HP PCL/PJL reference (PCL 5 Color) - Technical Reference Manual

EN Raster Graphics 6-5
An image’s width and height define the extent of the image, that is,
the number of pixels in a row and the number of rows in the image.
The following commands and features eliminate the need to send
blocks of zero pixel values:
1 The Y Offset command can be used to direct the printer to set all
the pixel values in the given number of rows to zero.
2 If not enough data is sent for all the pixels in a row, the remaining
pixels are set to zero.
3 If the image is ended before all rows have been received, the
values of the pixels in the remaining rows will be set to zero.
Furthermore, if too much data is sent the data extending outside the
raster area is clipped and discarded.