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A general description is here.
‘dev.capabilities()’ reports on device support for the new features.
The ‘grid’ package has new functions for drawing isolated groups: ‘grid.group()’, ‘grid.define()’, and ‘grid.use()’. These functions add compositing operators and affine transformations to R's graphics capabilities.
The ‘grid’ package also has new functions for stroking and filling paths: ‘grid.stroke()’, ‘grid.fill()’, and ‘grid.fillStroke()’.
A new function ‘as.path()’ allows the user to specify the fill rule for a path that is to be used for clipping, stroking, or filling; available options are ‘"winding"’ and ‘"evenodd"’. A new function ‘as.mask()’ allows the user to specify the type of a mask; available options are ‘"alpha"’ and ‘"luminance"’.
These new features are only supported so far (at most) on the Cairo-based graphics devices and on the ‘pdf()’ device.