r <- raster(nrows = 480 ,ncols = 736 ,ext = extent(0,600,0,450))

Tx <- (xmax(r)-xmin(r))/3            # Define periods
Ty <- (ymax(r)-ymin(r))/3

x <- rasterToPoints(r)[,1]           # get x coordinates
y <- rasterToPoints(r)[,2]           # get y coordinates
z <- cos(2*pi*x/Tx)+cos(2*pi*y/Ty)   # compute z

r[] <- z
plot(r)