HP PCL 5 Comparison Manual page 63

Placement Variations
PCL and HP-GL/2 (see following pages for HP-GL/2 pixel
placement command description) provide two pixel
placement modes: grid intersection (the default) and grid
centered. Grid intersection places pixels on the
intersections of the grid (see Figure 2-2). Grid centered
places pixels in the center of the grid. In Figure 2-2, a
rectangle extends from position (1,1) to (3,4). The grid
centered model produces a rectangle one dot thinner and
one dot shorter then the grid intersection model.
When rectangular area fills are used and grid intersection
is used, an overlapping of pixels can occur if rectangular
area fills are placed adjacent to one another (as shown
below). Depending on the raster operation presently in
effect, this overlap can produce undesirable results in the
final printed image. To avoid this problem, use the grid
centered method.
Note
Since PCL printers print only at intersections, grid centered
is implemented as shown on the right.
Figure 2-2. Pixel Placement Variations
Printer-Specific Differences 2-27