Polarity Reversal

The Reversal checkbox in the GBRUnion dialog box is intended to reverse the polarity of the resulting file. Digitized data (normally considered :dark") is XOR'd with a frame and the result is that digitized areas become "clear" and non digitized areas (out to the frame) become dark.

  board before polarity reversal

A standard PCB layout with normal polarity (i.e. digitized items are dark)

By checking the reversal option and say a RS274X output, the resulting file will be unionized and the figures are subtracted from a frame - the frame can be user defined using the Window option or if not, the extents of the data are calculated and that extents is used as the frame.

  after polarity reversal

After reversal the clear areas are dark. Note that there is no "margin" at the edges.

Often, a margin is needed. The user can enter both an X and a Y margin. The "frame" used to calculate the reversal is increased on each side by the margin amount. In the screenshot below, an X and Y margin of 0.25 inch as entered.

  after polarity reversal and margin

The reversal with a margin.