gEDA-user: 274x Gerber Files

[DJ Delorie]

> I guess I didn't think about it that way. When I fill in my order, I
> am required to check the polarity of each file submitted. So you are
> telling me that my soldermask would positive then - right?

Welcome to the confusing world of gerber.

The soldermask plots are negatives, so check the negatives box.  The
other layers are positives.

The confusion is that the polarity of a layer is NOT the same as what
the layer ends up looking like.  It's a means of defining the layer;
either by drawing the parts you want or by drawing the parts you don't
want.  In the case of soldermask, we draw the parts we don't want -
the holes.  But the layer is the mask, hence we tell it we're drawing
HOLES in the mask, not the mask itself.  If the layer were positive,
we'd have to draw a lot of complex polygons to define the mask shape.

Also, sublayers within each gerber can have a dark or light polarity,
which basically lets you draw or erase within each layer.  In theory,
we could have had a positive mask layer, drawn a big (dark polarity)
rectangle, then erased (light polarity) the holes.