[vtk-developers] Depth buffer and near plane clipping

[Biddiscombe, John A.]

Paul

I think the main reason is that the calculation for depth is done by Val/NearPlaneDist - when the near plane is very close to zero, the accuracy of the overall Z depth starts to fall apart. [you need (far-near)/near bins of Z and only 32 bits to store them in - large far and small near causes trouble] - my memory is failing and I expect you'll get a better answer once someone reads this and gasps in horror at what I've written.

Having said that, if you are having trouble with near objects being clipped and have a better algorithm to throw away far ones, then implement it, add an option to turn it on and contribute it as I'm sure others will like to try it.
NB. I didn't read the bug report carefully - but it looks like a fix for parallel projection mode is  there - I wonder why it hasn't been looked at in a year?

JB




From: vtk-developers-bounces at vtk.org [mailto:vtk-developers-bounces at vtk.org] On Behalf Of Paul Harris
Sent: 03 November 2009 07:51
To: vtk-developers at vtk.org
Subject: [vtk-developers] Depth buffer and near plane clipping

Hi all,

Could someone please explain to me this bit of code from vtkRenderer.cxx, about line 1162

  if (range[0] < this->NearClippingPlaneTolerance*range[1])
    {
    range[0] = this->NearClippingPlaneTolerance*range[1];
    }

I've been thinking about it, in relation to this bug report:
http://public.kitware.com/Bug/view.php?id=7823

I'm interested in WHY the decision has been made to "push" the near-field clipping plane out further when the depth range is too broad.
Why don't we bring the far-field clipping plane back towards us, and clip the stuff that is a long way away?
If we do that, then what should range[1] be reset to when range[0] is zero ?  (ie we have objects at or behind the camera, I would expect to see all the objects immediately in front of the camera).

I find it annoying when you have an object close to the camera (but not behind the camera) that is clipped when you also have object a long long way away.  My focus is on the near-field object, not the far-field object, yet it is the one that is 'punished'.

The Tolerance for 16-bit depth buffers is set to 0.001, which seems arbitrary to me.  Is this a value based on experience rather than calculated?

thanks,
Paul
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