LOD tech notes

Unreal Geomorph Level-Of-Detail Notes

Erik de Neve - Epic Games, Inc.

Last updated May 19 1999

LOD tech introduction

The Unreal engine has always been able to handle large open spaces quite well, but with large sized scenes, and with multi-player deathmatch and team games as well, there is an increased risk of many actors being visible at the same time.
By using distance-dependent simplification, the speed penalty for rendering actor meshes diminishes greatly as their smaller size farther away allows them to be drawn using fewer polygons.

Unreal's new continuous-LOD (Level-Of-Detail) rendering offers a way to make the rendering of all polygonal actors in the engine scaleable, fully compatible with all existing static and animating mesh content.

Polygons gradually disappear in a sequence which is fixed at content-compilation time (building the .U files) for each mesh. Such a continuous sequence of shapes, known as a 'geomorph', is constructed by automatic evaluation of the shape of a mesh, notably edge sizes and surface curvature.

Unreal's unique smooth LOD morphing ability makes the culling of polygons less conspicuous, notably close up, and is achieved by preemptive shrinkage of mesh faces in an overlapping a-synchronous fashion, while the texture mapping is smoothly adapted also.

Version 224 LOD note for licensees and mod authors

The mesh LOD as active in Unreal 224 does not give the optimal achievable speedup, as it uses very conservative default LOD strength settings. This was necessary to achieve backward compatibility with the entire Unreal 1 content without the team having to go back and check/tweak the settings for individual meshes.
There is one arbitrary hard-coded global LOD factor which licensees can change in UnMeshRnLod.h:
FLOAT MeshVertLOD = 430.f ..........
Useful settings fall between 150 and 450; smaller numbers result in more aggressive LOD culling.The MLOD console command can be helpful for testing adjustment of this global strength interactively.
The optimal tradeoff between speed and detail for individual meshes can be tweaked to satisfaction further by using the mesh LOD script parameters explained below.

Console commands

Mainly for in-game testing purposes; to see the LOD-culling and morphing in action,set MLMODE 3, then zoom in with the sniper rifle (or directly using the FOV command) to see how the distant meshes are simplified, then fly closer in slomo+ghost mode to see it morph into full detail again.
On the UT bots, you can see the backpack appear, an antenna suddenly extends upwards, the microphone snakes its way back around the head, etc.
Click here for some sample screenshots.

MLOD X
A strength scaling factor centered on 1.0, to scale the strength of the LOD culling -
higher values strengthen the LOD fall-off with distance.

MLMODE X
0    Draw everything at full (or at MLFIX-ed) detail.
1    Normal level-of-detail mode
2    Disable smooth morphing.
3    Ignore the field-of-view effect on LOD (great for debugging!)
4    Disable smooth morphing, ignore field-of-view.

MLFIX X
A float in the range 0.0-1.0 which, when in MLMODE 0, causes
all meshes to be drawn using this proportion of their vertices.

Mesh LOD script parameters

Here's a specific script variable example. None of these extra parameters are required.
LOD importing can be on or off by default (depending on how it is hardcoded in UnEdSrv.cpp)

#exec MESH IMPORT MESH=LodMesh ANIVFILE=MODELS\AnyMesh_a.3D DATAFILE=MODELS\AnyMesh_d.3D X=0 Y=0 Z=0 MLOD=1 LODSTYLE=0 LODFRAME=19
#exec MESH LODPARAMS MESH=LodMesh STRENGTH=1.0 MINVERTS=14 MORPH=0.25 ZDISP=90.0

MESH IMPORT parameters:

MLOD	Boolean , meshLOD switch ( if 0, the mesh will always be rendered at full detail ). Always use MLOD=0 if you want to disable any detail loss, rather than putting extreme STRENGTH or MINVERTS values in the LODPARAMS.
LODSTYLE	A bit field with switches to change the style of building the pre-collapse sequence. By default, none of these bits are set. Useful ones right now are 8 and 2 in particular. 1: Value curvature over length. 2: Protect the edges of doublesided polygons - like the CTF flags. 4: Try to respect texture seams more. Use this if there is too much 'stretching' going on. 8: Value length over curvature. Needed for unwelded, multi-element, or otherwise 'open' meshes like the new superhealth. 16: Translucent polygons will be collapsed less early in the LOD sequence. (Broken in the 224 code base.)
LODFRAME	When there are multiple animation frames, you can pick a representative one other than the default 0th frame to be sampled for generating the collapse sequence.
LODNOTEX	If set, causes texture coordinates to be ignored when importing. Results in modest memory and performance savings for complex environment-mapped meshes.
LODOLD	Maintain the original order of animation vertices as they are imported. If active, this means some internal reorganization will be done at load time, instead of compile time. Included to make differential patching of older content feasible.

MESH LODPARAMS parameters:

STRENGTH	Per-mesh linear LOD scaler. Default = 1.0. Tweak this higher for more aggressive LOD-culling, lower if you see the mesh distort too early .
MINVERTS	The minimum number of vertices you want your mesh to always retain at the farthest distances. Useful for some really simple objects (boxes, trees, birds...) that need to look good even at a fair distance.
MORPH	0.0 for no morphing, any other value up to 1.0 indicates the proportion of vertices that are allowed to contribute to the smooth morphing. More means less popping, but at the cost of more CPU time. Useful values are 0.25 - 0.4, the default is currently 0.3.
ZDISP	In world units ( the standard player height is 60 ) this pushes away the distance at which the LOD culling kicks in. More precisely, the fraction of vertices drawn scales with the distance Z as follows: 1 / ( STRENGTH * (Z - ZDISP) ), where (Z-ZDISP) is clamped at a minimum of 1.0.