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5-24 The PCL Print Model EN
Pixel Placement
HP PCL 5 printers place pixels at the intersection of the squares of a
theoretical, device-dependent grid covering the printable area on the
page. Depending on the image and the logical operation in effect, a
problem may occur when the sides of two polygons touch each
other—the pixels along the common border may be printed twice or
not at all. For example, a source rectangle consisting of all 1’s that is
XORed with a destination consisting of all 1’s produces a white
rectangle; but if another source rectangle is placed on the page
touching the first rectangle, the two rectangles will be white-filled
except at their common border ( (1^1) ^ 1 = 1).
To correct situations where this problem occurs, the PCL printer
language provides a choice of pixel placement models: grid
intersection and grid centered. The grid intersection model is the
default: pixels are rendered on the intersections of the
device-dependent grid covering the page. In the grid-centered model,
the number of rows and columns are each reduced by one, and pixels
are placed in the center of the squares, rather than at the
intersections.
The following example illustrates the concepts of the two models (see
Figure 5-5). Assume a rectangle extends from coordinate position
(1,1) to position (3,4). As shown below, for the same coordinates, the
grid-centered model produces a rectangle that is one dot row thinner
and one dot row shorter than the grid intersection model. Thus, the
grid-centered model should be selected when two or more polygons
on a page may share a common border.
Since PCL printers print only at the intersections of the grid, the
actual implementation of the grid-centered model is shown on the
right.