gEDA-dev: code sprint ideas

[Bernd Jendrissek]

On 10/15/07, Peter Clifton <pcjc2 at cam.ac.uk> wrote:
> With square + rounded, the misalignment is not allowed to be as big in
> both directions at the same time. (Eg.. if the registration is out both
> in the X and Y directions at once, it might mean mask covers the corner
> of the pad whilst the X and Y registrations are otherwise in tolerance.
> (It seems likely to me that the X and Y registrations are independent
> error terms, not some "distance" offset error in registration.

IF your X and Y errors are normally distributed, then exactly because
they are independent variables, you can indeed do a coordinate
transform into (r, theta) and take advantage of the rotational
symmetry of the error distribution: your X and Y error limits would
then be equivalent to a limit on "distance" error.

OTOH, if the fab house achieves its tolerances only by grepping out
the out-of-error-band machine settings, then you'd need a
square+square pattern to survive their out-of-control process with its
rectangular error distribution.