[ES] texture alpha channels

[michaelrw]

Hey all, Im looking for some additional information from experienced artists about the various uses of the alpha channel in the different maps used in skyrim and other TES games.

My main confusion stems from using an alpha channel in a diffuse map for transparency on non-transparent meshes. Landscape textures are a good example of this. 

 

 

Example 1: architecture\whiterun\wrfieldgrass.dds

 

a pretty standard grass texture, nothing special. if you look in the alpha youll find what appears to be outlines of the walkway stones. It seems that this alpha is used in places where the grass meets the road, giving the effect of the grass growing around the road stones.

 

 

Example 2: landscape\rocks01.dds

 

a standard rocks texture. The alpha channel looks pretty much like a grayscale rendition of the RGB, with a little bit of blur and decreased contrast. From my testing, this alpha channel seems to be used in conjunction with NiAlphaProperty on the mesh it's being applied to. a lot of the rock and mountain meshes use this texture on the edge/trim as a transition into the next type of land texture. I think the alpha is used to determine how much or which parts of the texture are used (based on the alpha threshold set in the mesh). This is on a hypothesis based on some of my observations. Any clarification here would be very much appreciated. 

 

 

Example 3:

landscape\coastbeach01.dds

landscape\dirt02.dds

landscape\fallforestgrass01.dds

etc.

 

these textures all have alpha channels that are basically just grayscale version of the RGB. unlike the previous examples, there is no obvious reason why they would have an alpha channel. There is no transparency (i know this because i cant see through the ground ). They arent used on any meshes that i can readily open in nifskope and investigate. I also have never seen textures in this group be applied only partially and/or as trim on a mesh. My only thought is the alpha is used to sort of "dull down"  the appearance of the texture in-game. This is probably not correct, but it is the only thing i can think of. Any clarification here would be greatly appreciated as well.

 

Thanks

[neomonkeus]

The two criterion for enable alpha values to be used are a NiAlphaProperty and the shader flags. 

 

You have more or less covered the texture part but there is also the vertex colors have an alpha component, If vertex colors are enabled in the shader then these will also use the alpha. Your example given above is excellent for such a case, as the mesh itself will fade out if it has vertex_alpha. This is used in combination with the alpha technique on the texture layer. 

 

Also just an aside, NifSkope is a best effort render, it will attempt to render the mesh, but that might not accurately reflect how it will look in-game as that depends on the implementation of the game engine, as many share the same format but render implementation varies.

[_echo]

IIRC, the alpha channel also determines the level of "shiny" an texture has. Full white is dull and flat, black is bright and shiny, or was it the other way around?

Basically the textures that don't seem to need it use it to done down the shine due to directional lighting "reflecting" off it. In the case of stones and grass, they don't want much shine.

[michaelrw]

The two criterion for enable alpha values to be used are a NiAlphaProperty and the shader flags. 

 

You have more or less covered the texture part but there is also the vertex colors have an alpha component, If vertex colors are enabled in the shader then these will also use the alpha. Your example given above is excellent for such a case, as the mesh itself will fade out if it has vertex_alpha. This is used in combination with the alpha technique on the texture layer. 

 

Also just an aside, NifSkope is a best effort render, it will attempt to render the mesh, but that might not accurately reflect how it will look in-game as that depends on the implementation of the game engine, as many share the same format but render implementation varies.

great! some new information to look into!

Ive seen the vertex colors shader flag before and even enabled/disabled it a few times depending on what i was doing. however ive never really known what vertex colors are or what they do. I will do some research though there should be some information on it.

 

Also, you say "Your example given above is excellent for such a case"  .. im not entirely sure which of the examples youre referring to.

Thanks for the reply, very much appreciated.

[michaelrw]

IIRC, the alpha channel also determines the level of "shiny" an texture has. Full white is dull and flat, black is bright and shiny, or was it the other way around?

Basically the textures that don't seem to need it use it to done down the shine due to directional lighting "reflecting" off it. In the case of stones and grass, they don't want much shine.

yeah the shininess is usually referred to as the specularity and the map for that is stored in the alpha channel. but i have only ever seen specular maps in the alpha channel of normal maps, never in diffuse. I dont think skyrim is even set up to read specularity from diffuse maps even if you wanted it to. i could be wrong though

 

you got it backwards btw, .. white is shiny black is dull

Edited August 31, 2013 by michaelrw