Re: [Algorithms] Up Vector in Projection Matrix

[Pai-Hung C. <pa...@ac...>]

> I'm no D3D expert - but I'll presume this is similar to the OpenGL
'glLookAt'
> stuff.
>
> When you specify a point or direction to "look at",  you have only
specified
> two of the degrees of freedom of the system.  That object could be in the
> middle of the screen - either upside-down - or right way up - or at any
> 'roll' angle in between.
>
> The 'up' vector only has to be enough to resolve that issue - and only one
> of its degrees of freedom is needed.  Hence, there are a wide variety of
> vectors that will do the job.  The only constraint is that the up vector
> and the 'look direction' are not parallel.
>
> Hence, so long as your character can't look absolutely vertically upwards
> or absolutely vertically downwards, an 'up' vector of (0,1,0) will do
fine.

That explains everything (though I don't know why/how the underlying math
works).  An ever-confusing question got explained!  Thanks a lot! :-)

Pai-Hung Chen