Alpha masking
From TrainzOnline
Alpha masking refers to a special case of alpha blending where all pixels are rendered as either fully opaque (100% alpha) or fully transparent (0% alpha) with no partial transparency. This can be treated as a special case because it does not require alpha sorting in order to produce visually correct results. This provides a significant performance win over an equivalent alpha sorted material.
TBD: Materials which support alpha masking.
TBD: Control of alpha masking via texture.txt file
TBD: Image of alpha mask and the in-game result, versus alpha blending.
TBD: Discussion of using alpha testing without a b&w texture.
TBD: Overdraw concerns
TBD: alpha masking vs mip mapping