[vtkusers] My investigation results on the three vtkOpenGLGPUVolumeRayCastMapper bugs. Please read!
AGPX
agpxnet at yahoo.it
Thu Aug 12 08:54:28 EDT 2010
Not fully closed. The third bug still alive! I'm working on it... :)
________________________________
Da: Dave Partyka <dave.partyka at kitware.com>
A: AGPX <agpxnet at yahoo.it>
Cc: vtk vtk <vtkusers at vtk.org>
Inviato: Gio 12 agosto 2010, 14:51:47
Oggetto: Re: [vtkusers] My investigation results on the three
vtkOpenGLGPUVolumeRayCastMapper bugs. Please read!
Wow! Thank you for the bug fixes! Can you send the changes as a git patch. See
here for how to create one.
http://vtk.org/Wiki/VTK/Git#Submitting_a_patch
Also can you provide the bug tracker numbers so we can mark them as closed.
Thanks!
On Thu, Aug 12, 2010 at 5:29 AM, AGPX <agpxnet at yahoo.it> wrote:
Hi to all,
>
>I have the time to address the 3 ancients bugs that affect the mapper
>vtkOpenGLGPUVolumeRayCastMapper, discovered and signaled on mantis bugtracer by
>me on February 2009 (bugs never solved). These bugs make the mapper quite
>unusable and this is a serious issue because that mapper is the best compromise
>between quality and speed for volume rendering.
>OK, we have 3 different issue. Let me talk about the three.
>
>1) Plane clipping bugs (to better figure out the problem, here a video of the
>problem: http://www.youtube.com/watch?v=NjUTTA86yTM&feature=related). I have
>solved this bug. The problem is in the method:
>vtkOpenGLGPUVolumeRayCastMapper::RenderClippedBoundingBox. That method check the
>correct winding order of the polygon and fails its job. Actually the algorithm
>check the relationship between the polygon plane and the SPATIAL center of the
>volume. Note that the volume is always convex. The problem is that you haven't
>to use spatial center, but center of mass. If you use spatial center (center of
>bounding box) you wrong because it not always lie inside the convex polyhedra:
>it could lie on a polygon boundary plane. Let me show this in 2D:
>
> __________
>| /
>| /
>| /
>| /
>| x
>| /
>| /
>| /
>| /
>|/
>
>If you have a triangle, the spatial center lie on an edge and due to floating
>point inaccuracy this point could lie in front or behind the plane. When point
>lie behind the plane the algorithm flip the vertex order resulting in back-faced
>polygon. The solution is to compute center of mass (average of vertices):
>
> __________
>| /
>| /
>| /
>| /
>| x /
>| /
>| /
>| /
>| /
>|/
>
>The center of mass always lie inside the convex polyhedra and this solve the
>problem.
>
>Here my fix (in vtkOpenGLGPUVolumeRayCastMapper::RenderClippedBoundingBox):
>
> double center[3] = {0,0,0};
> double min[3] = {VTK_DOUBLE_MAX, VTK_DOUBLE_MAX, VTK_DOUBLE_MAX};
> double max[3] = {VTK_DOUBLE_MIN, VTK_DOUBLE_MIN, VTK_DOUBLE_MIN};
>
> // First compute center point
> npts = points->GetNumberOfPoints();
> for ( i = 0; i < npts; i++ )
> {
> double pt[3];
> points->GetPoint( i, pt );
> for ( j = 0; j < 3; j++ )
> {
> // AGPX MODIFIED
>// min[j] = (pt[j]<min[j])?(pt[j]):(min[j]);
>// max[j] = (pt[j]>max[j])?(pt[j]):(max[j]);
> center[j] += pt[j];
> // AGPX MODIFIED
> }
> }
>
> // AGPX MODIFIED
> //center[0] = 0.5*(min[0]+max[0]);
> //center[1] = 0.5*(min[1]+max[1]);
> //center[2] = 0.5*(min[2]+max[2]);
> center[0] /= ((double)npts);
> center[1] /= ((double)npts);
> center[2] /= ((double)npts);
> // AGPX MODIFIED
>
>This completely solve this issue. Now go to the next bug.
