# vtkAppendPolyData

**URL:** https://discourse.vtk.org/t/vtkappendpolydata/1495
**Category:** Development
**Created:** [August 6, 2019, 12:24pm UTC](https://discourse.vtk.org/t/vtkappendpolydata/1495 "2019-08-06T12:24:55Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![at123](https://discourse.vtk.org/user_avatar/discourse.vtk.org/at123/32/748_2.png) [@at123](https://discourse.vtk.org/u/at123)
#### Post date: [August 6, 2019, 12:24pm UTC](https://discourse.vtk.org/t/vtkappendpolydata/1495/1 "2019-08-06T12:24:55Z")

</div>

Hi,

I am inputting multiple contours into a vtkAppendPolyData object, does it matter what order I add these contours? I want to create a convex hull. Say from left to right, I create a large circle, small circle, and large circle. I would like to form a convex hull that looks like a hourglass. I tried using vtkDelaunay3D to render it, but I ended up getting a hull that looks like a cylinder.

Any help would be appreciated. Thanks!

---

<div class="post-metadata">

### Author: ![lassoan](https://discourse.vtk.org/user_avatar/discourse.vtk.org/lassoan/32/50_2.png) [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [August 6, 2019, 12:51pm UTC](https://discourse.vtk.org/t/vtkappendpolydata/1495/2 "2019-08-06T12:51:39Z")

</div>

> [@at123](#):
>
> I would like to form a convex hull that looks like a hourglass.

This is impossible because convex hull of an hourglass shape is a cylinder. You might be able to reconstruct hourglass shape from points using vrkDelaunay3D using non-zero alpha parameter or using other methods described in this post:

> [@Problems using vtkContourWidget, vtkAppendPolyData, and vtkDelaunay3D](https://discourse.vtk.org/t/problems-using-vtkcontourwidget-vtkappendpolydata-and-vtkdelaunay3d/1480/2):
>
> It seems that a convex hull is generated, which is what vtkDelaunay3D is expected to do. You can adjust alpha parameters on the vtkDelaunay3D filter, to make it generate concave objects, but it may make mesh generation fail. Alternatively, if the shape is just slightly concave then you may get away without using alpha shapes and applying vtkButterflySubdivisionFilter on the output. You can try both these options in 3D Slicer’s Markups To Model module (install 3D Slicer and then in the Slicer …