Raster engraving traces the laser across the surface in a
back-and-forth slowly advancing linear pattern that will remind one of the
printhead on an inkjet or similar printer. The pattern is usually optimized by
the controller/computer so that areas to either side of the pattern which
aren't to be engraved are ignored and the trace across the material is thus
shortened for better efficiency. The amount of advance of each line is normally
less than the actual dot-size of the laser; the engraved lines overlap just
slightly to create a continuity of engravure. As is true of all rasterized
devices, curves and diagonals can sometimes suffer if the length or position of
the raster lines varies even slightly in relation to the adjacent raster scan;
therefore exact positioning and repeatability are critically important to the
design of the machine.
The advantage of rasterizing is the near effortless
"fill" it produces. Most images to be engraved are bold letters or
have large continuously engraved areas, and these are well-rasterized. Photos
are rasterized (as in printing), with dots larger than that of the laser's
spot, and these also are best engraved as a raster image. Almost any
page-layout software can be used to feed a raster driver for an X-Y or drum
laser engraver. While traditional sign and plaque engraving tended to favor the
solid strokes of vectors out of necessity, modern shops tend to run their laser
engravers mostly in raster mode, reserving vector for a traditional outline
"look" or for speedily marking out lines or "hatches" where
a plate is to be cut.