Making Semi-transparent textures

written by Springheel

I don't know if people already know about this, but it's news to me.

When I was working on the lantern textures, I noticed that if I used the following shader the texture turned out to be transparent.

textures/darkmod/metal/flat/lamp_glass_lit_transparent     //glass is partially transparent
{
	glass
	translucent // avoids implicit opaque black stage used for filling the depth buffer

	{
		blend   add 
		map	models/darkmod/props/textures/lamp_glass_lit
		
	 }
}

Better yet, it wasn't totally transparent--you could still see the texture itself, but you could see through it. The effect was pretty neat, so I tried it on a simple brush in a map. Here's the result.

You can clearly see the texture, but you can also nicely see through it. Imagine this as a window. And no vertex shaders necessary, so it shouldn't have conflicts with water (though I haven't tested it). With the "translucent" keyword, you can also add a specular and bumpmap to the texture if you wish. I think this might be a cool way to provide transparent windows but still keep the warm glowy effect. You wouldn't be able to use it for all window textures (since any frame or crosspiece in the texture would also become transparent unless they were straight black) but it will be very effective for some models, and might be useful in other places I haven't thought of yet (an underwater overlay comes to mind).

An example of it in models (the two outside ones).

Pinkdot pointed out that the "blend add" effect will brighten whatever the player sees through the texture. This would work well for looking into well-lit homes or for stained glass. By using the same technique, but replacing "add" with "filter" you can get semi-transparent textures that darken what you see through them, perfect for looking outside or into unlit homes.

It's possible to put the lit texture on one side of a brush and the unlit on the other and have both work, so windows could have the proper appearance depending on whether you are looking inside or outside.

New Findings

Real Translucency

Rich_is_Bored discovered that a technique for translucent lighting discussed on Polygon forums could be implemented in Doom 3 by setting the vertex color gray and using an inverted normalmap for the inversevertexcolor.

Rebb found that you could use the scale keyword to programmatically invert the normalmap.

Finally, Rich_is_Bored found that all surfaces can use the effect if you simply set the diffuses stage to rgb 0.5 which is essentially the same as gray vertex color.

Final example:

translucent/example

{

    // first pass

    {
        blend    bumpmap
        map    normalmap.tga
    }
    {
        blend    diffusemap
        map    diffusemap.tga
        rgb    0.5
    }
    
    {

      blend specularmap
      map specularmap.tgs
    }

    // second pass
    // the scale function here is "inverting" the normal map so that it catches light from the opposite side.

    {
        blend    bumpmap
        map    scale(normalmap.tga, 1, 1, 0, 1)
    }
    {
        blend    diffusemap
        map    diffusemap.tga
        rgb    0.5
    }

    // TDM Ambient Method Related

    {                            
        if (global5 == 1)        
        blend add                
        map              diffusemap.tga      
        scale            1, 1        
        red              global2    
        green            global3    
        blue             global4    
    }                            

}