After graphical imagery has been rendered to an X window, the user may want the window contents to be captured and sent to a printer for hard copy output. The most widespread technique for doing this is to perform a screen dump, that is, to read back the window pixels with XGetImage, and to send the resulting image to the printer. To fit the image to the size of the printed page, some image resampling may be necessary. This can introduce aliasing artifacts into the image.
Another print capture technique that is growing in popularity in the X11 community is to re-render the graphics through a special printer graphics API. This API supports the standard Xlib graphics calls. It converts these calls into a page description language (PDL) format and sends it to the appropriate print spooler. The advantage of this technique is that the graphics can be scaled to fit the printed page by scaling the coordinates themselves rather than the pixels after scan conversion has been applied. As a result, aliasing artifacts are minimized.
The print API technique has a significant drawback when applied to an overlay/underlay window pair. Most PDLs only support the notion of opaque paint; they do not provide for the marking of transparent paint. In the PostScript PDL, for example, the marked pixels always supersede what was previously marked. Given such a limitation, it is not always possible to capture the imagery in an overlay/underlay window pair using this technique. Certainly, in applications where the background of the overlay is completely transparent and only opaque paint is drawn to it, the underlay could be marked first and the overlay marked second. But if transparent paint was drawn to the overlay, erasing other opaque paint in the overlay, this would not work.