No subject

may have manufacturing/solderability problems, and it is often
necessary to modify a footprint after a board has been laid out.
Granted, for a small board, it's not much of a problem, and you can
generally do some magic with search and replace.  However, boards are
getting more complicated, and replacing footprints for 200 resistor
instances manually is quite painful and risks screwing up the board in
the process.

I want to help fix this issue and submit some patches; unfortunately,
it is not simple and will most likely require significant changes to
the file format and architecture.  The ideas document has a few
potential fixes, and it would be great to see some discussion and
hopefully reach a consensus on what really needs to be changed and how
to proceed about it.

This is one proposal from the ideas document:
---------------------------------
  proposal:  store a board side and rotation flag for the element as a
  whole.  The internal contents of the element will show top layer
  and 0 rotation.

harry: comment
  The internal contents should remain rotated and on the layer they are on.
  This avoids constantly re-computing these values for every re-draw for
  example. The flags can inform the system how to transform the component
  back (or transform a replacement version forward) to the "standard"
  format. Presumably this would be an uncommon operation in which the
  computations take place.
end-comment harry
---------------------------------

My opinion:
This proposal would certainly be a step in the right direction.  We
should copy each library footprint into the document once, and then
reference it by name with an appropriate transformation matrix to map
the component coordinates to the board coordinates.  A mirror flag
could indicate that the component should be flipped layer-wise.

I slightly disagree with Harry on the topic of storing the rotated
elements.  It would be OK to store a pre-computed representation
internally, but it should be strictly a cache feature, and should not
ever make it to the .pcb file.  A rotation or layer change command
should be performed by modifying the rotation matrix and invalidating
the cached representation.   This yields several useful features: it
will be possible to display and change any component's rotation angle
to an exact value with arbitrary precision; it makes the .pcb file
more compact and less redundant; it makes it much easier to reconcile
the embedded components with the parts library.

The second proposal in the file entails maintaining a list of changes
to the library footprint for each instance.  This begs the question:
is this even necessary?  The vast majority of commercial PCB layout
packages do not permit footprint modification on the board level.
Permitting an individual instance of a footprint to be modified would
let the PCB designer essentially change the netlist and component
specifications at will, which seems pretty dangerous and semantically
wrong.  In almost all cases where a footprint needs to be changed, the
safest and most appropriate way to do it would be to create a
different version of it.

I would love to hear everyone's thoughts and opinions on this.  Is
there something I am not seeing here?

Thanks,
Igor