I need a simple and fast way to find out how big a 3D bounding box appears on screen (for LOD calculation) by using OpenGL Modelview and Projection matrices and the OpenGL Viewport dimensions.
My first intention is to project all 8 box corners on screen by using gluProject() and calculate the area of the convex hull afterwards. This solution works only with bounding boxes that are fully within the view frustum.
But how can a get the covered area on screen for boxes that are not fully within the viewing volume? Imaging a box where 7 corners are behind the near plane and only one corner is in front of the near plane and thus within the view frustum.
I have found another very similar question Screen Projection and Culling united but it does not cover my problem.
what about using queries and get samples that passes rendering?
http://www.opengl.org/wiki/Query_Object and see GL_SAMPLES_PASSED,
that way you could measure how many fragments are rendered and compare it for proper LOD selection.