    paper.checkViewport()

For every view in the paper, determine if it fits into the viewport specified by `paper.options.viewport` function. Views that fit (views for which that function returns `true`) are attached to the DOM; views that do not are detached.

While `async` papers do this automatically, synchronous papers require an explicit call to this method for this functionality to be applied. To show all views again, use `paper.dumpViews()`.

    paper.checkViewport(opt)

If `opt.viewport` is provided, it is used as the callback function instead of `paper.options.viewport`. The format of the callback method is described in the option's documentation, as are examples of usage.

In `async` papers, providing `opt.viewport` causes viewport to be temporarily recalculated according to the provided callback function. This may be useful in situations where you need to show different views depending on context (e.g. viewing a section of the diagram vs. printing all of it).