Sets the transparency mode for this object, 0 is opaque, 1 is alpha blended, 2 is additive blended, 3 is custom blending. Using a transparency value greater than 0 will slow down rendering, it also doesn't write transparent objects to the Z buffer so it might cause some depth ordering problems. There is an alternative form of transparency called Alpha Masking that is useful if you only need fully transparent or fully opaque pixels but no blending in between. See SetObjectAlphaMask for more details.
When using mode 3 (custom) the blend values must be specified with SetObjectBlendModes otherwise the object will not be transparent.
SetObjectTransparency( objID, mode )