HP 5961-0509 manual Pen Number Represents a pixel in the pattern being defined, White

Note	A pixel is equal to the size of one dot at the current printer resolution.
	z Pen Number - Represents a pixel in the pattern being defined
	and indicates its color (black or white).
	0 -	White
	>0 -	Black

The pen number parameter defines pixels left to right, top to bottom. The total number of pen number parameters should be equal to the width times height parameters. For example, to define a pattern that is 8 x 16 pixels, you need 128 pen number parameters. If you do not include enough pen number parameters, the rest of the pixels are assumed to be white (zero). Patterns are printed in rows parallel to the plotter-unit X-axis.

Table 22-16 Example: Creating and Printing a Fill Pattern

?E	Reset the printer.
?%0B	Enter HP-GL/2 mode.
IN;	Initialize HP-GL/2 mode.
SP1;	Select pen number 1. Even though there is no
	physical pen, the SP command must be used
	to enable printing.
PU5,5;	Lift the pen and move to absolute position
	(5,5).
PA3500,2500;	Specify absolute plotting and move to
	(3500,2500).
RF2,8,4,	Define a raster fill pattern (index number 2) that
0,0,0,0,0,0,0,0,	is 8 dots wide by 4 dots high.
0,0,0,1,1,0,0,0,
0,0,0,1,1,0,0,0,
0,0,0,0,0,0,0,0;
FT11,2;	Select the user-defined pattern having an index
	number of 2.

EN RF, Raster Fill Definition 22-33