Fax

An FFGL effect for Resolume Arena and Avenue. A Group 3 fax doesn't send pixels. It turns each scan line into black and white, 1728 pels across 215 mm, and sends the lengths of the runs as variable-length codes, one line at a time, each finished with an EOL. In MR most lines are sent as the differences from the line above, with a plain one-dimensional line every K lines so an error can't run on forever.

Fax pushes your clip through an actual T.4 coder, a noisy line and a decoder, and the fax look comes out of that rather than being drawn on afterwards. A bit error makes the decoder read the wrong code lengths, so it streaks from the error to the right-hand edge of its line and stops at the next EOL. In MR the damage runs downwards too, because every following line is decoded against a spoilt one, until the next one-dimensional line turns up: a wedge at most 2, 4 or 8 lines tall. A receiver that can tell a line is bad repeats the line above, which is that familiar vertical stretch.

Standard resolution is 3.85 lines per millimetre against 8 pels across, so a standard fax comes out squashed and gets doubled up vertically. In Page mode the page arrives a line at a time at the line rate, quick over blank paper and slow over detail, and never faster than the 10 ms a scan line is allowed. In Live mode every frame is a brand new page. The sending machine prints its header line across the top before coding the page, so that gets mangled on the way like everything else.

The code tables were typed in from the Recommendation by hand (about as much fun as it sounds) and are checked against two independent transcriptions, Go's x/image/ccitt and pdfminer.six. The plugin's own output is decoded back by Go's CCITT decoder, which shares no code with it. The print side draws the received page by how much area the black pels cover, on warm-grey thermal paper or plain paper.