# Issues with vtkImageCanvasSource2D

**URL:** https://discourse.vtk.org/t/issues-with-vtkimagecanvassource2d/6729
**Category:** Support
**Created:** [September 29, 2021, 11:18pm UTC](https://discourse.vtk.org/t/issues-with-vtkimagecanvassource2d/6729 "2021-09-29T23:18:35Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![kynetyk](https://discourse.vtk.org/user_avatar/discourse.vtk.org/kynetyk/32/3052_2.png) [@kynetyk](https://discourse.vtk.org/u/kynetyk)
#### Post date: [September 29, 2021, 11:18pm UTC](https://discourse.vtk.org/t/issues-with-vtkimagecanvassource2d/6729/1 "2021-09-29T23:18:35Z")

</div>

Hello!

I’m writing a PyQt5 application to draw on medical images using `vtkImageCanvasSource2D`, however I’m having issues with oriented images.

It seems that when I connect a `vtkImageCanvasSource2D` to a mapper using `GetOutputPort()`, the spacing and origin information is lost. Since this class is used to draw a mask, the output is not correctly overlaid on the underlying image.

I’m attaching a small snippet hoping that it illustrates well my issue. In the code below, I would expect that the second file would have preserved it’s spacing information.

(python 3.8.10, vtk 9.0.3, nibabel 3.2.1)

```python
import nibabel as nib
import vtk
from vtkmodules.util import vtkConstants

def run():

    inp = vtk.vtkImageData()
    inp.SetDimensions(10, 10, 10)
    inp.SetSpacing(1, 2, 3)
    inp.SetOrigin(-1, -3, -2)
    inp.SetDirectionMatrix(0, -1, 0, 1, 0, 0, 0, 0, 1)
    inp.AllocateScalars(vtkConstants.VTK_UNSIGNED_CHAR, 0)

    canvas = vtk.vtkImageCanvasSource2D()
    canvas.InitializeCanvasVolume(inp)

    writer = vtk.vtkNIFTIImageWriter()

    f1 = "f1.nii.gz"
    f2 = "f2.nii.gz"

    writer.SetFileName(f1)
    writer.SetInputData(inp)
    writer.Write()

    writer.SetFileName(f2)
    writer.SetInputConnection(canvas.GetOutputPort())
    writer.Write()

    print(nib.load(f1).header.get_zooms())
    print(nib.load(f2).header.get_zooms())

if __name__ == " __main__":
    run()

```

This outputs the following

```bash
(1.0, 2.0, 3.0)
(1.0, 1.0, 1.0)

```

Reading the source code, I see that `RequestInformation` sets the spacing and origin to default values. Would that be the source of the issue?

> <https://github.com/Kitware/VTK/blob/d64da3fa9d235a852012828cd5753df2d1315da1/Imaging/Sources/vtkImageCanvasSource2D.cxx#L1505>

Thanks for helping!

---

<div class="post-metadata">

### Author: ![kynetyk](https://discourse.vtk.org/user_avatar/discourse.vtk.org/kynetyk/32/3052_2.png) [@kynetyk](https://discourse.vtk.org/u/kynetyk)
#### Post date: [September 30, 2021, 2:20pm UTC](https://discourse.vtk.org/t/issues-with-vtkimagecanvassource2d/6729/2 "2021-09-30T14:20:02Z")

</div>

After some more digging I realized that I could create a separate image algorithm which resets the direction matrix.

```python
from vtkmodules.util.vtkAlgorithm import VTKPythonAlgorithmBase
from vtkmodules.vtkCommonDataModel import vtkDataObject

class SetImageDirection(VTKPythonAlgorithmBase):
    def __init__ (self, ref):
        super(). __init__ (
            nInputPorts=1,
            inputType="vtkImageData",
            nOutputPorts=1,
            outputType="vtkImageData",
        )
        self.direction = (-1, 0, 0, 0, -1, 0, 0, 0, 1)

    def RequestData(self, request, inInfo, outInfo):
        inp = vtk.vtkImageData.GetData(inInfo[0])
        opt = vtk.vtkImageData.GetData(outInfo)
        opt.ShallowCopy(inp)
        opt.SetDirectionMatrix(self.direction)
        return 1

    def RequestInformation(self, request, inInfo, outInfo):
        info = outInfo.GetInformationObject(0)
        info.Set(vtkDataObject.DIRECTION(), self.direction, 9)
        return 1

```

Chaining this filter in combination with `vtkImageChangeInformation` after algorithms that do not transmit the direction matrix seems to solve my problem. I’m linking to that [blog post](https://blog.kitware.com/vtkpythonalgorithm-is-great/) that came in handy, for reference.