I looked here:http://eigen.tuxfamily.org/dox/TutorialGeometry.html and tried to do this:

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Quaternion<float> q = AngleAxis<float>(angleX * M_PI / 180, Vector3f::UnitX());

I returned a long log of errors with this one:

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../src/camera/Camera.cpp:23: error: conversion from `Eigen::AngleAxis<float>' to non-scalar type `Eigen::Quaternion<float, 0>' requested

How to initialize the quaternion?

ok, this is an annoying c++ issue, while the syntax Type obj = other; makes a call to the Type ctor (and not operator=), c++ does not accept to call an explicit ctor in this case, you either have to write:

Quaternion<float> q(AngleAxis<float>(angleX * M_PI / 180, Vector3f::UnitX()));

or

Quaternion<float> q = Quaternion<float>(AngleAxis<float>(angleX * M_PI / 180, Vector3f::UnitX()));


I'll fix the doc and put a warning about that very common issue.

Ok. In the same document:

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vec2 = gen1 * vec1;

Thus I do:

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   Vector3d         frustumDirection;
.
.
.
   Quaternion<double> q(AngleAxis<double>(angleX * M_PI / 180, Vector3d::UnitX()));
   frustumDirection = q * frustumDirection;


And is there more optimized method of transformation a vector, for instance:

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frustumDirection *= q;
OR
frustumDirection.transform(q);


frustumDirection *= q;

would means:

frustumDirection = frustumDirection * q;

that is not the same. And no there is no such shortcut. Note that in this case that would only optimize the writing, not the performance.