# How to convert the coordinate between pixel and physical (vtkImageReslice)?

**URL:** https://discourse.vtk.org/t/how-to-convert-the-coordinate-between-pixel-and-physical-vtkimagereslice/1391
**Category:** Support
**Created:** [July 21, 2019, 8:11am UTC](https://discourse.vtk.org/t/how-to-convert-the-coordinate-between-pixel-and-physical-vtkimagereslice/1391 "2019-07-21T08:11:40Z")
**Posts on this page:** 1
**Showing post:** 4

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### Author: ![dgobbi](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dgobbi/32/18_2.png) [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [July 23, 2019, 7:43pm UTC](https://discourse.vtk.org/t/how-to-convert-the-coordinate-between-pixel-and-physical-vtkimagereslice/1391/4 "2019-07-23T19:43:32Z")

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> [@zhang-qiang-github](#):
>
> However, when the center point is [100, 255.5 40], the coordinate in 3D volume for the origin of resliced image is [255.5, -255.5000152]. The analytical 3D coordinate of origin should be [255.5 -255.5]. I don’t know why the result is different only when center point is [100 255.5 40]?

You should expect small errors in some results, because floating-point arithmetic has finite precision. However, for `double` the error should be much smaller than what you saw. I think that you found a bug. The VTK python wrappers seem to convert the numbers to `float` when `vtkMatrix4x4.MultiplyPoint()` is called. Because of this, only single-precision is used instead of double-precision. I will file a bug report so that this will be fixed.

For now, please use `vtkMatrix4x4.MultiplyDoublePoint()` instead of `vtkMatrix4x4.MultiplyPoint()`. This will force the use of double-precision.

> [@](#):
>
> Actually, the [location[0]-origin[0], location[1]-origin[1], location[2]-origin[2]] should be [0,0,0], but the result is: `[-1.2e-5, -7.4e-7, 0]`. Is that right?

Use `vtkMatrix4x4.MultiplyDoublePoint()` and the error will be much smaller. But it will not always be zero, because floating-point math is not exact.

> [@](#):
>
> In addition, you said that add the output origin to the pixel index. Is that mean: [x+origin[0], y+origin[1], origin[2]], the z of pixel should be 0?

Yes. If `reslice.SetOutputDimensionality(2)` is used, then the z of the output pixels is always zero. If `reslice.SetOutputDimensionality(3)` is used, then z can be anything.

* * *

I recommend that you try vtkImageReslice without `SetAutoCropOutput(True)` and without `SetOutputDimensionality(2)`. If you do this, you can set the `OutputOrigin` to zero, which will simplify the transformations. The following code shows the necessary changes to the ResliceAxes matrix elements:

```
reslice = vtk.vtkImageReslice() 
reslice.SetInputData(vtkImg) 
#reslice.SetAutoCropOutput(True)
#reslice.SetOutputDimensionality(2) 
reslice.SetInterpolationModeToCubic() 
reslice.SetSlabNumberOfSlices(1) 
reslice.SetOutputSpacing(1.0,1.0,1.0) 
reslice.SetOutputOrigin(0.0,0.0,0.0)
# choose your own output dimensions
reslice.SetOutputExtent(0,511,0,511,0,0)

import math
angle = math.pi/4
x = [math.cos(angle),math.sin(angle),0.0]
y = [-math.sin(angle),math.cos(angle),0.0]
z = [0.0,0.0,1.0]
# the center of the slice in input coordinates
center = [255.5, 255.5, 40.0]
# the center of the slice in output coordinates
ss = [255.5, 255.5, 0.0]
axialElement = [
    x[0], y[0], z[0], center[0] - ss[0]*x[0] - ss[1]*y[0] - ss[2]*z[0],
    x[1], y[1], z[1], center[1] - ss[0]*x[1] - ss[1]*y[1] - ss[2]*z[1],
    x[2], y[2], z[2], center[2] - ss[0]*x[2] - ss[1]*y[2] - ss[2]*z[2],
    0.0, 0.0, 0.0, 1.0
]

```

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