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There are several methods to access certain positions at a path. At first there
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are `atbegin` and `atend` methods, which return a coordinate tuple. ...
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! In this example, we have defined a small helper function `mark` to which we
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can pass the return value of the `at...` methods. We do this by transforming
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the sequence in to positional argument of the function call. This Python
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language feature is available by the `*` syntax in the call.
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! The coordinates returned by the `at...` methods are PyX lengths in the
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unscaleable true units similar to the return value of the `arclen` method.
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For the left path `p1`, we also show how to use the `at` method,
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which can be used to get the coordinates of a certain point of the path
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depending on the arc length along the path.
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A different use of the `at` function is shown for the right path `p2`: It is
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also possible to pass parametrization instances of the path to the `at` method.
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In the shown case we first fetch such parametrization instances for the
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beginning and the end of the path by the `begin` and `end` methods. Note that
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the `atbegin` method is equal to call `at` with the result value of the `begin`
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method (except for optimizations: atbegin is faster than the two calls).
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Similar `atend` could be rewritten using the result value of `end`. As shown in
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the example you can use the parametrization instances to add and substract arc
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lengths from selected point.
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! The `at` method of a path instance can not only handle a single value, in
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which case it returns a single result tuple. You can also pass a list to the
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method and the return value will contain a list of coordinate tuples. You
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should bear in mind that passing a list is much more efficient than multiple
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calls of the method performing single conversions at each time.