PMDF System Manager's Guide

PMDF-FAX generates FAX pages of a fixed length L_0 . When these pages are passed to a FAX modem for transmission, the modem prepends a header (i.e., adds additional scan lines) to the top of the page. This extends the length of each page to the length L_0+d_tx , d_tx>0 . Then, when the FAX is received at a remote station, some additional scan lines containing reception information (e.g., local time, page number, etc.) may be added to the top or bottom (or both) of each received page. Thus, the final length of a received page is L_rcv=L_0 +d_tx+ d_rcv , d_rcv >= 0 .

Now, PMDF-FAX has no control over the values d_tx and d_rcv , and no way of knowing a priori the expected value for d_rcv . Given this, it should come as no surprise that in some cases L_rcv will exceed a desired length L such as 11 inches. The best that can be done is to choose a value for L_0 which will yield, in most cases, L_rcv < = L .

By default, the PS_TO_G3 channel uses L_0=10.75 inches ( 27.305 cm). However, some sites may wish to adjust this length. Two common reasons for wanting to do so are: (1) to produce an overall page size closer to that of an A4 sheet of paper (29.7 cm), or (2) so that a full page of graphics may be presented. (With the defaults, the top 1/4 inch of an 11 inch long page will be lost.)

To accomplish goals such as these, the PS_TO_G3 channel options BITMAP_HEIGHT_IN_PIXELS (number of scanlines per page) and BITMAP_VERTICAL_DPI (number of scanlines per inch) should be adjusted. By default, the settings

BITMAP_HEIGHT_IN_PIXELS=2107 BITMAP_VERTICAL_DPI=196

are used. These produce a 10.75 inch long page (2107 scanlines divided by 196 scanlines per inch = 10.75 inch). Now, to simply produce an 11 inch long page, specify, in the ps_to_g3_option file,

! 11 inch long page BITMAP_HEIGHT_IN_PIXELS=2156 BITMAP_VERTICAL_DPI=196

Or for a 29.7 centimeter long page (A4), use

! 29.7 cm long page BITMAP_HEIGHT_IN_PIXELS=2291 BITMAP_VERTICAL_DPI=196

In both cases, the value for BITMAP_HEIGHT_IN_PIXELS was determined by multiplying the desired page length, in inches, with BITMAP_VERTICAL_DPI.

Now, suppose you want to image a full 11 inch page of graphics (i.e., L_0=11 , but then want to shrink that page a bit so that it is received as an 11 inch or shorter page (i.e., L_rcv=L_0 +d_tx+ d_rcv < = 11 . One such case where this might arise is when full page graphics are being transmitted to a plain paper FAX machine which will output two sheets of paper for every received page whose length exceeds the length of a single sheet. To accomodate such a situation, you can use a trick: decrease BITMAP_VERTICAL_DPI. This will cause fewer scanlines per vertical inch to be generated. This trick works because FAX machines themselves support only two vertical resolutons: 98 or 196 scanlines per inch. So, for instance, if you generate 2107 scanlines at 191.5454 lines per inch (11 inches total) and transmit them at high resolution (196 lines per inch) to a remote FAX machine, they will be output at 196 lines per inch thereby producing a 10.75 inch long page. The proper option settings in the PS_TO_G3 channel option file would be, for this case,

BITMAP_HEIGHT_IN_PIXELS=2107 BITMAP_VERTICAL_DPI=191.5454

The general formula for computing these two numbers is given by
BITMAP_HEIGHT_IN_PIXELS= R L
BITMAP_VERTICAL_DPI= R L / L_0 where L is the desired length for the received FAX page, L_0 is page length to generate when the FAX is created, and R is the resolution at which the FAX is to be transmitted (either 98 (low) or 196 (high) lines per inch).