GE_CM_DRAWMODE_COPY New pixels overwrite old ones.
GE_CM_DRAWMODE_BLEND New pixels blend with old ones, taking the alpha value of the new pixels in account as well as the blend factor specified by the parameter. The value specified by the parameter is the opacity of the source (bitmap). It ranges from 0 to 256, where 256 equals 1.0 and means opaque.
GE_CM_DRAWMODE_MASK New pixels overwrite the old ones at the bits where the mask specified by the parameter equals 1.