[vtkusers] FW: ResetClippingpLane

[John Platt]

-----Original Message-----
From: James C Robinson [mailto:j.robinson at kepler-systems.com] 
Sent: 11 May 2005 14:39
To: 'John Platt'
Subject: ResetClippingpLane

 

Dear John,

 

I can't seem to send the message below to the group (something is
blocking it as spam?).

 

Could you send it on if possible (and any comments welcomed).

 

Regards,

 

Jim

 

 

Dear All,

 

I have been having a problem with the AutoResetCameraClippingRange
facility, in that when viewing objects of certain dimension, zooming in
causes detail to be lost when it is too close to the camera. This is, of
course, deliberate behaviour, in that objects that are between the
camera and the near clipping plane are not rendered. However, the issue
is with the distance of the near clipping plane from the camera (when in
auto mode). It is set to 0.01 times (1%) the distance of the far
clipping plane (which is related to the perpendicular distance of the
farthest bounding box vertex from the camera..). The reason for this is
related to Z buffering and the quality of images when the near clipping
plane is too close to the camera (I think - pure layman's understanding
- that it may be to do with integer maths etc.). Anyway, on
investigating deeper into the VTK code for void
vtkRenderer::ResetCameraClippingRange I see what looks like an error...

 

  // Make sure near is at least some fraction of far - this prevents
near

  // from being behind the camera or too close in front. How close is
too

  // close depends on the resolution of the depth buffer

  int ZBufferDepth = 16;

  if (this->RenderWindow)

    {

      ZBufferDepth = this->RenderWindow->GetDepthBufferSize();

    }

  //

  if ( ZBufferDepth <= 16 )

    {

    range[0] = (range[0] < 0.01*range[1])?(0.01*range[1]):(range[0]);

    }

  else if ( ZBufferDepth <= 24 )

    {

    range[0] = (range[0] < 0.01*range[1])?(0.01*range[1]):(range[0]);

    }

  else

    {

    range[0] = (range[0] < 0.01*range[1])?(0.01*range[1]):(range[0]);

    }

The error is that (I may be misreading the code in my haste to find a
solution) range[0] will be reset to 1% range[1] regardless of the value
of ZBufferDepth. I would have thought that (a) if this is the case,
there is no need for these 10 lines and (b) the larger z buffer depth
(number of bits in the z buffer - usually 24 nowadays?) would mean that
the near plane could be significantly closer to the camera? Say 0.01% -
which would suit my needs as I am viewing objects with a dimension of up
to 3000 units and I would like to be able to see detail in objects
within as close as 0.3 units.

 

 

Please advise.

 

Regards,

 

Jim

 

 

James C Robinson, 

*    42 Rivergrove,

         Glanmire,

         Co. Cork,

         Eire

'     +353 21 4822028

         +353 87 2393010

*    jrobinson at eircom.net> 

  _____  

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.vtk.org/pipermail/vtkusers/attachments/20050511/b20fb059/attachment.htm>