>
>2) "Volume disappearing". Suddenly, if you fly-through inside a volume, it can
>disappear. The problem is related to the near clipping plane. Actually the
>method vtkOpenGLGPUVolumeRayCastMapper::ClipBoundingBox, clip the volume box
>with the near clipping plane (and eventually also with custom clip planes) in
>order to avoid that the volume results opened. The problem is that due to
>floating point inaccuracy the resulting clipped convex polyhedra could be still
>opened! The problem could be solved pushing the near clipping plane slightly
>ahead. That is:
>
>if(this->NearPlane==0)
> {
> this->NearPlane= vtkPlane::New();
> }
>
> // AGPX ADDED
> const double offset = 0.01;
> camNearPoint[0] += camPlaneNormal[0] * offset;
> camNearPoint[1] += camPlaneNormal[1] * offset;
> camNearPoint[2] += camPlaneNormal[2] * offset;
> // AGPX ADDED
>
> this->NearPlane->SetOrigin( camNearPoint );
> this->NearPlane->SetNormal( camPlaneNormal );
> this->Planes->AddItem(this->NearPlane);
>
>The problem here is to establish how much you have to push plane ahead. 0.01 is
>working for me. Ok, following me to the last (hard) bug.
>
>3) Volume Rendering Deformation. This bug show a non correct rendering of the
>volume when you perform a fly-through inside a volume (here
>http://www.youtube.com/watch?gl=IT&hl=it&v=C_GA6UHbUGo you can see a video of
>the problem). Actually this bug is unsolved and here I need some help from
>authors. Look like a bad entry-ray and/or ray-direction calculation. The problem
>is showed only when the near clipping plane clips the volume box. The entry
>point is given by the texture coordinate of the point being rasterized (a 3D
>texture coordinate is applied to the vertices of the rendered volume box). This
>look like correct. The ray direction is given by the difference between the
>entry point and the camera position in texture space. This should be always ok.
>The method vtkOpenGLGPUVolumeRayCastMapper::LoadProjectionParameters computed
>the camera position in texture space and looks correct (notice that I'm always
>talking about a perspective camera):
>
>double *bounds=this->CurrentScalar->GetLoadedBounds();
>
> double dx=bounds[1]-bounds[0];
> double dy=bounds[3]-bounds[2];
> double dz=bounds[5]-bounds[4];
>
>
> // Perspective projection
>
> // Compute camera position in texture coordinates
> // Position of the center of the camera in world frame
> double cameraPosWorld[4];
> // Position of the center of the camera in the dataset frame
> // (the transform of the volume is taken into account)
> double cameraPosDataset[4];
> // Position of the center of the camera in the texture frame
> // the coordinates are translated and rescaled
> double cameraPosTexture[4];
>
> ren->GetActiveCamera()->GetPosition(cameraPosWorld);
> cameraPosWorld[3]=1.0; // we use homogeneous coordinates.
>
> datasetToWorld->MultiplyPoint(cameraPosWorld,cameraPosDataset);
>
> // From homogeneous to cartesian coordinates.
> if(cameraPosDataset[3]!=1.0)
> {
> double ratio=1/cameraPosDataset[3];
> cameraPosDataset[0]*=ratio;
> cameraPosDataset[1]*=ratio;
> cameraPosDataset[2]*=ratio;
> }
>
> cameraPosTexture[0] = (cameraPosDataset[0]-bounds[0])/dx;
> cameraPosTexture[1] = (cameraPosDataset[1]-bounds[2])/dy;
> cameraPosTexture[2] = (cameraPosDataset[2]-bounds[4])/dz;
>
>Ray direction is computed by the shader
>(vtkGPUVolumeRayCastMapper_PerspectiveProjectionFS.glsl):
>
>// Entry position (global scope)
>vec3 pos;
>// Incremental vector in texture space (global scope)
>vec3 rayDir;
>// Camera position in texture space
>uniform vec3 cameraPosition;
>
>// Defined in the right projection method.
>void incrementalRayDirection()
>{
> // Direction of the ray in texture space, not normalized.
> rayDir=pos-cameraPosition;
> ...
>}
>
>and looks correct.
>
>The entry point seems correct, because if I use a totally opaque volume (in
>order to avoid ray traversal, that is the algorithm stop immediately on the
>first hitted voxel) and clipped with a plane to enter inside it, the rendering
>is always ok. That is entry point looks correct. When the volume isn't fully
>opaque the result is wrong. So, I suspect that the ray direction is wrong. Note
>that the problem disappear (or at least is not much noticeable) IF I try push
>the near clipping plane ahead of a greater quantity (>> 0.01). If I push it
>ahead of 16.0 instead of 0.01 (as stated in the previous bug), the deformation
>is not very noticeable. Note that 16.0 is an empirically determinated value for
>my tested datasets and reasonably it's not a valid choice for every dataset (a
>way to determine a good value?). It's not clear why the near clipping plane
>cause this problem. The texture coordinate seems to be interpolated correctly
>and apparently there are no reason why the method should not works. Here I'm
>stuck and I need some help from some CG Guru! Please support!
>
>Thanks in advance for your attention.
>
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