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The technique was first used in Day of Defeat's snow maps, such as Kalt.
The green channel of DOD's ground\snowblendtexture.

$blendmodulatetexture is a material parameter for the WorldVertexTransition shader. It changes the transition between the textures from a smooth, linear gradient to one "stamped" with a pattern.

You should use $blendmodulatetexture wherever possible, as the improvement in visual quality is immense!

Blend modulate textures

Tip:The best way to create a modulation texture is to import the two albedos to an image editor as layers, and create an alpha channel there by hand.

Only two colour channels are read from the texture:

Biases toward $basetexture2. 128 is normal.
The idea is to paint noise and patterns into this channel, so that the edge between the two textures becomes irregular. Valve's snow texture (see right) has lots of ragged edges in the green channel, and even some bootprints. There are also lines painted along the edges of the albedo's bricks that give them a tendency to have snow along their edges.
Sharpens. 0 makes the transition binary, 255 effectively disables modulation.
The red channel of Valve's snow modulation texture is a constant 99, which produces a fairly sharp falloff. Using different values in different places has the potential to produce a more complex effect.

There is not always a need for the texture to be high resolution - high frequency blends can be produced from low-re textures.