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.
5-24 The PCL Print Model | EN |
