$brightness

This entity is Obsolete. Its use is discouraged, and it may only exist/function in older engine branches.

$brightness is a deprecated shader parameter referenced only in the pre-release HDR version of the UnlitGeneric shader.

It would contribute additively to the brightness of the alpha channel of the main texture. However, it is effectively commented out and unused.

References to it can still be found in a couple materials in the final game; it does nothing.

"UnlitGeneric"
{
	// Original shader: AddTexture
	"$basetexture" "effects/spark"
	"$brightness" "effects/spark_brightness"
	"$additive" 1
	"$vertexcolor" 1
}

• 

  spark.vtf

• 

  spark_brightness.vtf