kicad-developers team mailing list archive

[Dick Hollenbeck <dick@xxxxxxxxxxx>]

>>
>> The largest distance that wx can support is INT_MAX, since it represents
>> distance often in a wxCoord or wxSize. As a scalar, a distance is always
>> positive. On most machines which run KiCad, int is 32 bits and INT_MAX is
>> 2147483647. The most difficult distance for a virtual (world) cartesian
>> space is the hypotenuse, or diagonal measurement at a 45 degree angle. This
>> puts the most stress on the distance magnitude within the bounded virtual
>> space. So if we allow this distance to be our constraint of <= INT_MAX, this
>> constraint then propagates to the maximum distance in X and in Y that can be
>> supported on each axis. Remember that the hypotenuse of a 1x1 square is
>> sqrt( 1x1 + 1x1 ) = sqrt(2) = 1.41421356.
>>
>> hypotenuse of any square = sqrt(2) * deltaX;
>>
>> Let maximum supported hypotenuse be INT_MAX, then:
>>
>> MAX_AXIS = INT_MAX / sqrt(2) = 2147483647 / 1.41421356 = 1518500251
>>
>> This maximum distance is imposed by wxWidgets, not by KiCad. The imposition
>> comes in the form of the data structures used in the graphics API at the
>> wxDC level. Obviously when we are not interacting with wx we can use double
>> to compute distances larger than this. For example the computation of the
>> total length of a net, can and should be done in double, since it might
>> actually be longer than a single diagonal line.
>>
>> The next choice is what to use for internal units (IU), sometimes called
>> world units.  If nanometers, then the virtual space must be limited to
>> about 1.5 x 1.5 meters square.  This is 1518500251 divided by 1e9 nm/meter.
>>
>> The maximum zoom factor then depends on the client window size.  If we ask
>> wx to handle something outside INT_MIN to INT_MAX, there are unreported
>> problems in the non-Debug build because wxRound() goes silent.
>>
>> Let:
>> const double MAX_AXIS = 1518500251;
>>
>> Then a maximum zoom factor for a screen of 1920 pixels wide is
>>     1518500251 / 1920 = 790885.
>>
>> The largest ZOOM_FACTOR in above table is ZOOM_FACTOR( 300 ), which computes
>> out to 762000 just below 790885.


If we don't drop the bucket on our toes, proceed carefully, watch where we are going, it
looks like it will hold water using a virtual world sized at 2.14 meters x 2.14 meters.

So far so good.

Dick