grid.clip. The key difference is that instead of
applying a new clipping context to the viewport, we apply a new
masking context.
pushMask(mask = NULL, label = NULL, name = NULL, draw = TRUE)
popMask()mask object, used as the definition of an opacity mask.
pushMask grob. The value is returned invisibly.
label is specified, uses a mask that has been supplied to
registerMask. If mask is specified it will be used as
the new masking context for the current viewport. If both are
specified, it will attempt to define the mask with the given label, as
well as applying the mask as the new masking context for the current
viewport. popMask is an alias for popContext.
mask, registerMask,
grid.mask, popContext.