$lightwarptexture

The technique was first used in

Day of Defeat: Source's snow maps, such as Kalt.

$lightwarptexture is a cornerstone of

Team Fortress 2's illustrative character rendering.

Posterized color can be achieved, but lighting must be tightly controlled.

Template:Shaderparam It tints texels depending on their brightness. It can be thought of as localized color correction.

Caveats

Important:$lightwarptexture must be specified after all other textures, otherwise it will overwrite them.

Important:Make sure the texture is clamped in the VTF otherwise it will render incorrectly.

Important:$lightwarptexture on VertexLitGeneric requires a $bumpmap. If not specified, a default one will be used instead! It will be as if a $bumpmap is in the VMT!

Warning: This means that all parameters that don't work with $bumpmap also won't work with $lightwarptexture.

$detailblendmode 5-9 won't work - 5 & 6 will work when $phong is used.
$envmapmask won't work, except in Mapbase.
$selfillum_envmapmask_alpha doesn't work in any branch.

Note:Does not work without $phong on VertexLitGeneric in

Counter-Strike: Source and

CS:GO engine branch.

Note:$lightwarptexture does not seem to exist at all for LightmappedGeneric in

CS:GO engine branch.

Bug:Does not work when using $detail or $bumpmap2 on both LightmappedGeneric and WorldVertexTransition. [todo tested in ?]

Bug:Does not work when using $selfillumfresnel without also using $phong. [todo tested in ?]

Bug:Does not work when using $seamless_scale. [todo tested in ?]

Lightwarp textures

A lightwarp texture is a one-dimensional strip of pixels. Dark texels will be tinted with the colour on the left-hand side, and bright texels with the colour on the right. 50% gray means no tint.

VTF parameters

A lightwarp texture should use uncompressed BGR888 format to avoid undesired color banding on lighting gradients^[1], and should be clamped.

If using VTEX, use these compile parameters:

nocompress 1 // disable texture compression, to prevent undesired color banding
nomip 1 // disable mipmaps, which are unnecessary in this instance
nolod 1 // always use the full resolution version of this texture (due to being low-resolution to begin with)
clamps 1 // prevent tints at lighting extremes from blending together
clampt 1 // optional?
stripalphachannel 1 // remove unnecessary alpha channel, if present

Examples

snow_warp: $dod\materials\ground\snow_warp$; This is the lightwarp texture used in the Day of Defeat: Source screen above. It is designed to add a subtle bluish tint to darker snow without affecting brighter regions.
pyro_lightwarp: $tf\materials\models\player\pyro\pyro_lightwarp$; This is applied to all Team Fortress 2 characters, not just the Pyro. It creates the high-contrast shadows with red-tinged terminators typical of the illustrative art the game apes.
Posterized: This custom warp texture creates a posterized "line art" style. Enabling lightwarp alone isn't enough however: ambient lighting must be at 0 to make unlit areas black, and world lighting must become viewer-dependent to ensure that shadows are always cast.
Cel Shading: Cel shading can make objects look more cartoony by replacing the smooth lighting gradient with sharp steps.