White holes

[jpt13653903]

Clone of this issue on SourceForge, reproduced here for convenience.

The Gerber file format assumes an imaging model where the result can be drawn and erased in so-called "dark" and "clear" levels. Unfortunately, PDF does not support this model. There is no way to erase something that has been drawn.

The opacity is not part of the colour, but rather a property of the graphics state. The resulting opacity after a blend is always a union between the source and destination opacities. The consequence of this is that once the opacity is unity, it will remain unity. There is no way to clear it.

For aperture macros, I make use of clipping paths to obtain the correct result, but this method does not work for copper areas. Once the aperture has been created, there is no way to use that result to influence a lower-level clipping path. To obtain the correct visual result, I cheat and draw "clear" apertures in white.

This only works when a single layer is drawn onto a white page. When drawing a layer with large areas over another layer, the holes in that area are still being drawn as white, which gives the wrong result.

Any suggestions on how to fix this would be much appreciated.

References:

• Gerber Format Specification
• PDF Reference Page
• PDF Reference version 1.5
• Discussion with Adobe