[vtkusers] UTM coordinates, double precision, structured and unstructured grids

[Aashish Chaudhary]

Hi,

This is a problem with graphics board using single precision for
vertices positions. OpenGL uses floats as the data type for rendering
(positions, texture coordinates) and if your dataset is far far away
from origin (0,0,0) it loses precision and hence you see vertices jump
around. There are various ways to solve this issue. If applying offset
works for you then I would stick with it. This is a known problem when
dealing with UTM coordinates.

This is commonly known as jitter (if you search with this keyword you
will find many references).

Thanks


On Wed, Jul 11, 2012 at 6:35 PM, Richard R McDonald <rmcd at usgs.gov> wrote:
> Hello,
>
> We have an application that provides pre- and post- processing capabilities
> for computational models of flow and sediment transport in Rivers.  The
> application is built with Qt and VTK.  We use both structured and
> unstructured grids.  The grid x,y locations, stored in vtkPoints and set to
> datatype double, are specified in UTM coordinates. When zooming in close and
> rotating the the grid, the point locations jump around as if truncated to
> single precision.  Using the same data sets with a subtracted offset to
> bring the x,y locations down to single precision eliminates the problem.
> Can anyone lend some insight as to why this might be happening?  Is there a
> truncation issue in VTK or is it a product of the graphics board?  Any help
> would be greatly appreciated.
>
> Thank you,
> Rich McDonald
> Research Hydrologist
> USGS Geomorphology and Sediment Transport Laboratory
> 4620 Technology Drive, Suite 400
> Golden, CO 80403
> 303 278 7952
> rmcd at usgs.gov
>
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-- 
| Aashish Chaudhary
| R&D Engineer
| Kitware Inc.
| www.kitware.com