[Replying to my own question for future reference of others.]
On close inspection of the documentation, it seems that image_pattern is the
best way to use a grayscale image as an opacity map:
texture {
image_pattern { png "image.png" }
texture_map {
[0 texture { pigment {transmit 1 } } ]
[1 OtherTexture ]
}
}
Or for simple colors:
pigment {
image_pattern { png "image.png" }
pigment_map {
[0 transmit 1 ]
[1 OtherPigment ]
}
}
