LOD

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A Level of Detail (LOD) system can be used to bring more details to the screen while still speeding up rendering. See Wikipedia for an introduction.

Contents

The new LOD System - Overview

TheDarkMod LOD System Overview.png

SVG version of the above graphic here.

Note: Fade in and fade out do only work on entities via alpha-fading (e.g. with alphatest materials and an alpha-channel in the texture that contains a range from 0..1.0). This is due to a deficit in the D3 engine and can only be fixed after D3 has gone open source.

How to use LOD in your TDM map

To enable the LOD system, use either one of the pre-made LOD-enabled entities in your map (most of these are defined in def/tdm_lod.def) or set dist_check_period to a number greater than 0.

Note: The dist_check_period spawnarg needs to be combined with one of "hide_distance" or "lod_X_distance" and "model_lod_X", "skin_lod_X" or "noshadows_lod_X" to have any effect!

See also LOD Bias for how to hide individual entities depending on the menu setting.

GUI Setting

There is a new GUI setting, called Object detail . You'll find it under Settings - Video - Advanced. It consist of multiple levels, where Normal is the default. Lower settings cause the LOD distances to shrink, meaning entities switch sooner to the low-quality version and vanish sooner. Higher settings are the opposite, increasing the quality, but may be decreasing the FPS.

Mappers should design and test their map with the Normal setting on a typical modern PC (e.g. not too old and not high-end gaming rig either), and preferable also test on some slower and faster machines.

Entities

Example entities:

  • atdm:nature_pine_skeleton (just the tree trunk)
  • atdm:nature_pine (Pine + particle leaves slowly swaying)
  • atdm:nature_tree_stump

More can be found in the "Nature" folder with the DarkRadiant "Create entity" command.

Spawnargs

min_lod_bias/max_lod_bias

See LOD Bias for how to hide individual entities depending on the menu setting.

hide_distance

In D3 units. When set to a value > 0, then the entity will be hidden if it is further away than this distance from the player.

Note: This might cause "popping", e.g. the entity will suddenly vanish or appear. See lod_fadeout_range for how to avoid this.

The actual value used from this spawnarg is modified by the menu setting "Object detail", so make to sure to test with this setting at "Normal". Also see min_show_distance:

lod_normal_distance

From TDM v1.05 onwards: This spawnarg defines a minimum distance from the player where the entity will ignore settings lower than Normal. This can be used to enforce a certain minimum distance so that players who set the detail level to Low or Lowest still do not experience entities popping in and out when being very close.

If this spawnarg is not set, it defaults to 500 units. Avoid setting this spawnarg too high or even equal or higher than "hide_distance". This would only result in the menu setting "Object detail" being fixed at "Normal", e.g. players could never lower the quality settings to achive better frame rates. Do so only in rare cases where important entities would otherwise vanish in front of the eyes of the player.

Setting min_show_distance to very low values (e.g. 100 units) is ok for things that really should vanish close, like grass or small decals. Setting it to f.i. to 1000 for a big statue can avoid popping.

lod_hide_probability

If lod_hide_distance is true, this spawnarg controls what the probabiliy of this entity vanishing is. Values range from 0.0 (will never vanish) to 1.0 (will certainly vanish). Whether a particular entity vanishes is computed at map start, so an entity either stays or vanishes, but does not change during the map play time.

Example:

"lod_hide_probability" "0.2"

If you place X identical entities, about 20% of them will vanish at the hide_distance and 80% will stay.

This can be used to "thin out" forests at a distance without having them vanish completely.

lod_X_distance

In D3 units. Sets the distance from where on LOD X is used. Will be valid until the next defined lod_(X+1)_distance, or infinty if there is no further LOD distance set.

To change the appearance of the entity inside this LOD, use the following spawnargs with a matching X:

model_lod_X

Set the model the entity displays inside this LOD. If not set, uses the model from LOD 0 e.g. the one from the "model" spawnarg.

skin_lod_X

Set the skin the entity displays uses this LOD. If not set, uses the skin from LOD 0 e.g. the one from the "skin" spawnarg.

noshadows_lod_X

Boolean. If set, the entity casts no shadows while in this LOD zone.

lod_fadein_range

Note: Fade in and fade out do only qwork via alpha-fading. This can only be fixed after D3 has gone open source.

In D3 units. If set to non-zero, then the object will start hidden and begin to fade in at lod_1_distance - lod_fadein_range and be completely (100% opacity) visible at lod_1_distance. This can be used to create entities that only appear in some distance but not close up.

lod_fadeout_range

Note: Fade in and fade out do only qwork via alpha-fading. This can only be fixed after D3 has gone open source.

In D3 units. If set to non-zero, then the object will not vanish suddenly at hide_distance, but start to fade out and be completely invisible at hide_distance + lod_fadeoutrange. Defaults to 0.

Increasing the LOD steps

The number of possible LOD steps is fixed in code and defined in game/misc.h as a constant. The current value is 7, meaning that there are 5 different rendering possibilities beside level 0 (the original) and level 6 (the object is hidden). That should be enough even for huge outdoor maps.

Performance

Profiling showed that 3000 thinking entities consume about 7% CPU time. So if you use only a few hundred or thousand LOD entities, the performance of the extra thinking time is very small compared to the benefits of having only a few entities actually visible around the player.

History

Doom3 did not have any LOD system at all. The first steps of such a system were added in TDM by supporting a hide_distance spawnarg, which made entities hide or portals close once they were further away than this distance. The LOD system in TDM was totally rewritten for v1.04.

Comparison

Here we compare the new (v1.04) to the old system (until v1.02):

  • The old system had only two levels: 0 (original model), and 1 (entity is hidden)
  • Due to a typo, all thinking for the LOD entities happened in the first half of the interval. That means if you set the intervall to 1 s, all entities did their distance check between 0 and 0.5 seconds, and none between 0.5 and 1 second and so on. This lead to fluctuating frame rates.

The new system:

  • Supports up to 7 levels (last is hide), more can easily be done with a recompile
  • You can use less levels if you want
  • Thinking is distributed over the entire interval
  • makes scenes with thousands of entities possible
  • Can not only switch the model, but also change the skin (f.i. use less expensive glass shaders) and turn shadows off
  • Can hide only a fraction of all entities of the same class (thinning out)
  • Allows the player (via the settings menu) to control the quality vs. speed

Note: The new system still works only for func_static (e.g. not for lights, AI etc)!

See also

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