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PP (Pixel
Placement)
When printing, the printer places pixels at the intersection
of the squares of a theoretical, device-dependent grid
covering the printable area on a page. When the sides of
two HP-GL/2 polygons touch each other, the pixels along
the border may be printed twice or not at all—depending on
the logical operation in effect. For example, if a source
rectangle consisting of all 1’s is XORed with a destination
consisting of all 1’s, a white rectangle is printed. If another
source rectangle is placed on the page touching the first
rectangle, the two rectangles are white-filled except at their
common border: that is, (1^1)^1 = 1.
To correct this situation, two models of pixel placement are
used: 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. 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, when two or more
polygons on a page share a common border, grid centering
(value=1) can be turned on.
Since PCL printers print only at the intersections of the
grid, the actual implementation of the grid-centered model
is also shown in the following illustration.
Color Vector Graphics (HP-GL/2) 7-19Color Vector Graphics (HP-GL/2) 7-19