 4 5 5 6 6 7
```
---
### Author:  [@olesenm](https://discourse.vtk.org/u/olesenm)
#### Post date: [June 11, 2024, 2:04pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/5 "2024-06-11T14:04:45Z")
Perhaps it could be useful to try some simple reverse engineering? Scribble together what you want as a VTK legacy format, load in paraview and save in XML format and see what it generates.
---
### Author:  [@jpmorr](https://discourse.vtk.org/u/jpmorr)
#### Post date: [June 11, 2024, 2:28pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/6 "2024-06-11T14:28:42Z")
Hi Mark, the problems for me isn’t the difference between legacy and xml formats (I’m not familiar with the legacy format anyway) but just how to specify the `offsets` and `connectivity` for a VTK file as this should be the same for a polydata file in either legacy (.vtk) or xml (.vtp) format.
I understand how to specify the `offsets` and `connectivity` to generate a single `polyline` using all the data points or how to create multiple individual lines which requires duplicating shared points but is not ideal for very large datasets where a single point can be the start/end point of up to 20 lines. This could make files many times larger than necessary.
If you know how to create the legacy vtk file that achieves this, that would also be very helpful because it should have the same `connectivity` and `offsets` and as you mentioned, I could just just open in something like Parview and save to a different format.
---
### Author:  [@Aron\_Helser](https://discourse.vtk.org/u/Aron_Helser)
#### Post date: [June 11, 2024, 4:44pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/7 "2024-06-11T16:44:44Z")
I should probably find if this is explained in a manual somewhere 😃
No, you want to leave the `offsets` array alone, that was correct. It specifies where in the connectivity array to start looking for each cell. Since your cells are lines, each cell has 2 points, so the offsets should be `2 4 6 8 ...`
Now you know the first two entries (from `offsets`) in the `connectivity` array define the first cell, a line. So the first two numbers there are indices of the points that define that line. If they are `0 1`, then points 0 and 1 will define that first line. If the next two entries are `5 6` then points 5 and 6 will define the next line.
To play with this more, I used ParaView, and I suggest you try it out. Here’s the file I produced by creating a `Line` source, turning off the `Use regular refinement` checkbox, then choosing `File .. Save Data`, and choosing a VTP polydata file in `ascii` mode.
```auto
0 0 1 0
0
1
0.37692267 0.2668703 0.04731071 0.07376513 0.45690945 0.070647135
0.46425074249
0.46818645384
0.46425074249
0.46818645384
0 1
2
```
Note that if you turn on the `Use regular refinement` checkbox, and save again, you get a connectivity array like `0 1 2 3 4 5 6` and an offsets array of `7`, which is a 7 element line strip, so you can do that, too.
---
### Author:  [@jpmorr](https://discourse.vtk.org/u/jpmorr)
#### Post date: [June 11, 2024, 7:16pm UTC](https://discourse.vtk.org/t/individual-coloured-lines-in-polydata-file/14007/8 "2024-06-11T19:16:58Z")
@Aron_Helser Thank you so much for explaining. I misunderstood that it was the connectivity that I was supposed to change while leaving the offsets alone. Using this knowledge I was example to make the example work…eventually.
I took me quite a while to spot the _ **obvious** _ mistake preventing it from working:
```auto
```
which clearly should have `NumberOfLines="6"`
If this is in the documentation, then a lot of people have missed it. I’ve only ever seen the `.vtp` files written with the points duplicated but I guess this is the same way elements in structured grids are also defined. It’s just not obvious that it also applies here for simple lines.
I’m finally learning vtk in a bit of detail instead of blindly writing data in in-efficient ways.