To main page
is a kind of texture map
, which shows which part of material reflect the light.
Basically, if you have a rusted barrel - you don't want rust to be shiny, thefore you paint it black in specular map, the parts that are shiny are painted white.
Like most of the maps, you need to program the shader
to interpret the specular map yourself.
This article is a stub, it contains information that doesn't explain subject fully.