Difference between revisions of "Fake contrast"

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The system, however, is not perfect. It only works for orthogonal geometry, even though getting free of the orthogonality constraint present in older raycaster engines (such as [[Wolfenstein 3D]]) was one of the main points of the Doom engine. The relatively steep difference detracts from it as well, and has even been perceived as a bug.
 
The system, however, is not perfect. It only works for orthogonal geometry, even though getting free of the orthogonality constraint present in older raycaster engines (such as [[Wolfenstein 3D]]) was one of the main points of the Doom engine. The relatively steep difference detracts from it as well, and has even been perceived as a bug.
  
While [[Heretic]] and [[Strife]] retain this feature, [[Hexen]] disabled it entirely. Amongst [[source port]]s, [[Doomsday]], the [[Eternity Engine]], and [[ZDoom]] feature options to completely disable fake contrast or to make it gradually change with wall orientation.
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While [[Heretic]] and [[Strife]] retain this feature, [[Hexen]] disables it entirely. Amongst [[source port]]s, [[Doomsday]], the [[Eternity Engine]], and [[ZDoom]] feature options to completely disable fake contrast or to make it gradually change with wall orientation.
  
 
== Technical details ==
 
== Technical details ==

Revision as of 12:03, 26 July 2014

Screenshot from the beginning of E2M2: Containment Area (Doom): the crate maze in this level is a good example of how fake contrast enhances the appearance of the game.
Same scene from E2M2 taken in a hacked engine with fake contrast disabled.

Fake contrast is a feature of the original vanilla Doom engine, which consists in making walls oriented parallel to the East-West axis brighter, while walls parallel to the North-South axis are darker. The aim of fake contrast is to help accentuate the angles in the map's geometry, because with the simple lighting system of the Doom engine (ambient omnidirectional light, no shadows) and the low-resolution paletted textures, the angles could seem flat in rooms.

The system, however, is not perfect. It only works for orthogonal geometry, even though getting free of the orthogonality constraint present in older raycaster engines (such as Wolfenstein 3D) was one of the main points of the Doom engine. The relatively steep difference detracts from it as well, and has even been perceived as a bug.

While Heretic and Strife retain this feature, Hexen disables it entirely. Amongst source ports, Doomsday, the Eternity Engine, and ZDoom feature options to completely disable fake contrast or to make it gradually change with wall orientation.

Technical details

In vanilla, it corresponds to a part of the functions R_RenderMaskedSegRange and R_StoreWallRange in r_segs.c, specifically code segments like this: <syntaxhighlight lang="C">

   // Calculate light table.
   // Use different light tables
   //   for horizontal / vertical / diagonal. Diagonal?
   // OPTIMIZE: get rid of LIGHTSEGSHIFT globally
   curline = ds->curline;
   frontsector = curline->frontsector;
   backsector = curline->backsector;
   texnum = texturetranslation[curline->sidedef->midtexture];
   lightnum = (frontsector->lightlevel >> LIGHTSEGSHIFT)+extralight;
   if (curline->v1->y == curline->v2->y)

lightnum--;

   else if (curline->v1->x == curline->v2->x)

lightnum++;

   if (lightnum < 0)		

walllights = scalelight[0];

   else if (lightnum >= LIGHTLEVELS)

walllights = scalelight[LIGHTLEVELS-1];

   else

walllights = scalelight[lightnum]; </syntaxhighlight> The vanilla engine has 16 light levels corresponding to the 256 light values, so increasing or decreasing lightnum as done here corresponds to increasing or decreasing the effective sector light for the wall by 16. A perfectly "horizontal" wall is brightened, while a perfectly "vertical" one is darkened; every other wall uses the light level of the front sector.

Previous implementations

The idea for fake contrast predates Doom and extends back to the earliest id Software first-person shooters. In Hovertank 3D, a simple flat-shaded raycasting engine, walls in alternate directions are shaded a darker or lighter shade to create contrast. Its texture-mapped successor, Catacomb 3-D, alternates differently shaded textures instead, though there seems to be flexibility in the assignment. In Wolfenstein 3D, this is accomplished automatically - texture themes are assigned per cell in the world map, and these will automatically include the darker and brighter versions of each texture on the walls facing the appropriate axis in the coordinate system.

Sources

This article incorporates text from the open-content ZDoom documentation project article Fake contrast.