Spell animation BAM and Translucent (7) flag

@ScottBrooks
@Avenger_teambg
@Galactygon

I would like to know how the engine implements transparency/translucency (from a mathematical standpoint) into BAMs that themselves store no Alpha channel transparency information.

Let's take the animation for Magic Missile (SPMAGMIS.BAM) as an example:

This BAM is called via a Projectile (MAGICMIS.PRO) which has the "Translucent (7)" flag set. This flag effectively turns

into

What is the equation that calculates the alpha channel levels (or is there some quasi-standard filter that is used)?

TIA,
Sam_