I have a 3D floating-point matrix, in worst-case scenario the size could be (200000x1000000x100), I want to visualize this matrix using Qt/OpenGL.
Since the number of elements is extremely high, I want to render them in a way that when the camera is far away from the matrix, I just show a number of interesting points that gives an approximation of how the matrix look like. When the camera gets closer, I want to get more details and hence more elements are calculated.
I would like to know if there are techniques that deals with this kind of visualization.
The general idea is called level-of-detail rendering and is a whole science in itself.
For your domain i would recommend two steps:
1) Reduce the number of cells by averaging (arithmetic-mean function) them in cubes of different sizes and caching those cubes (on disk as well as RAM). “Different” means here, that you have the same data in multiple sizes of cubes, e.g. coarse-grained cubes of 10000x10000x10000 and finer cubes of 100x100x100 cells resulting in multiple levels-of-detail. You have to organize these in a hierarchical structure (the larger ones containing multiple smaller ones) and for this i would recommend an Octree:
http://en.wikipedia.org/wiki/Octree
2) The second step is to actually render parts of this Octree:
To do this use the distance of your camera-point to the sub-cubes. Go through the cubes and decide to either enter the sub-cube or render the larger cube by using this distance-function and heuristically chosen or guessed threshold-values.
(2) can be further optimized but this is optional: To optimize this rendering organize the to-be-rendered cube’s into layers: The direction of the layers (whether it is in x, y, or z-slices) depends on your camera-viewpoint to which it should be near-perpendicular. Then render each slice into a texture and voila you only have to render a single quad with that texture for each slice, 1000 quads are no problem to render.