gradientPage <- linearGradient(c("black", "white", "black"), 
                           gradientUnits="coords",
                           x0=0, y0=.5, x1=1, y1=.5)


pushViewport(viewport(width=1/3, name="vp"))
registerGradientFill("g", gradientPage)
upViewport()
grid.rect(1:2/3, 1:2/3, width=1/3, height=.2, name="r2")
grid.gradientFill("r2", label="g")
grid.export()


pdf(NULL, width=2, height=1)
grid.rect(gp=gpar(col="grey"))
pushViewport(viewport(width=1/3, name="vp"))
registerGradientFill("g", gradientPage)
upViewport()
grid.rect(1:2/3, 1:2/3, width=1/3, height=.2, name="r2")
grid.gradientFill("r2", label="g")
grid.export()
downViewport("vp")
grid.rect(gp=gpar(lty="dotted"))
grid.export("Figures/export-gradient-fill-register.svg")
dev.off()
system("inkscape --export-pdf=Figures/export-gradient-fill-register.pdf Figures/export-gradient-fill-register.svg")
system("convert Figures/export-gradient-fill-register.pdf Web/export-gradient-fill-register.png")