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Submitted by Steve Streeting, posted on March 19, 2003

Image Description, by Steve Streeting

This is a shot demonstrating the automated level-of-detail reduction on arbitrary meshes in OGRE. I haven't submitted an IOTD in a long time, but I thought it was about time: I skipped submitting images of our terrain engine plugin(s), because people have seen enough of those I think :)

The automatic mesh reduction algorithm takes account of the curvature of the surface, manifold edges and seams in textures and vertex colours in order to work out the 'least cost' way to reduce the mesh (which it does by collapsing vertices onto each other). You'll see from the images above that flatter areas are simplified more than angular areas, and borders (like the edges of the eyes) are maintained in order to retain the general shape of flat model sections. The result is that if these lower LOD meshes were displayed in the distance, you can barely tell the difference between this and the original model. Broadly speaking the algorithm is along the lines of Garland and Heckbert (SIGGRAPH 97), with some extensions to avoid collapsing manifold edges.

OGRE automatically uses the appropriate LOD mesh at runtime; the distance at which this happens is is largely determined when you generate the LODs, but you can also bias the decision per object and per camera, in order to weight the results appropriately.

If you don't like the automatic LODs, OGRE also allows you to assign manual LODs, including alternative skeletons for animated objects, if you wish.

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