# Extract opacity value at 3d point (in a form of world coordinate) on vtkImageData?

**URL:** https://discourse.vtk.org/t/extract-opacity-value-at-3d-point-in-a-form-of-world-coordinate-on-vtkimagedata/6687
**Category:** Development
**Created:** [September 24, 2021, 8:50am UTC](https://discourse.vtk.org/t/extract-opacity-value-at-3d-point-in-a-form-of-world-coordinate-on-vtkimagedata/6687 "2021-09-24T08:50:35Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![YuezhenQin](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/y/f1d935/32.png) [@YuezhenQin](https://discourse.vtk.org/u/YuezhenQin)
#### Post date: [September 24, 2021, 8:50am UTC](https://discourse.vtk.org/t/extract-opacity-value-at-3d-point-in-a-form-of-world-coordinate-on-vtkimagedata/6687/1 "2021-09-24T08:50:35Z")

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Given that ijk[3] is the world coordinate of a point and xyz[3] is transformed by (connector-\>GetOutput()-\>TransformPhysicalPointToContinuousIndex(ijk[0], ijk[1], ijk[2], xyz);

I have tried the following ways:

1. volume-\>GetProperty()-\>GetScalarOpacity(…);
2. volume-\>GetProperty()-\>GetScalarOpacity()-\>GetValue(…);

I have no idea how to input parameters in these function. Can anyone help me?