Prefer zooming on the screen scaling factor instead of the fov value

[bmatthieu3]

I think it would be more stable if the zooming directly influences the scaling factor of the screen image and not the fov which gives different scaling factors in function of the projection used. Internally aladin lite would have to compute the fov from a specific scaling factor.

[tboch]

Can you elaborate on this? Would that imply having the FoV value changed when changing projection?

[bmatthieu3]

No because when changing the projection we can still reset the screen scaling factor of the screen with respect to the current fov.
I noticed a delta of fov degrees (i.e. one wheel event) influences differently the screen scaling factor in function of the projection chosen (this is especially true for all sky view). It provides a different zooming effect (more or less fast) in function of the projection. It is more relevant and projection agnostic to treat the wheel events as a way to directly change the scaling factor (in the screen space) instead of the fov degrees (which is a sky quantity).