6.1.1 How to define the observer position ?

When creating an image from a model, one has to choose the observer position, the look at point and the orientation of the head. The user has tree possibilities to define these parameters :

Define manually the observer matrix obs.obs is a $4\times3$ array matrix. Meaning of the four vectors composing this matrix is given in the following table :

`obs`	Meaning
obs[0]	position of the observer
obs[1]	position of the look at point (with respect to the position of the observer)
obs[2]	position of the head (with respect to the position of the observer)
obs[3]	position of the arm (with respect to the position of the observer)

Using the parameters xp, x0 and alpha, where x0 is the observer position, xp is the look at point and alpha the angle between the head and the axis.
Using the parameters view and r_obs. view can be equal to 'xy', 'xz' or 'yz', the projection being parallel to one of the main axis. r_obs gives the distance between the observer and the look at point.

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