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idTech4 (aka Doom3) supports only an alpha threshold. That means the shader for a material sets an arbitrary threshold, and values in the alpha map below that value render opaque, and above that value transparent.

The results in pixels either being transparent, or opaque. The only other way to achive translucency is to set the translucent keyword in a shader. This however, fixes the translucency to 50%.

So there is no way to make models to appear fading-in, or fading-out, except by varying the alpha threshold. Together with a good alpha map with ramps, this can make things appear to burn-away (an effect used by D3 f.i. for the imps "burning" away after you killed them), or to "grow thinner" with distance.

Here is a crude example of such an alpha map (the black parts are transparent, white opaque):

Left: Diffuse Center: Normal alpha Right: Alpha with ramp

(note: the greyscale images above represents the alpha channel in a diffuse texture)

Example Material with alpha-test:

        qer_editorimage textures/darkmod/decals/vegetation/cattail_atlas
        description "foliage"

        //twosided (we use rotated patches to save one surface hence one drawcall)

                blend diffusemap
         map    textures/darkmod/decals/vegetation/cattail_atlas
                alphatest 0.5

        // TDM Ambient Method Related
                if (global5 == 1)               
                blend add                               
                map                             textures/darkmod/decals/vegetation/cattail_atlas                
                scale                   1, 1            
                red                             global2 
                green                   global3 
                blue                    global4 

Note: This section:

                if (global5 == 2)               
                blend add                               
                program ambientEnvironment.vfp  
                vertexParm              0               1, 1, 1, 1              // UV Scales for Diffuse and Bump       
                vertexParm              1               1, 1, 1, 1      // (X,Y) UV Scale for specular          
                vertexParm              2               global2, global3, global4, 1    
                fragmentMap             0               cubeMap env/gen1                                
                fragmentMap             1               _flat                   // Bump                         
                fragmentMap             2               textures/darkmod/decals/vegetation/cattail_atlas                        // Diffuse                      
                fragmentMap             3               _black                  // Specular                     

has been removed as it is no longer needed in TDM 2.0 (Standalone)

Example LOD Entity Def:

entityDef atdm:nature_grass_base
	"inherit"				"atdm:nature_base"
	"editor_displayFolder"	"Nature/Grass"

	"editor_color"			".4 0.8 0.1"
	"editor_usage"			"Don't use, base for all TDM LOD grass patches."

	"solid"					"0"

	// grass models usually do not have a clipmodel, resulting in "size == 0"
	// Adjust their density thus:
	"seed_base_density"		"0.01"

	// grass may grow through cracks on gravel, but it certainly grows less dense
	"seed_material_gravel"	"0.05"

	// turn off shadows from this distance on
//	"lod_1_distance"		"250"
//	"model_lod_1"			""
//	"noshadows_lod_1"		"1"

	// hide it from this distance on (it is a small object, so can vanish fast)
	"hide_distance"			"1000"

	"lod_fadeout_range"		"600"

	// by default use more dense grass
	"skin"					"nature/long_grass_dense"

See also