[PD] Free rotation in GEM

[Mathieu Bouchard]

On Thu, 17 Jul 2008, PSPunch wrote:

> My primary goal is to place an object you can rotate by dragging the mouse.

Ok, if it's through a user interface like that, for computing just one 
rotation matrix, I guess you could use [accumrotate] unless it accumulates 
too much error. Suppose that it consistently gets a relative error of 
2**-24 downwards on every rotation. Then after 1000000 rotations it gets 
its values 6% wrong. If rotating using doubles (float64) instead of 
float32, then you'd hardly notice anything after 1000000000000 rotations. 
Note that this is just a rule of thumb of worst cases. It's better to try 
it. [accumrotate] doesn't try to compensate for rounding errors because 
they aren't quite significant in practice in this case. (it's possible to 
do the theory for figuring out average realistic error but it's easier and 
more accurate to just try the implementation.)

> Using procedures completely different from the one mentioned abouve, I 
> was able to achieve the expected behavior based on [accumrotate] but I 
> found no simple way of managing the current angle.

What do you mean "managing the current angle" ??

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