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Re: gEDA-user: pcb crooked traces
Rick Collins wrote:
While accumulation of error is an issue with floating point, the
connector or any predefined
shape is a particularely bad example:
At least I wouln't convert the spacing and then sum up the erratic
value but convert
all the original values, which gives exactly 1 conversion error per
position, irrespective
of the number of pins/position. And that's exactly how the read
routines operate.
To do it the wrong way, they would need to be fortune tellers,
because there is
no 'array' element in new-lib files.
Then please come up with some of your own. I am sure there is no
shortage of examples just because I haven't thought of them.
I believe such an example will be hard to find in a well programmed CAD
application.
The reason is, that significant accumulation of discretization errors
happens when
nummerous small values get added to a large value or with small
differences of
large numbers.
Typical examples for these problems are:
*) solving large badly conditioned linear equation systems
*) numerical treatment of initial value problems of ordinary
differential equations
*) statistical computations
Even for those apps there are algorithms that account for the problem -
skilled developers are aware of it and avoid it.
The general procedure in a CAD system is, to place "primitives" in a virtual
space with as few operations as possible and derive some data (screen view,
gerbers, drill coordinates,...) from this model as straight forward as
possible,
again involving only a hand full of operations per primitive. Since the
primitives are independent of each other, there is no accumulation of error.
One example of the tiny difference of large numbers is, to fill an area
with parallel traces, using the same trace width as center distance.
Depending on the display scale you might see chinks. As far as I know,
this is known bad practice. I'm not a developer of pcb, so if there
really are issues with this, others speak up please.
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