Level of detail and geometries merger

The Level Of Detail avoids to display a far object with lot of details. In fact, if an object is far, we can't see the complexity of the object. So the aim of the LOD methods is to decrease the complexity of the object depending of the distance on the point of view.

A model with two resolutions. The studs are composed of 16 quads or 64 quads.
A model with two resolutions. The studs are composed of 16 quads or 64 quads.

Ldraw uses a basic LOD: many objects may use two different resolutions (it concerns cylinders and curved surfaces). On the picture, meshes of stud are more detailed in the second case. The LDraw format file has been created before the 3D evolution in computer sciences, so it considers non smoothed primitive. With this technical limitation, if you want great visual result, you need to increase the number for primitives. So using smoothed surface may resolve the using of detailed meshes. Next, the Ldraw format is a tree structure. Each file contains an average of 15 primitives (quads, triangles, lines) and a list of links to other files. This structure is convenient if you have memory limitation but today graphical cards prefer big meshs than lot of little parts of an object. The program wastes time during tree traversal, transformation selection and culling. The best way is certainly to merge the object whose the memory size is relatively small. So the culling time is insignificant and the global time is lower even with a high resolution.

These screenshot show the computation time to display the same object. The top left image corresponds to the low resolution. At the top-right, the merged low resolution. At the bottom-left, the high resolution. At the bottom-right, the merged high resolution case.
These screenshot show the computation time to display the same object. The top left image corresponds to the low resolution. At the top-right, the merged low resolution. At the bottom-left, the high resolution. At the bottom-right, the merged high resolution case.

The screen-shots show that we obtain speed-up of 25 and 33 for respectively the low and high resolution.

  • low resolution non-merged: 10.4 + 4.5 = 14.9ms
  • low resolution merged: 0.2 + 0.4 = 0.6ms
  • high resolution non-merged: 29 + 14.2 = 43.2ms
  • high resolution merged: 0.2 + 1.1 = 1.3ms

Three points needs to be implemented:

  • LDraw format proposes two type of line (line and optional line) which are merged. These type of line need to be splitted to use them correctly.
  • The second point concern the color, it exists two type of objects: the monochrome parts and the part with motifs (patern). Commonly, monochrome part use the color 16 for each primitive. So color mechanism don't need to be implemented as a color vector. But for patern part, object should be splitted to separate monochrome color and motif.
  • The third point, concern the propagation of the color during the merge. An color indexes vector exists but we don't use it because there are difficulty which the color 16. During the merge, the color 16 may needs to be replace by an other color and I'm sceptique about the interrest to use indexes vectors to group colors.
  • From the application point of view, we need to create a mechanism to exclude a part of the object during the merge. Many times, we will need to separated parts of an object (for example: a door need to be be separate from an house to allows interaction with the door).

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