[osg-users] Volume Rendering and Depth Buffer

[Chris Hanson]

Just out of curiousity, what are you volume rendering and what are your
requirements? We did some interesting volume rendering work a few years ago
for Iowa State University's Ames Lab.

On Mon, Apr 15, 2019 at 1:08 PM Anna Osvin <annieloveowls at gmail.com> wrote:

> Hi. In a project that I'm working on I have an osgVolume::VolumeScene
> containing one volume model and some polygonal models. I need to do some
> depth buffer checks on that scene, so I've attached depth buffer to viewer
> camera. When I fetch osg::Image representing depth buffer content,
> everything is fine, except background values are not calculated properly
> (they should be 1, but instead it's ~0.15). And when I add osg::Depth(
> osg::Depth::Function::LESS, zNear, zFar ) attribute to the camera,
> background values are calculated correctly, but I lose all of the polygonal
> models data (they are no longer visible, and not presented in depth buffer).
>
> Here is function that does depth buffer checking:
>
> Code:
>
> bool pickZBufferIntersection( osgViewer::View& view, bool
> perspectiveProjection, const osg::Vec2& point2d, osg::Vec3& pickedPoint )
> {
>     osg::Camera* camera = view.getCamera();
>     if ( camera == nullptr )
>     {
>         Q_ASSERT_X( false, "bool pickZBufferIntersection( ... )", "View
> has no camera." );
>         return false;
>     }
>
>     const osg::Image* zImage =
> camera->getBufferAttachmentMap()[osg::Camera::BufferComponent::DEPTH_BUFFER]._image;
>     if ( zImage == nullptr )
>     {
>         Q_ASSERT_X( false, "bool pickZBufferIntersection( ... )", "ZBuffer
> is not attached to view camera." );
>         return false;
>     }
>
>     osg::Vec3f cameraPos;
>     osg::Vec3f center;
>     osg::Vec3f up;
>     camera->getViewMatrixAsLookAt( cameraPos, center, up );
>
>     osg::Matrixd prInv = osg::Matrixd::inverse(
> camera->getProjectionMatrix() );
>     osg::Matrixd viewInv = osg::Matrixd::inverse( camera->getViewMatrix()
> );
>
>     const osg::Viewport* viewPort = camera->getViewport();
>
>     int x = point2d.x() - viewPort->x();
>     int y = point2d.y() - viewPort->y();
>
>     osg::Vec3 s(0, 0, -1);
>     s[0] = (point2d[0] / viewPort->width()) * 2.0 - 1.0;
>     s[1] = (point2d[1] / viewPort->height()) * 2.0 - 1.0;
>     s = s * prInv * viewInv;
>
>     osg::Vec3d v = s - cameraPos;
>
>     double zNear = 1.0f;
>     double zFar = 10000.0f;
>
>     static uint pickNumber = 0;
>
>     double zV, z_n, z;
>     osg::Vec3f point;
>
>     zV = ( (float*)zImage->data( x, y ) )[ 0 ];
>
>     if( perspectiveProjection )
>     {
>         z_n = 2.0*zV - 1.0;
>         z = 2.0 * zNear * zFar / ( zFar + zNear - z_n * ( zFar - zNear ) );
>
>         point = cameraPos + v*z;
>     }
>     else
>     {
>         z = ( zFar - zNear )*zV;
>         point = s + ( center - cameraPos )*z;
>     }
>
>     if( z > zNear + 1e-8 && z < zFar - 1 - 1e-8 )
>     {
>         pickedPoint = point;
>         return true;
>     }
>     return false;
> }
>
>
>
>
> And here is viewer setup bit:
>
> Code:
>
> QWidget* createViewWidget(osgQt::GraphicsWindowQt* gw,
> osgVolume::VolumeScene& scene)
> {
>     osgViewer::View* view = new osgViewer::View;
>     /* ... */
>     osg::Camera* camera = view->getCamera();
>     camera->setGraphicsContext(gw);
>
> camera->setComputeNearFarMode(osg::CullSettings::DO_NOT_COMPUTE_NEAR_FAR);
>
>     const osg::GraphicsContext::Traits* traits = gw->getTraits();
>     camera->setClearColor(osgColor( SCENE3D_BACKGROUND_COLOR ));
>     camera->setViewport(new osg::Viewport(0, 0, traits->width,
> traits->height));
>
>     osg::ref_ptr<osg::Image> zImage = new osg::Image();
>     zImage->allocateImage(1000, 1000, 1, GL_DEPTH_COMPONENT, GL_FLOAT);
>     camera->attach(osg::Camera::DEPTH_BUFFER, zImage);
>
>     const int orthoRange = 120;
>     const float zNear = 1.0f;
>     const float zFar = 10000.0f;
>     camera->setProjectionMatrixAsOrtho(-orthoRange, orthoRange,
> -orthoRange, orthoRange, zNear, zFar);
>     //camera->getOrCreateStateSet()->setAttribute( new osg::Depth(
> osg::Depth::Function::LESS, zNear, zFar ), osg::StateAttribute::ON |
> osg::StateAttribute::OVERRIDE );
>
>     /* ... */
>
>     view->setSceneData(&scene);
>     view->addEventHandler(new osgViewer::StatsHandler);
>
>     gw->setTouchEventsEnabled(true);
>
>     return gw->getGLWidget();
> }
>
>
>
>
> ...
>
> Thank you!
>
> Cheers,
> Annie[/img]
>
> ------------------
> Read this topic online here:
> http://forum.openscenegraph.org/viewtopic.php?p=75829#75829
>
>
>
>
> Attachments:
> http://forum.openscenegraph.org//files/depth_buffer_depthrange_185.png
> http://forum.openscenegraph.org//files/depth_buffer_154.png
>
>
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>


-- 
Chris 'Xenon' Hanson, omo sanza lettere. Xenon at AlphaPixel.com
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