I have a few questions about the way OSG handles depth maps, and how the
range of depth values relates to the near and far calculation done by
OSG.
Firstly, just to confirm my understanding. I have assumed that the
purpose of OSG calculating near&far values is so that depth map values
will range between 0.0 and 1.0 for pixels rendered between the near and
far range. Oh.. and for clipping too!? Please shoot me now if that's not
correct. ;-)
As a test case, I constructed a very simple scene comprised of a large
cuboid with a number of smaller cuboids within it.
When I move the camera inside the large cuboid, I had expected the depth
map rendered by an RTT camera to show depth values of all the smaller
cuboids in the scene, but it doesn't, it just seems to contain a solid
black texture. (you can easily display an inverted representation of the
depth map of a scene using the osgViewer option '--wowvx-20')
Now, the odd thing is that if I change the nearfar calculation mode of
the RTT camera to:
camera->setComputeNearFarMode(osg::CullSettings::ComputeNearFarMode::COMPUTE_NEAR_FAR_USING_PRIMITIVES)
instead of the default, which is COMPUTE_NEAR_FAR_USING_BOUNDING_VOLUMES
,the smaller cubes now appear as I would have expected in the depthmap.
I have been looking for an explantion for this difference in the OSG
code
in:
bool CullVisitor::updateCalculatedNearFar(const osg::Matrix&
matrix,const
osg::Drawable& drawable, bool isBillboard)
It seemed to me that there may be an issue with the code:
---snip---
//if (d_near<0.0) osg::notify(osg::WARN)<<" 3) set near with
d_near="<<d_near<< std::endl;
_computed_znear = d_near;
---snip---
which allows negative values of 'd_near' to be assigned to
'_computed_znear' and hence the near clipping plane to be behind the
camera. In fact, the code a few lines above this snippet does ignore
values of d_near < 0.0, so I wondered if this snippet should have the
same
logic. In fact, if I change the snippet above (by removing the comment
and
putting in an else),
---new snip---
if (d_near<0.0) osg::notify(osg::WARN)<<" 3) set near with
d_near="<<d_near<< std::endl;
else _computed_znear = d_near;
---new snip---
the depth map seems correct for my test example, yippie! Let me know if
I
need to put this change on the submissions list.
Ah ha.... nope.. I haven't finished with your attention yet ;-)
What follows is a different issue, only loosely related to the above:
I can see that I should expect to get a kind of 'popping' effect in a
depth map range due to culling of objects as I move through as scene. It
seemed to me that getting 'smooth' transitions in near/far calculations
would be 'at least in concept' a difficult/impossible problem to solve.
You are probably wondering why I care about this, when a even large
jitters in depth map range does not seem to affect the rendered scene
much. The answer is, because the depth map is used as part of the input
to
a WoWvx autostereoscopic display. The problem is that to transfer the
depthmap data to the stereoscopic display, the depthmap gets rendered as
a
greyscale image, presumably losing a lot of precision in converting to
int
0-255 range greyscale color value. The effect I get as I move through a
scene is that the contrast in the depthmap image seems to fade in and
out
as I move the camera past objects in the scene, and this affects the 3D
effect you get from the WoWvx.
I can see that the WoWvx implementation in OSG does include some
disparity
calculation shader code that allows adjustment of the conversion from
depthmap value to greyscale, but it is difficult to see how to calculate
the parameters 'on the fly' as you move through a scene. Does anyone
have
any suggestions or advice on how dynamically keep more contrast in
greyscale rendering of a depthmap?
Chris.
